From: "Matthew Wilcox (Oracle)" <[email protected]>
When populating the page cache for readahead, mappings that use
->readpages must populate the page cache themselves as the pages are
passed on a linked list which would normally be used for the page cache's
LRU. For mappings that use ->readpage or the upcoming ->readahead method,
we can put the pages into the page cache as soon as they're allocated,
which solves a race between readahead and direct IO. It also lets us
remove the gfp argument from read_pages().
Use the new readahead_page() API to implement the repeated calls to
->readpage(), just like most filesystems will. This iterator also
supports huge pages, even though none of the filesystems have been
converted to use them yet.
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
---
include/linux/pagemap.h | 20 +++++++++++++++++
mm/readahead.c | 48 +++++++++++++++++++++++++----------------
2 files changed, 49 insertions(+), 19 deletions(-)
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 55fcea0249e6..4989d330fada 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -647,8 +647,28 @@ struct readahead_control {
/* private: use the readahead_* accessors instead */
pgoff_t _index;
unsigned int _nr_pages;
+ unsigned int _batch_count;
};
+static inline struct page *readahead_page(struct readahead_control *rac)
+{
+ struct page *page;
+
+ BUG_ON(rac->_batch_count > rac->_nr_pages);
+ rac->_nr_pages -= rac->_batch_count;
+ rac->_index += rac->_batch_count;
+ rac->_batch_count = 0;
+
+ if (!rac->_nr_pages)
+ return NULL;
+
+ page = xa_load(&rac->mapping->i_pages, rac->_index);
+ VM_BUG_ON_PAGE(!PageLocked(page), page);
+ rac->_batch_count = hpage_nr_pages(page);
+
+ return page;
+}
+
/* The number of pages in this readahead block */
static inline unsigned int readahead_count(struct readahead_control *rac)
{
diff --git a/mm/readahead.c b/mm/readahead.c
index 83df5c061d33..aaa209559ba2 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -113,15 +113,14 @@ int read_cache_pages(struct address_space *mapping, struct list_head *pages,
EXPORT_SYMBOL(read_cache_pages);
-static void read_pages(struct readahead_control *rac, struct list_head *pages,
- gfp_t gfp)
+static void read_pages(struct readahead_control *rac, struct list_head *pages)
{
const struct address_space_operations *aops = rac->mapping->a_ops;
+ struct page *page;
struct blk_plug plug;
- unsigned page_idx;
if (!readahead_count(rac))
- return;
+ goto out;
blk_start_plug(&plug);
@@ -130,23 +129,23 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages,
readahead_count(rac));
/* Clean up the remaining pages */
put_pages_list(pages);
- goto out;
- }
-
- for (page_idx = 0; page_idx < readahead_count(rac); page_idx++) {
- struct page *page = lru_to_page(pages);
- list_del(&page->lru);
- if (!add_to_page_cache_lru(page, rac->mapping, page->index,
- gfp))
+ rac->_index += rac->_nr_pages;
+ rac->_nr_pages = 0;
+ } else {
+ while ((page = readahead_page(rac))) {
aops->readpage(rac->file, page);
- put_page(page);
+ put_page(page);
+ }
}
-out:
blk_finish_plug(&plug);
BUG_ON(!list_empty(pages));
- rac->_nr_pages = 0;
+ BUG_ON(readahead_count(rac));
+
+out:
+ /* If we were called due to a conflicting page, skip over it */
+ rac->_index++;
}
/*
@@ -165,9 +164,11 @@ void __do_page_cache_readahead(struct address_space *mapping,
LIST_HEAD(page_pool);
loff_t isize = i_size_read(inode);
gfp_t gfp_mask = readahead_gfp_mask(mapping);
+ bool use_list = mapping->a_ops->readpages;
struct readahead_control rac = {
.mapping = mapping,
.file = filp,
+ ._index = index,
._nr_pages = 0,
};
unsigned long i;
@@ -184,6 +185,8 @@ void __do_page_cache_readahead(struct address_space *mapping,
if (index + i > end_index)
break;
+ BUG_ON(index + i != rac._index + rac._nr_pages);
+
page = xa_load(&mapping->i_pages, index + i);
if (page && !xa_is_value(page)) {
/*
@@ -191,15 +194,22 @@ void __do_page_cache_readahead(struct address_space *mapping,
* contiguous pages before continuing with the next
* batch.
