2024-02-03 08:32:21

by Christoph Hellwig

[permalink] [raw]
Subject: [PATCH 12/13] writeback: add a writeback iterator

Refactor the code left in write_cache_pages into an iterator that the
file system can call to get the next folio for a writeback operation:

struct folio *folio = NULL;

while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
error = <do per-foli writeback>;
}

The twist here is that the error value is passed by reference, so that
the iterator can restore it when breaking out of the loop.

Handling of the magic AOP_WRITEPAGE_ACTIVATE value stays outside the
iterator and needs is just kept in the write_cache_pages legacy wrapper.
in preparation for eventually killing it off.

Heavily based on a for_each* based iterator from Matthew Wilcox.

Signed-off-by: Christoph Hellwig <[email protected]>
---
include/linux/writeback.h | 4 +
mm/page-writeback.c | 192 ++++++++++++++++++++++----------------
2 files changed, 118 insertions(+), 78 deletions(-)

diff --git a/include/linux/writeback.h b/include/linux/writeback.h
index f67b3ea866a0fb..9845cb62e40b2d 100644
--- a/include/linux/writeback.h
+++ b/include/linux/writeback.h
@@ -82,6 +82,7 @@ struct writeback_control {
/* internal fields used by the ->writepages implementation: */
struct folio_batch fbatch;
pgoff_t index;
+ int saved_err;

#ifdef CONFIG_CGROUP_WRITEBACK
struct bdi_writeback *wb; /* wb this writeback is issued under */
@@ -366,6 +367,9 @@ int balance_dirty_pages_ratelimited_flags(struct address_space *mapping,

bool wb_over_bg_thresh(struct bdi_writeback *wb);

+struct folio *writeback_iter(struct address_space *mapping,
+ struct writeback_control *wbc, struct folio *folio, int *error);
+
typedef int (*writepage_t)(struct folio *folio, struct writeback_control *wbc,
void *data);

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 3abb053e70580e..5fe4cdb7dbd61a 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2325,18 +2325,18 @@ void __init page_writeback_init(void)
}

/**
- * tag_pages_for_writeback - tag pages to be written by write_cache_pages
+ * tag_pages_for_writeback - tag pages to be written by writeback
* @mapping: address space structure to write
* @start: starting page index
* @end: ending page index (inclusive)
*
* This function scans the page range from @start to @end (inclusive) and tags
- * all pages that have DIRTY tag set with a special TOWRITE tag. The idea is
- * that write_cache_pages (or whoever calls this function) will then use
- * TOWRITE tag to identify pages eligible for writeback. This mechanism is
- * used to avoid livelocking of writeback by a process steadily creating new
- * dirty pages in the file (thus it is important for this function to be quick
- * so that it can tag pages faster than a dirtying process can create them).
+ * all pages that have DIRTY tag set with a special TOWRITE tag. The caller
+ * can then use the TOWRITE tag to identify pages eligible for writeback.
+ * This mechanism is used to avoid livelocking of writeback by a process
+ * steadily creating new dirty pages in the file (thus it is important for this
+ * function to be quick so that it can tag pages faster than a dirtying process
+ * can create them).
*/
void tag_pages_for_writeback(struct address_space *mapping,
pgoff_t start, pgoff_t end)
@@ -2434,69 +2434,68 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
}

/**
- * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
+ * writeback_iter - iterate folio of a mapping for writeback
* @mapping: address space structure to write
- * @wbc: subtract the number of written pages from *@wbc->nr_to_write
- * @writepage: function called for each page
- * @data: data passed to writepage function
+ * @wbc: writeback context
+ * @folio: previously iterated folio (%NULL to start)
+ * @error: in-out pointer for writeback errors (see below)
*
- * If a page is already under I/O, write_cache_pages() skips it, even
- * if it's dirty. This is desirable behaviour for memory-cleaning writeback,
- * but it is INCORRECT for data-integrity system calls such as fsync(). fsync()
- * and msync() need to guarantee that all the data which was dirty at the time
- * the call was made get new I/O started against them. If wbc->sync_mode is
- * WB_SYNC_ALL then we were called for data integrity and we must wait for
- * existing IO to complete.
- *
- * To avoid livelocks (when other process dirties new pages), we first tag
- * pages which should be written back with TOWRITE tag and only then start
- * writing them. For data-integrity sync we have to be careful so that we do
- * not miss some pages (e.g., because some other process has cleared TOWRITE
- * tag we set). The rule we follow is that TOWRITE tag can be cleared only
- * by the process clearing the DIRTY tag (and submitting the page for IO).
- *
- * To avoid deadlocks between range_cyclic writeback and callers that hold
- * pages in PageWriteback to aggregate IO until write_cache_pages() returns,
- * we do not loop back to the start of the file. Doing so causes a page
- * lock/page writeback access order inversion - we should only ever lock
- * multiple pages in ascending page->index order, and looping back to the start
- * of the file violates that rule and causes deadlocks.
+ * This function returns the next folio for the writeback operation described by
+ * @wbc on @mapping and should be called in a while loop in the ->writepages
+ * implementation.
*
- * Return: %0 on success, negative error code otherwise
+ * To start the writeback operation, %NULL is passed in the @folio argument, and
+ * for every subsequent iteration the folio returned previously should be passed
+ * back in.
+ *
+ * If there was an error in the per-folio writeback inside the writeback_iter()
+ * loop, @error should be set to the error value.
+ *
+ * Once the writeback described in @wbc has finished, this function will return
+ * %NULL and if there was an error in any iteration restore it to @error.
+ *
+ * Note: callers should not manually break out of the loop using break or goto
+ * but must keep calling writeback_iter() until it returns %NULL.
+ *
+ * Return: the folio to write or %NULL if the loop is done.
*/
-int write_cache_pages(struct address_space *mapping,
- struct writeback_control *wbc, writepage_t writepage,
- void *data)
+struct folio *writeback_iter(struct address_space *mapping,
+ struct writeback_control *wbc, struct folio *folio, int *error)
{
- int ret = 0;
- int error;
- struct folio *folio;
- pgoff_t end; /* Inclusive */
-
- if (wbc->range_cyclic) {
- wbc->index = mapping->writeback_index; /* prev offset */
- end = -1;
- } else {
- wbc->index = wbc->range_start >> PAGE_SHIFT;
- end = wbc->range_end >> PAGE_SHIFT;
- }
- if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
- tag_pages_for_writeback(mapping, wbc->index, end);
-
- folio_batch_init(&wbc->fbatch);
+ if (!folio) {
+ folio_batch_init(&wbc->fbatch);
+ wbc->saved_err = *error = 0;