*/
- read_pages(&rac, &page_pool, gfp_mask);
+ read_pages(&rac, &page_pool);
continue;
}
page = __page_cache_alloc(gfp_mask);
if (!page)
break;
- page->index = index + i;
- list_add(&page->lru, &page_pool);
+ if (use_list) {
+ page->index = index + i;
+ list_add(&page->lru, &page_pool);
+ } else if (add_to_page_cache_lru(page, mapping, index + i,
+ gfp_mask) < 0) {
+ put_page(page);
+ read_pages(&rac, &page_pool);
+ continue;
+ }
if (i == nr_to_read - lookahead_size)
SetPageReadahead(page);
rac._nr_pages++;
@@ -210,7 +220,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
* uptodate then the caller will launch readpage again, and
* will then handle the error.
*/
- read_pages(&rac, &page_pool, gfp_mask);
+ read_pages(&rac, &page_pool);
}
/*
--
2.25.0
On 2/19/20 1:00 PM, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <[email protected]>
>
> When populating the page cache for readahead, mappings that use
> ->readpages must populate the page cache themselves as the pages are
> passed on a linked list which would normally be used for the page cache's
> LRU. For mappings that use ->readpage or the upcoming ->readahead method,
> we can put the pages into the page cache as soon as they're allocated,
> which solves a race between readahead and direct IO. It also lets us
> remove the gfp argument from read_pages().
>
> Use the new readahead_page() API to implement the repeated calls to
> ->readpage(), just like most filesystems will. This iterator also
> supports huge pages, even though none of the filesystems have been
> converted to use them yet.
>
> Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
> ---
> include/linux/pagemap.h | 20 +++++++++++++++++
> mm/readahead.c | 48 +++++++++++++++++++++++++----------------
> 2 files changed, 49 insertions(+), 19 deletions(-)
>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 55fcea0249e6..4989d330fada 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -647,8 +647,28 @@ struct readahead_control {
> /* private: use the readahead_* accessors instead */
> pgoff_t _index;
> unsigned int _nr_pages;
> + unsigned int _batch_count;
> };
>
> +static inline struct page *readahead_page(struct readahead_control *rac)
> +{
> + struct page *page;
> +
> + BUG_ON(rac->_batch_count > rac->_nr_pages);
> + rac->_nr_pages -= rac->_batch_count;
> + rac->_index += rac->_batch_count;
> + rac->_batch_count = 0;
Is it intentional, to set rac->_batch_count twice (here, and below)? The
only reason I can see is if a caller needs to use ->_batch_count in the
"return NULL" case, which doesn't seem to come up...
> +
> + if (!rac->_nr_pages)
> + return NULL;
> +
> + page = xa_load(&rac->mapping->i_pages, rac->_index);
> + VM_BUG_ON_PAGE(!PageLocked(page), page);
> + rac->_batch_count = hpage_nr_pages(page);
> +
> + return page;
> +}
> +
> /* The number of pages in this readahead block */
> static inline unsigned int readahead_count(struct readahead_control *rac)
> {
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 83df5c061d33..aaa209559ba2 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -113,15 +113,14 @@ int read_cache_pages(struct address_space *mapping, struct list_head *pages,
>
> EXPORT_SYMBOL(read_cache_pages);
>
> -static void read_pages(struct readahead_control *rac, struct list_head *pages,
> - gfp_t gfp)
> +static void read_pages(struct readahead_control *rac, struct list_head *pages)
> {
> const struct address_space_operations *aops = rac->mapping->a_ops;
> + struct page *page;
> struct blk_plug plug;
> - unsigned page_idx;
>
> if (!readahead_count(rac))
> - return;
> + goto out;
>
> blk_start_plug(&plug);
>
> @@ -130,23 +129,23 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages,
> readahead_count(rac));
> /* Clean up the remaining pages */
> put_pages_list(pages);
> - goto out;
> - }
> -
> - for (page_idx = 0; page_idx < readahead_count(rac); page_idx++) {
> - struct page *page = lru_to_page(pages);
> - list_del(&page->lru);
> - if (!add_to_page_cache_lru(page, rac->mapping, page->index,
> - gfp))
> + rac->_index += rac->_nr_pages;
> + rac->_nr_pages = 0;
> + } else {
> + while ((page = readahead_page(rac))) {
> aops->readpage(rac->file, page);
> - put_page(page);
> + put_page(page);
> + }
> }
>
> -out:
> blk_finish_plug(&plug);
>
> BUG_ON(!list_empty(pages));
> - rac->_nr_pages = 0;
> + BUG_ON(readahead_count(rac));
> +
> +out:
> + /* If we were called due to a conflicting page, skip over it */
Tiny documentation nit: What if we were *not* called due to a conflicting page?