- for (;;) {
- folio = writeback_get_folio(mapping, wbc);
- if (!folio)
- break;
+ /*
+ * For range cyclic writeback we remember where we stopped so
+ * that we can continue where we stopped.
+ *
+ * For non-cyclic writeback we always start at the beginning of
+ * the passed in range.
+ */
+ if (wbc->range_cyclic)
+ wbc->index = mapping->writeback_index;
+ else
+ wbc->index = wbc->range_start >> PAGE_SHIFT;

- error = writepage(folio, wbc, data);
+ /*
+ * To avoid livelocks when other processes dirty new pages, we
+ * first tag pages which should be written back and only then
+ * start writing them.
+ *
+ * For data-integrity writeback we have to be careful so that we
+ * do not miss some pages (e.g., because some other process has
+ * cleared the TOWRITE tag we set). The rule we follow is that
+ * TOWRITE tag can be cleared only by the process clearing the
+ * DIRTY tag (and submitting the page for I/O).
+ */
+ if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
+ tag_pages_for_writeback(mapping, wbc->index,
+ wbc_end(wbc));
+ } else {
wbc->nr_to_write -= folio_nr_pages(folio);

- if (error == AOP_WRITEPAGE_ACTIVATE) {
- folio_unlock(folio);
- error = 0;
- }
+ WARN_ON_ONCE(*error > 0);

/*
* For integrity writeback we have to keep going until we have
@@ -2510,33 +2509,70 @@ int write_cache_pages(struct address_space *mapping,
* wbc->nr_to_write or encounter the first error.
*/
if (wbc->sync_mode == WB_SYNC_ALL) {
- if (error && !ret)
- ret = error;
+ if (*error && !wbc->saved_err)
+ wbc->saved_err = *error;
} else {
- if (error || wbc->nr_to_write <= 0)
+ if (*error || wbc->nr_to_write <= 0)
goto done;
}
}

- /*
- * For range cyclic writeback we need to remember where we stopped so
- * that we can continue there next time we are called. If we hit the
- * last page and there is more work to be done, wrap back to the start
- * of the file.
- *
- * For non-cyclic writeback we always start looking up at the beginning
- * of the file if we are called again, which can only happen due to
- * -ENOMEM from the file system.
- */
- folio_batch_release(&wbc->fbatch);
- if (wbc->range_cyclic)
- mapping->writeback_index = 0;
- return ret;
+ folio = writeback_get_folio(mapping, wbc);
+ if (!folio) {
+ /*
+ * To avoid deadlocks between range_cyclic writeback and callers
+ * that hold pages in PageWriteback to aggregate I/O until
+ * the writeback iteration finishes, we do not loop back to the
+ * start of the file. Doing so causes a page lock/page
+ * writeback access order inversion - we should only ever lock
+ * multiple pages in ascending page->index order, and looping
+ * back to the start of the file violates that rule and causes
+ * deadlocks.
+ */
+ if (wbc->range_cyclic)
+ mapping->writeback_index = 0;
+
+ /*
+ * Return the first error we encountered (if there was any) to
+ * the caller.
+ */
+ *error = wbc->saved_err;
+ }
+ return folio;