(And what is a "conflicting page", in this context, btw?) The next line unconditionally
moves the index ahead, so the "if" part of the comment really confuses me.
> + rac->_index++;
> }
>
> /*
> @@ -165,9 +164,11 @@ void __do_page_cache_readahead(struct address_space *mapping,
> LIST_HEAD(page_pool);
> loff_t isize = i_size_read(inode);
> gfp_t gfp_mask = readahead_gfp_mask(mapping);
> + bool use_list = mapping->a_ops->readpages;
> struct readahead_control rac = {
> .mapping = mapping,
> .file = filp,
> + ._index = index,
> ._nr_pages = 0,
> };
> unsigned long i;
> @@ -184,6 +185,8 @@ void __do_page_cache_readahead(struct address_space *mapping,
> if (index + i > end_index)
> break;
>
> + BUG_ON(index + i != rac._index + rac._nr_pages);
> +
> page = xa_load(&mapping->i_pages, index + i);
> if (page && !xa_is_value(page)) {
> /*
> @@ -191,15 +194,22 @@ void __do_page_cache_readahead(struct address_space *mapping,
> * contiguous pages before continuing with the next
> * batch.
> */
> - read_pages(&rac, &page_pool, gfp_mask);
> + read_pages(&rac, &page_pool);
> continue;
> }
>
> page = __page_cache_alloc(gfp_mask);
> if (!page)
> break;
> - page->index = index + i;
> - list_add(&page->lru, &page_pool);
> + if (use_list) {
> + page->index = index + i;
> + list_add(&page->lru, &page_pool);
> + } else if (add_to_page_cache_lru(page, mapping, index + i,
> + gfp_mask) < 0) {
I still think you'll want to compare against !=0, rather than < 0, here.
> + put_page(page);
> + read_pages(&rac, &page_pool);
Doing a read_pages() in the error case is because...actually, I'm not sure yet.
Why do we do this? Effectively it's a retry?
> + continue;
> + }
> if (i == nr_to_read - lookahead_size)
> SetPageReadahead(page);
> rac._nr_pages++;
> @@ -210,7 +220,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
> * uptodate then the caller will launch readpage again, and
> * will then handle the error.
> */
> - read_pages(&rac, &page_pool, gfp_mask);
> + read_pages(&rac, &page_pool);
> }
>
> /*
>
Didn't spot any actual errors, just mainly my own questions here. :)
thanks,
--
John Hubbard
NVIDIA
On 2/20/20 7:43 PM, Matthew Wilcox wrote:
> On Thu, Feb 20, 2020 at 07:19:58PM -0800, John Hubbard wrote:
>>> +static inline struct page *readahead_page(struct readahead_control *rac)
>>> +{
>>> + struct page *page;
>>> +
>>> + BUG_ON(rac->_batch_count > rac->_nr_pages);
>>> + rac->_nr_pages -= rac->_batch_count;
>>> + rac->_index += rac->_batch_count;
>>> + rac->_batch_count = 0;
>>
>>
>> Is it intentional, to set rac->_batch_count twice (here, and below)? The
>> only reason I can see is if a caller needs to use ->_batch_count in the
>> "return NULL" case, which doesn't seem to come up...
>
> Ah, but it does. Not in this patch, but the next one ...
>
> + if (aops->readahead) {
> + aops->readahead(rac);
> + /* Clean up the remaining pages */
> + while ((page = readahead_page(rac))) {
> + unlock_page(page);
> + put_page(page);
> + }
>
> In the normal case, the ->readahead method will consume all the pages,
> and we need readahead_page() to do nothing if it is called again.