done:
folio_batch_release(&wbc->fbatch);
if (wbc->range_cyclic)
mapping->writeback_index = folio->index + folio_nr_pages(folio);
+ return NULL;
+}
+
+/**
+ * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
+ * @mapping: address space structure to write
+ * @wbc: subtract the number of written pages from *@wbc->nr_to_write
+ * @writepage: function called for each page
+ * @data: data passed to writepage function
+ *
+ * Return: %0 on success, negative error code otherwise
+ *
+ * Note: please use writeback_iter() instead.
+ */
+int write_cache_pages(struct address_space *mapping,
+ struct writeback_control *wbc, writepage_t writepage,
+ void *data)
+{
+ struct folio *folio = NULL;
+ int error;
+
+ while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
+ error = writepage(folio, wbc, data);
+ if (error == AOP_WRITEPAGE_ACTIVATE) {
+ folio_unlock(folio);
+ error = 0;
+ }
+ }
+
return error;
}
EXPORT_SYMBOL(write_cache_pages);
--
2.39.2



2024-02-05 11:39:55

by Jan Kara

[permalink] [raw]
Subject: Re: [PATCH 12/13] writeback: add a writeback iterator

On Sat 03-02-24 08:11:46, Christoph Hellwig wrote:
> Refactor the code left in write_cache_pages into an iterator that the
> file system can call to get the next folio for a writeback operation:
>
> struct folio *folio = NULL;
>
> while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> error = <do per-foli writeback>;
> }
>
> The twist here is that the error value is passed by reference, so that
> the iterator can restore it when breaking out of the loop.
>
> Handling of the magic AOP_WRITEPAGE_ACTIVATE value stays outside the
> iterator and needs is just kept in the write_cache_pages legacy wrapper.
> in preparation for eventually killing it off.
>
> Heavily based on a for_each* based iterator from Matthew Wilcox.
>
> Signed-off-by: Christoph Hellwig <[email protected]>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <[email protected]>

Honza

> ---
> include/linux/writeback.h | 4 +
> mm/page-writeback.c | 192 ++++++++++++++++++++++----------------
> 2 files changed, 118 insertions(+), 78 deletions(-)
>
> diff --git a/include/linux/writeback.h b/include/linux/writeback.h
> index f67b3ea866a0fb..9845cb62e40b2d 100644
> --- a/include/linux/writeback.h
> +++ b/include/linux/writeback.h
> @@ -82,6 +82,7 @@ struct writeback_control {
> /* internal fields used by the ->writepages implementation: */
> struct folio_batch fbatch;
> pgoff_t index;
> + int saved_err;
>
> #ifdef CONFIG_CGROUP_WRITEBACK
> struct bdi_writeback *wb; /* wb this writeback is issued under */
> @@ -366,6 +367,9 @@ int balance_dirty_pages_ratelimited_flags(struct address_space *mapping,
>
> bool wb_over_bg_thresh(struct bdi_writeback *wb);
>
> +struct folio *writeback_iter(struct address_space *mapping,
> + struct writeback_control *wbc, struct folio *folio, int *error);
> +
> typedef int (*writepage_t)(struct folio *folio, struct writeback_control *wbc,
> void *data);
>
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 3abb053e70580e..5fe4cdb7dbd61a 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -2325,18 +2325,18 @@ void __init page_writeback_init(void)
> }
>
> /**
> - * tag_pages_for_writeback - tag pages to be written by write_cache_pages
> + * tag_pages_for_writeback - tag pages to be written by writeback
> * @mapping: address space structure to write
> * @start: starting page index
> * @end: ending page index (inclusive)
> *
> * This function scans the page range from @start to @end (inclusive) and tags
> - * all pages that have DIRTY tag set with a special TOWRITE tag. The idea is
> - * that write_cache_pages (or whoever calls this function) will then use
> - * TOWRITE tag to identify pages eligible for writeback. This mechanism is
> - * used to avoid livelocking of writeback by a process steadily creating new
> - * dirty pages in the file (thus it is important for this function to be quick
> - * so that it can tag pages faster than a dirtying process can create them).
> + * all pages that have DIRTY tag set with a special TOWRITE tag. The caller
> + * can then use the TOWRITE tag to identify pages eligible for writeback.
> + * This mechanism is used to avoid livelocking of writeback by a process
> + * steadily creating new dirty pages in the file (thus it is important for this
> + * function to be quick so that it can tag pages faster than a dirtying process
> + * can create them).
> */
> void tag_pages_for_writeback(struct address_space *mapping,
> pgoff_t start, pgoff_t end)
> @@ -2434,69 +2434,68 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
> }
>
> /**
> - * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
> + * writeback_iter - iterate folio of a mapping for writeback
> * @mapping: address space structure to write
> - * @wbc: subtract the number of written pages from *@wbc->nr_to_write
> - * @writepage: function called for each page
> - * @data: data passed to writepage function
> + * @wbc: writeback context
> + * @folio: previously iterated folio (%NULL to start)
> + * @error: in-out pointer for writeback errors (see below)
> *
> - * If a page is already under I/O, write_cache_pages() skips it, even
> - * if it's dirty. This is desirable behaviour for memory-cleaning writeback,
> - * but it is INCORRECT for data-integrity system calls such as fsync(). fsync()
> - * and msync() need to guarantee that all the data which was dirty at the time
> - * the call was made get new I/O started against them. If wbc->sync_mode is
> - * WB_SYNC_ALL then we were called for data integrity and we must wait for
> - * existing IO to complete.
> - *
> - * To avoid livelocks (when other process dirties new pages), we first tag
> - * pages which should be written back with TOWRITE tag and only then start
> - * writing them. For data-integrity sync we have to be careful so that we do
> - * not miss some pages (e.g., because some other process has cleared TOWRITE
> - * tag we set). The rule we follow is that TOWRITE tag can be cleared only
> - * by the process clearing the DIRTY tag (and submitting the page for IO).
> - *
> - * To avoid deadlocks between range_cyclic writeback and callers that hold
> - * pages in PageWriteback to aggregate IO until write_cache_pages() returns,
> - * we do not loop back to the start of the file. Doing so causes a page
> - * lock/page writeback access order inversion - we should only ever lock
> - * multiple pages in ascending page->index order, and looping back to the start
> - * of the file violates that rule and causes deadlocks.
> + * This function returns the next folio for the writeback operation described by
> + * @wbc on @mapping and should be called in a while loop in the ->writepages
> + * implementation.
> *
> - * Return: %0 on success, negative error code otherwise
> + * To start the writeback operation, %NULL is passed in the @folio argument, and
> + * for every subsequent iteration the folio returned previously should be passed
> + * back in.
> + *
> + * If there was an error in the per-folio writeback inside the writeback_iter()
> + * loop, @error should be set to the error value.
> + *
> + * Once the writeback described in @wbc has finished, this function will return
> + * %NULL and if there was an error in any iteration restore it to @error.
> + *
> + * Note: callers should not manually break out of the loop using break or goto
> + * but must keep calling writeback_iter() until it returns %NULL.
> + *
> + * Return: the folio to write or %NULL if the loop is done.
> */
> -int write_cache_pages(struct address_space *mapping,
> - struct writeback_control *wbc, writepage_t writepage,
> - void *data)
> +struct folio *writeback_iter(struct address_space *mapping,
> + struct writeback_control *wbc, struct folio *folio, int *error)
> {
> - int ret = 0;
> - int error;
> - struct folio *folio;
> - pgoff_t end; /* Inclusive */
> -
> - if (wbc->range_cyclic) {
> - wbc->index = mapping->writeback_index; /* prev offset */
> - end = -1;
> - } else {
> - wbc->index = wbc->range_start >> PAGE_SHIFT;
> - end = wbc->range_end >> PAGE_SHIFT;
> - }
> - if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
> - tag_pages_for_writeback(mapping, wbc->index, end);
> -
> - folio_batch_init(&wbc->fbatch);
> + if (!folio) {
> + folio_batch_init(&wbc->fbatch);
> + wbc->saved_err = *error = 0;
>
> - for (;;) {
> - folio = writeback_get_folio(mapping, wbc);
> - if (!folio)
> - break;
> + /*
> + * For range cyclic writeback we remember where we stopped so
> + * that we can continue where we stopped.
> + *
> + * For non-cyclic writeback we always start at the beginning of
> + * the passed in range.
> + */
> + if (wbc->range_cyclic)
> + wbc->index = mapping->writeback_index;
> + else
> + wbc->index = wbc->range_start >> PAGE_SHIFT;
>
> - error = writepage(folio, wbc, data);
> + /*
> + * To avoid livelocks when other processes dirty new pages, we
> + * first tag pages which should be written back and only then
> + * start writing them.
> + *
> + * For data-integrity writeback we have to be careful so that we
> + * do not miss some pages (e.g., because some other process has
> + * cleared the TOWRITE tag we set). The rule we follow is that
> + * TOWRITE tag can be cleared only by the process clearing the
> + * DIRTY tag (and submitting the page for I/O).
> + */
> + if (wbc->sync_mode == WB_SYNC_ALL || wbc->tagged_writepages)
> + tag_pages_for_writeback(mapping, wbc->index,
> + wbc_end(wbc));
> + } else {
> wbc->nr_to_write -= folio_nr_pages(folio);
>
> - if (error == AOP_WRITEPAGE_ACTIVATE) {
> - folio_unlock(folio);
> - error = 0;
> - }
> + WARN_ON_ONCE(*error > 0);
>
> /*
> * For integrity writeback we have to keep going until we have
> @@ -2510,33 +2509,70 @@ int write_cache_pages(struct address_space *mapping,
> * wbc->nr_to_write or encounter the first error.
> */
> if (wbc->sync_mode == WB_SYNC_ALL) {
> - if (error && !ret)
> - ret = error;
> + if (*error && !wbc->saved_err)
> + wbc->saved_err = *error;
> } else {
> - if (error || wbc->nr_to_write <= 0)
> + if (*error || wbc->nr_to_write <= 0)
> goto done;
> }
> }
>
> - /*
> - * For range cyclic writeback we need to remember where we stopped so
> - * that we can continue there next time we are called. If we hit the
> - * last page and there is more work to be done, wrap back to the start
> - * of the file.
> - *
> - * For non-cyclic writeback we always start looking up at the beginning
> - * of the file if we are called again, which can only happen due to
> - * -ENOMEM from the file system.
> - */
> - folio_batch_release(&wbc->fbatch);
> - if (wbc->range_cyclic)
> - mapping->writeback_index = 0;
> - return ret;
> + folio = writeback_get_folio(mapping, wbc);
> + if (!folio) {
> + /*
> + * To avoid deadlocks between range_cyclic writeback and callers
> + * that hold pages in PageWriteback to aggregate I/O until
> + * the writeback iteration finishes, we do not loop back to the
> + * start of the file. Doing so causes a page lock/page
> + * writeback access order inversion - we should only ever lock
> + * multiple pages in ascending page->index order, and looping
> + * back to the start of the file violates that rule and causes
> + * deadlocks.
> + */
> + if (wbc->range_cyclic)
> + mapping->writeback_index = 0;
> +
> + /*
> + * Return the first error we encountered (if there was any) to
> + * the caller.
> + */
> + *error = wbc->saved_err;
> + }
> + return folio;
>
> done:
> folio_batch_release(&wbc->fbatch);
> if (wbc->range_cyclic)
> mapping->writeback_index = folio->index + folio_nr_pages(folio);
> + return NULL;
> +}
> +
> +/**
> + * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
> + * @mapping: address space structure to write
> + * @wbc: subtract the number of written pages from *@wbc->nr_to_write
> + * @writepage: function called for each page
> + * @data: data passed to writepage function
> + *
> + * Return: %0 on success, negative error code otherwise
> + *
> + * Note: please use writeback_iter() instead.
> + */
> +int write_cache_pages(struct address_space *mapping,
> + struct writeback_control *wbc, writepage_t writepage,
> + void *data)
> +{
> + struct folio *folio = NULL;
> + int error;
> +
> + while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> + error = writepage(folio, wbc, data);
> + if (error == AOP_WRITEPAGE_ACTIVATE) {
> + folio_unlock(folio);
> + error = 0;
> + }
> + }
> +
> return error;
> }
> EXPORT_SYMBOL(write_cache_pages);
> --
> 2.39.2
>
--
Jan Kara <[email protected]>
SUSE Labs, CR