>
>>> + if (!rac->_nr_pages)
>>> + return NULL;
>
> ... admittedly I could do:
>
> if (!rac->_nr_pages) {
> rac->_batch_count = 0;
> return NULL;
> }
>
> which might be less confusing.
Yes, that would be a nice bit of polish if you end up doing another revision for other
reasons.
>
>>> @@ -130,23 +129,23 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages,
>>> readahead_count(rac));
>>> /* Clean up the remaining pages */
>>> put_pages_list(pages);
>>> - goto out;
>>> - }
>>> -
>>> - for (page_idx = 0; page_idx < readahead_count(rac); page_idx++) {
>>> - struct page *page = lru_to_page(pages);
>>> - list_del(&page->lru);
>>> - if (!add_to_page_cache_lru(page, rac->mapping, page->index,
>>> - gfp))
>>> + rac->_index += rac->_nr_pages;
>>> + rac->_nr_pages = 0;
>>> + } else {
>>> + while ((page = readahead_page(rac))) {
>>> aops->readpage(rac->file, page);
>>> - put_page(page);
>>> + put_page(page);
>>> + }
>>> }
>>>
>>> -out:
>>> blk_finish_plug(&plug);
>>>
>>> BUG_ON(!list_empty(pages));
>>> - rac->_nr_pages = 0;
>>> + BUG_ON(readahead_count(rac));
>>> +
>>> +out:
>>> + /* If we were called due to a conflicting page, skip over it */
>>
>> Tiny documentation nit: What if we were *not* called due to a conflicting page?
>> (And what is a "conflicting page", in this context, btw?) The next line unconditionally
>> moves the index ahead, so the "if" part of the comment really confuses me.
>
> By the end of the series, read_pages() is called in three places:
>
> 1. if (page && !xa_is_value(page)) {
> read_pages(&rac, &page_pool);
>
> 2. } else if (add_to_page_cache_lru(page, mapping, index + i,
> gfp_mask) < 0) {
> put_page(page);
> read_pages(&rac, &page_pool);
>
> 3. read_pages(&rac, &page_pool);
>
> In the first two cases, there's an existing page in the page cache
> (which conflicts with this readahead operation), and so we need to
> advance index. In the third case, we're exiting the function, so it
> does no harm to advance index one further.
OK, I see. As you know, I tend toward maybe over-documenting, but what about
adding just a *few* hints to help new readers, like this approximately (maybe
it should be pared down):
diff --git a/mm/readahead.c b/mm/readahead.c
index 9fb5f77dcf69..0dd5b09c376e 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -114,6 +114,10 @@ int read_cache_pages(struct address_space *mapping, struct list_head *pages,
EXPORT_SYMBOL(read_cache_pages);
+/*
+ * Read pages into the page cache, OR skip over a page if it is already in the
+ * page cache.
+ */
static void read_pages(struct readahead_control *rac, struct list_head *pages)
{
const struct address_space_operations *aops = rac->mapping->a_ops;
@@ -152,7 +156,11 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages)
BUG_ON(readahead_count(rac));
out:
- /* If we were called due to a conflicting page, skip over it */
+ /*
+ * This routine might have been called in order to skip over a page
+ * that is already in the page cache. And for other cases, the index is
+ * ignored by the caller. So just increment unconditionally:
+ */
rac->_index++;
}
?
>
>>> + } else if (add_to_page_cache_lru(page, mapping, index + i,
>>> + gfp_mask) < 0) {
>>
>> I still think you'll want to compare against !=0, rather than < 0, here.
>
> I tend to prefer < 0 when checking for an error value in case the function
> decides to start using positive numbers to mean something. I don't think
> it's a particularly important preference though (after all, returning 1
> might mean "failed, but for this weird reason rather than an errno").
>
>>> + put_page(page);
>>> + read_pages(&rac, &page_pool);
>>
>> Doing a read_pages() in the error case is because...actually, I'm not sure yet.
>> Why do we do this? Effectively it's a retry?