2024-02-05 15:40:37

by Brian Foster

[permalink] [raw]
Subject: Re: [PATCH 12/13] writeback: add a writeback iterator

On Sat, Feb 03, 2024 at 08:11:46AM +0100, Christoph Hellwig wrote:
> Refactor the code left in write_cache_pages into an iterator that the
> file system can call to get the next folio for a writeback operation:
>
> struct folio *folio = NULL;
>
> while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> error = <do per-foli writeback>;
> }
>
> The twist here is that the error value is passed by reference, so that
> the iterator can restore it when breaking out of the loop.
>
> Handling of the magic AOP_WRITEPAGE_ACTIVATE value stays outside the
> iterator and needs is just kept in the write_cache_pages legacy wrapper.
> in preparation for eventually killing it off.
>
> Heavily based on a for_each* based iterator from Matthew Wilcox.
>
> Signed-off-by: Christoph Hellwig <[email protected]>
> ---
> include/linux/writeback.h | 4 +
> mm/page-writeback.c | 192 ++++++++++++++++++++++----------------
> 2 files changed, 118 insertions(+), 78 deletions(-)
>
..
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 3abb053e70580e..5fe4cdb7dbd61a 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
..
> @@ -2434,69 +2434,68 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
> }
>
> /**
..
> */
> -int write_cache_pages(struct address_space *mapping,
> - struct writeback_control *wbc, writepage_t writepage,
> - void *data)
> +struct folio *writeback_iter(struct address_space *mapping,
> + struct writeback_control *wbc, struct folio *folio, int *error)
> {
..
> + } else {
> wbc->nr_to_write -= folio_nr_pages(folio);
>
> - if (error == AOP_WRITEPAGE_ACTIVATE) {
> - folio_unlock(folio);
> - error = 0;
> - }
> + WARN_ON_ONCE(*error > 0);

Why the warning on writeback error here? It looks like new behavior, but
maybe I missed something. Otherwise the factoring LGTM.

Brian

>
> /*
> * For integrity writeback we have to keep going until we have
> @@ -2510,33 +2509,70 @@ int write_cache_pages(struct address_space *mapping,
> * wbc->nr_to_write or encounter the first error.
> */
> if (wbc->sync_mode == WB_SYNC_ALL) {
> - if (error && !ret)
> - ret = error;
> + if (*error && !wbc->saved_err)
> + wbc->saved_err = *error;
> } else {
> - if (error || wbc->nr_to_write <= 0)
> + if (*error || wbc->nr_to_write <= 0)
> goto done;
> }
> }
>
> - /*
> - * For range cyclic writeback we need to remember where we stopped so
> - * that we can continue there next time we are called. If we hit the
> - * last page and there is more work to be done, wrap back to the start
> - * of the file.
> - *
> - * For non-cyclic writeback we always start looking up at the beginning
> - * of the file if we are called again, which can only happen due to
> - * -ENOMEM from the file system.
> - */
> - folio_batch_release(&wbc->fbatch);
> - if (wbc->range_cyclic)
> - mapping->writeback_index = 0;
> - return ret;
> + folio = writeback_get_folio(mapping, wbc);
> + if (!folio) {
> + /*
> + * To avoid deadlocks between range_cyclic writeback and callers
> + * that hold pages in PageWriteback to aggregate I/O until
> + * the writeback iteration finishes, we do not loop back to the
> + * start of the file. Doing so causes a page lock/page
> + * writeback access order inversion - we should only ever lock
> + * multiple pages in ascending page->index order, and looping
> + * back to the start of the file violates that rule and causes
> + * deadlocks.
> + */
> + if (wbc->range_cyclic)
> + mapping->writeback_index = 0;
> +
> + /*
> + * Return the first error we encountered (if there was any) to
> + * the caller.
> + */
> + *error = wbc->saved_err;
> + }
> + return folio;
>
> done:
> folio_batch_release(&wbc->fbatch);
> if (wbc->range_cyclic)
> mapping->writeback_index = folio->index + folio_nr_pages(folio);
> + return NULL;
> +}
> +
> +/**
> + * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
> + * @mapping: address space structure to write
> + * @wbc: subtract the number of written pages from *@wbc->nr_to_write
> + * @writepage: function called for each page
> + * @data: data passed to writepage function
> + *
> + * Return: %0 on success, negative error code otherwise
> + *
> + * Note: please use writeback_iter() instead.
> + */
> +int write_cache_pages(struct address_space *mapping,
> + struct writeback_control *wbc, writepage_t writepage,
> + void *data)
> +{
> + struct folio *folio = NULL;
> + int error;
> +
> + while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> + error = writepage(folio, wbc, data);
> + if (error == AOP_WRITEPAGE_ACTIVATE) {
> + folio_unlock(folio);
> + error = 0;
> + }
> + }
> +
> return error;
> }
> EXPORT_SYMBOL(write_cache_pages);
> --
> 2.39.2
>
>


2024-02-06 14:54:05

by Brian Foster

[permalink] [raw]
Subject: Re: [PATCH 12/13] writeback: add a writeback iterator

On Mon, Feb 05, 2024 at 10:33:52AM -0500, Brian Foster wrote:
> On Sat, Feb 03, 2024 at 08:11:46AM +0100, Christoph Hellwig wrote:
> > Refactor the code left in write_cache_pages into an iterator that the
> > file system can call to get the next folio for a writeback operation:
> >
> > struct folio *folio = NULL;
> >
> > while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> > error = <do per-foli writeback>;
> > }
> >
> > The twist here is that the error value is passed by reference, so that
> > the iterator can restore it when breaking out of the loop.
> >
> > Handling of the magic AOP_WRITEPAGE_ACTIVATE value stays outside the
> > iterator and needs is just kept in the write_cache_pages legacy wrapper.
> > in preparation for eventually killing it off.
> >
> > Heavily based on a for_each* based iterator from Matthew Wilcox.
> >
> > Signed-off-by: Christoph Hellwig <[email protected]>
> > ---
> > include/linux/writeback.h | 4 +
> > mm/page-writeback.c | 192 ++++++++++++++++++++++----------------
> > 2 files changed, 118 insertions(+), 78 deletions(-)
> >
> ...
> > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > index 3abb053e70580e..5fe4cdb7dbd61a 100644
> > --- a/mm/page-writeback.c
> > +++ b/mm/page-writeback.c
> ...
> > @@ -2434,69 +2434,68 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
> > }
> >
> > /**
> ...
> > */
> > -int write_cache_pages(struct address_space *mapping,
> > - struct writeback_control *wbc, writepage_t writepage,
> > - void *data)
> > +struct folio *writeback_iter(struct address_space *mapping,
> > + struct writeback_control *wbc, struct folio *folio, int *error)
> > {
> ...
> > + } else {
> > wbc->nr_to_write -= folio_nr_pages(folio);
> >
> > - if (error == AOP_WRITEPAGE_ACTIVATE) {
> > - folio_unlock(folio);
> > - error = 0;
> > - }
> > + WARN_ON_ONCE(*error > 0);
>
> Why the warning on writeback error here? It looks like new behavior, but
> maybe I missed something. Otherwise the factoring LGTM.