>
> Same as the reason we call read_pages() if we found a page in the page
> cache earlier -- we're sending down a set of pages which are consecutive
> in the file's address space, and now we have to skip one. At least one ;-)
>
Got it. Finally. :)
thanks,
--
John Hubbard
NVIDIA
On Wed, Feb 19, 2020 at 01:00:48PM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <[email protected]>
>
> When populating the page cache for readahead, mappings that use
> ->readpages must populate the page cache themselves as the pages are
> passed on a linked list which would normally be used for the page cache's
> LRU. For mappings that use ->readpage or the upcoming ->readahead method,
> we can put the pages into the page cache as soon as they're allocated,
> which solves a race between readahead and direct IO. It also lets us
> remove the gfp argument from read_pages().
>
> Use the new readahead_page() API to implement the repeated calls to
> ->readpage(), just like most filesystems will. This iterator also
> supports huge pages, even though none of the filesystems have been
> converted to use them yet.
>
> Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
> ---
> include/linux/pagemap.h | 20 +++++++++++++++++
> mm/readahead.c | 48 +++++++++++++++++++++++++----------------
> 2 files changed, 49 insertions(+), 19 deletions(-)
>
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 55fcea0249e6..4989d330fada 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -647,8 +647,28 @@ struct readahead_control {
> /* private: use the readahead_* accessors instead */
> pgoff_t _index;
> unsigned int _nr_pages;
> + unsigned int _batch_count;
> };
>
> +static inline struct page *readahead_page(struct readahead_control *rac)
> +{
> + struct page *page;
> +
> + BUG_ON(rac->_batch_count > rac->_nr_pages);
> + rac->_nr_pages -= rac->_batch_count;
> + rac->_index += rac->_batch_count;
> + rac->_batch_count = 0;
> +
> + if (!rac->_nr_pages)
> + return NULL;
> +
> + page = xa_load(&rac->mapping->i_pages, rac->_index);
> + VM_BUG_ON_PAGE(!PageLocked(page), page);
> + rac->_batch_count = hpage_nr_pages(page);
> +
> + return page;
> +}
> +
> /* The number of pages in this readahead block */
> static inline unsigned int readahead_count(struct readahead_control *rac)
> {
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 83df5c061d33..aaa209559ba2 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -113,15 +113,14 @@ int read_cache_pages(struct address_space *mapping, struct list_head *pages,
>
> EXPORT_SYMBOL(read_cache_pages);
>
> -static void read_pages(struct readahead_control *rac, struct list_head *pages,
> - gfp_t gfp)
> +static void read_pages(struct readahead_control *rac, struct list_head *pages)
> {
> const struct address_space_operations *aops = rac->mapping->a_ops;
> + struct page *page;
> struct blk_plug plug;
> - unsigned page_idx;
>
> if (!readahead_count(rac))
> - return;
> + goto out;
>
> blk_start_plug(&plug);
>
> @@ -130,23 +129,23 @@ static void read_pages(struct readahead_control *rac, struct list_head *pages,
> readahead_count(rac));
> /* Clean up the remaining pages */
> put_pages_list(pages);
> - goto out;
> - }
> -
> - for (page_idx = 0; page_idx < readahead_count(rac); page_idx++) {
> - struct page *page = lru_to_page(pages);
> - list_del(&page->lru);
> - if (!add_to_page_cache_lru(page, rac->mapping, page->index,
> - gfp))
> + rac->_index += rac->_nr_pages;
> + rac->_nr_pages = 0;
> + } else {
> + while ((page = readahead_page(rac))) {
> aops->readpage(rac->file, page);
> - put_page(page);
> + put_page(page);
> + }
> }
>
> -out:
> blk_finish_plug(&plug);
>
> BUG_ON(!list_empty(pages));
> - rac->_nr_pages = 0;
> + BUG_ON(readahead_count(rac));
> +
> +out:
> + /* If we were called due to a conflicting page, skip over it */
> + rac->_index++;
> }
>
> /*
> @@ -165,9 +164,11 @@ void __do_page_cache_readahead(struct address_space *mapping,
> LIST_HEAD(page_pool);
> loff_t isize = i_size_read(inode);
> gfp_t gfp_mask = readahead_gfp_mask(mapping);
> + bool use_list = mapping->a_ops->readpages;
I find this single use variable a little weird. Not a dealbreaker,
but just checking the methods would seem a little more obvious to me.
Except for this and the other nitpick the patch looks good to me:
Reviewed-by: Christoph Hellwig <[email protected]>