Err, sorry.. I glossed over the > 0 check and read it as < 0.
Disregard, this seems reasonable to me as long as we no longer expect
those AOP returns (which I'm not really clear on either, but anyways..):

Reviewed-by: Brian Foster <[email protected]>

>
> Brian
>
> >
> > /*
> > * For integrity writeback we have to keep going until we have
> > @@ -2510,33 +2509,70 @@ int write_cache_pages(struct address_space *mapping,
> > * wbc->nr_to_write or encounter the first error.
> > */
> > if (wbc->sync_mode == WB_SYNC_ALL) {
> > - if (error && !ret)
> > - ret = error;
> > + if (*error && !wbc->saved_err)
> > + wbc->saved_err = *error;
> > } else {
> > - if (error || wbc->nr_to_write <= 0)
> > + if (*error || wbc->nr_to_write <= 0)
> > goto done;
> > }
> > }
> >
> > - /*
> > - * For range cyclic writeback we need to remember where we stopped so
> > - * that we can continue there next time we are called. If we hit the
> > - * last page and there is more work to be done, wrap back to the start
> > - * of the file.
> > - *
> > - * For non-cyclic writeback we always start looking up at the beginning
> > - * of the file if we are called again, which can only happen due to
> > - * -ENOMEM from the file system.
> > - */
> > - folio_batch_release(&wbc->fbatch);
> > - if (wbc->range_cyclic)
> > - mapping->writeback_index = 0;
> > - return ret;
> > + folio = writeback_get_folio(mapping, wbc);
> > + if (!folio) {
> > + /*
> > + * To avoid deadlocks between range_cyclic writeback and callers
> > + * that hold pages in PageWriteback to aggregate I/O until
> > + * the writeback iteration finishes, we do not loop back to the
> > + * start of the file. Doing so causes a page lock/page
> > + * writeback access order inversion - we should only ever lock
> > + * multiple pages in ascending page->index order, and looping
> > + * back to the start of the file violates that rule and causes
> > + * deadlocks.
> > + */
> > + if (wbc->range_cyclic)
> > + mapping->writeback_index = 0;
> > +
> > + /*
> > + * Return the first error we encountered (if there was any) to
> > + * the caller.
> > + */
> > + *error = wbc->saved_err;
> > + }
> > + return folio;
> >
> > done:
> > folio_batch_release(&wbc->fbatch);
> > if (wbc->range_cyclic)
> > mapping->writeback_index = folio->index + folio_nr_pages(folio);
> > + return NULL;
> > +}
> > +
> > +/**
> > + * write_cache_pages - walk the list of dirty pages of the given address space and write all of them.
> > + * @mapping: address space structure to write
> > + * @wbc: subtract the number of written pages from *@wbc->nr_to_write
> > + * @writepage: function called for each page
> > + * @data: data passed to writepage function
> > + *
> > + * Return: %0 on success, negative error code otherwise
> > + *
> > + * Note: please use writeback_iter() instead.
> > + */
> > +int write_cache_pages(struct address_space *mapping,
> > + struct writeback_control *wbc, writepage_t writepage,
> > + void *data)
> > +{
> > + struct folio *folio = NULL;
> > + int error;
> > +
> > + while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> > + error = writepage(folio, wbc, data);
> > + if (error == AOP_WRITEPAGE_ACTIVATE) {
> > + folio_unlock(folio);
> > + error = 0;
> > + }
> > + }
> > +
> > return error;
> > }
> > EXPORT_SYMBOL(write_cache_pages);
> > --
> > 2.39.2
> >
> >
>
>


2024-02-07 08:42:49

by Jan Kara

[permalink] [raw]
Subject: Re: [PATCH 12/13] writeback: add a writeback iterator

On Tue 06-02-24 09:54:01, Brian Foster wrote:
> On Mon, Feb 05, 2024 at 10:33:52AM -0500, Brian Foster wrote:
> > On Sat, Feb 03, 2024 at 08:11:46AM +0100, Christoph Hellwig wrote:
> > > Refactor the code left in write_cache_pages into an iterator that the
> > > file system can call to get the next folio for a writeback operation:
> > >
> > > struct folio *folio = NULL;
> > >
> > > while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> > > error = <do per-foli writeback>;
> > > }
> > >
> > > The twist here is that the error value is passed by reference, so that
> > > the iterator can restore it when breaking out of the loop.
> > >
> > > Handling of the magic AOP_WRITEPAGE_ACTIVATE value stays outside the
> > > iterator and needs is just kept in the write_cache_pages legacy wrapper.
> > > in preparation for eventually killing it off.
> > >
> > > Heavily based on a for_each* based iterator from Matthew Wilcox.
> > >
> > > Signed-off-by: Christoph Hellwig <[email protected]>
> > > ---
> > > include/linux/writeback.h | 4 +
> > > mm/page-writeback.c | 192 ++++++++++++++++++++++----------------
> > > 2 files changed, 118 insertions(+), 78 deletions(-)
> > >
> > ...
> > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > > index 3abb053e70580e..5fe4cdb7dbd61a 100644
> > > --- a/mm/page-writeback.c
> > > +++ b/mm/page-writeback.c
> > ...
> > > @@ -2434,69 +2434,68 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
> > > }
> > >
> > > /**
> > ...
> > > */
> > > -int write_cache_pages(struct address_space *mapping,
> > > - struct writeback_control *wbc, writepage_t writepage,
> > > - void *data)
> > > +struct folio *writeback_iter(struct address_space *mapping,
> > > + struct writeback_control *wbc, struct folio *folio, int *error)
> > > {
> > ...
> > > + } else {
> > > wbc->nr_to_write -= folio_nr_pages(folio);
> > >
> > > - if (error == AOP_WRITEPAGE_ACTIVATE) {
> > > - folio_unlock(folio);
> > > - error = 0;
> > > - }
> > > + WARN_ON_ONCE(*error > 0);
> >
> > Why the warning on writeback error here? It looks like new behavior, but
> > maybe I missed something. Otherwise the factoring LGTM.
>
> Err, sorry.. I glossed over the > 0 check and read it as < 0.
> Disregard, this seems reasonable to me as long as we no longer expect
> those AOP returns (which I'm not really clear on either, but anyways..):
>
> Reviewed-by: Brian Foster <[email protected]>

So my understanding is that AOP_WRITEPAGE_ACTIVATE should be now handled
directly by the caller of ->writepage hook and not by writeback_iter()
which is the reason why the warning is here.

Honza
--
Jan Kara <[email protected]>
SUSE Labs, CR

2024-02-07 14:02:21

by Brian Foster

[permalink] [raw]
Subject: Re: [PATCH 12/13] writeback: add a writeback iterator

On Wed, Feb 07, 2024 at 09:42:24AM +0100, Jan Kara wrote:
> On Tue 06-02-24 09:54:01, Brian Foster wrote:
> > On Mon, Feb 05, 2024 at 10:33:52AM -0500, Brian Foster wrote:
> > > On Sat, Feb 03, 2024 at 08:11:46AM +0100, Christoph Hellwig wrote:
> > > > Refactor the code left in write_cache_pages into an iterator that the
> > > > file system can call to get the next folio for a writeback operation:
> > > >
> > > > struct folio *folio = NULL;
> > > >
> > > > while ((folio = writeback_iter(mapping, wbc, folio, &error))) {
> > > > error = <do per-foli writeback>;
> > > > }
> > > >
> > > > The twist here is that the error value is passed by reference, so that
> > > > the iterator can restore it when breaking out of the loop.
> > > >
> > > > Handling of the magic AOP_WRITEPAGE_ACTIVATE value stays outside the
> > > > iterator and needs is just kept in the write_cache_pages legacy wrapper.
> > > > in preparation for eventually killing it off.
> > > >
> > > > Heavily based on a for_each* based iterator from Matthew Wilcox.
> > > >
> > > > Signed-off-by: Christoph Hellwig <[email protected]>
> > > > ---
> > > > include/linux/writeback.h | 4 +
> > > > mm/page-writeback.c | 192 ++++++++++++++++++++++----------------
> > > > 2 files changed, 118 insertions(+), 78 deletions(-)
> > > >
> > > ...
> > > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > > > index 3abb053e70580e..5fe4cdb7dbd61a 100644
> > > > --- a/mm/page-writeback.c
> > > > +++ b/mm/page-writeback.c
> > > ...
> > > > @@ -2434,69 +2434,68 @@ static struct folio *writeback_get_folio(struct address_space *mapping,
> > > > }
> > > >
> > > > /**
> > > ...
> > > > */
> > > > -int write_cache_pages(struct address_space *mapping,
> > > > - struct writeback_control *wbc, writepage_t writepage,
> > > > - void *data)
> > > > +struct folio *writeback_iter(struct address_space *mapping,
> > > > + struct writeback_control *wbc, struct folio *folio, int *error)
> > > > {
> > > ...
> > > > + } else {
> > > > wbc->nr_to_write -= folio_nr_pages(folio);
> > > >
> > > > - if (error == AOP_WRITEPAGE_ACTIVATE) {
> > > > - folio_unlock(folio);
> > > > - error = 0;
> > > > - }
> > > > + WARN_ON_ONCE(*error > 0);
> > >
> > > Why the warning on writeback error here? It looks like new behavior, but
> > > maybe I missed something. Otherwise the factoring LGTM.
> >
> > Err, sorry.. I glossed over the > 0 check and read it as < 0.
> > Disregard, this seems reasonable to me as long as we no longer expect
> > those AOP returns (which I'm not really clear on either, but anyways..):
> >
> > Reviewed-by: Brian Foster <[email protected]>
>
> So my understanding is that AOP_WRITEPAGE_ACTIVATE should be now handled
> directly by the caller of ->writepage hook and not by writeback_iter()
> which is the reason why the warning is here.
>

Yeah, I wasn't really familiar with the AOP error codes, saw that
multiple existed and just assumed they might be arbitrarily relevant
across different aop callbacks (and so then filtered the check/clear for
WRITEPAGE_ACTIVATE out of my brain ;). On taking a closer look, it seems
like the only other one (AOP_TRUNCATED_PAGE) doesn't have any relevance
to ->writepage(), so this all makes more sense to me now. Thanks.

Brian

> Honza
> --
> Jan Kara <[email protected]>
> SUSE Labs, CR
>