From: "Matthew Wilcox (Oracle)" <[email protected]>
At allocation time, put the pages in the cache unless we're using
->readpages. Add the readahead_for_each() iterator for the benefit of
the ->readpage fallback. This iterator supports huge pages, even though
none of the filesystems to be converted do yet.
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
---
include/linux/pagemap.h | 24 ++++++++++++++++++++++++
mm/readahead.c | 34 +++++++++++++++++-----------------
2 files changed, 41 insertions(+), 17 deletions(-)
diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 982ecda2d4a2..3613154e79e4 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -639,8 +639,32 @@ struct readahead_control {
/* private: use the readahead_* accessors instead */
pgoff_t _start;
unsigned int _nr_pages;
+ unsigned int _batch_count;
};
+static inline struct page *readahead_page(struct readahead_control *rac)
+{
+ struct page *page;
+
+ if (!rac->_nr_pages)
+ return NULL;
+
+ page = xa_load(&rac->mapping->i_pages, rac->_start);
+ VM_BUG_ON_PAGE(!PageLocked(page), page);
+ rac->_batch_count = hpage_nr_pages(page);
+
+ return page;
+}
+
+static inline void readahead_next(struct readahead_control *rac)
+{
+ rac->_nr_pages -= rac->_batch_count;
+ rac->_start += rac->_batch_count;
+}
+
+#define readahead_for_each(rac, page) \
+ for (; (page = readahead_page(rac)); readahead_next(rac))
+
/* 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 bdc5759000d3..9e430daae42f 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -113,12 +113,11 @@ 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;
blk_start_plug(&plug);
@@ -127,19 +126,13 @@ 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))
+ } else {
+ readahead_for_each(rac, page) {
aops->readpage(rac->file, page);
- put_page(page);
+ put_page(page);
+ }
}
-out:
blk_finish_plug(&plug);
}
@@ -159,6 +152,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
unsigned long i;
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,
@@ -196,8 +190,14 @@ void __do_page_cache_readahead(struct address_space *mapping,
page = __page_cache_alloc(gfp_mask);
if (!page)
break;
- page->index = offset;
- list_add(&page->lru, &page_pool);
+ if (use_list) {
+ page->index = offset;
+ list_add(&page->lru, &page_pool);
+ } else if (add_to_page_cache_lru(page, mapping, offset,
+ gfp_mask) < 0) {
+ put_page(page);
+ goto read;
+ }
if (i == nr_to_read - lookahead_size)
SetPageReadahead(page);
rac._nr_pages++;
@@ -205,7 +205,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
continue;
read:
if (readahead_count(&rac))
- read_pages(&rac, &page_pool, gfp_mask);
+ read_pages(&rac, &page_pool);
rac._nr_pages = 0;
rac._start = ++offset;
}
@@ -216,7 +216,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
* will then handle the error.
*/
if (readahead_count(&rac))
- read_pages(&rac, &page_pool, gfp_mask);
+ read_pages(&rac, &page_pool);
BUG_ON(!list_empty(&page_pool));
}
--
2.25.0
On Mon, Feb 17, 2020 at 10:45:52AM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <[email protected]>
>
> At allocation time, put the pages in the cache unless we're using
> ->readpages. Add the readahead_for_each() iterator for the benefit of
> the ->readpage fallback. This iterator supports huge pages, even though
> none of the filesystems to be converted do yet.
This could be better written - took me some time to get my head
around it and the code.
"When populating the page cache for readahead, mappings that don't
use ->readpages need to have their pages added to the page cache
before ->readpage is called. Do this insertion earlier so that the
pages can be looked up immediately prior to ->readpage calls rather
than passing them on a linked list. This early insert functionality
is also required by the upcoming ->readahead method that will
replace ->readpages.
Optimise and simplify the readpage loop by adding a
readahead_for_each() iterator to provide the pages we need to read.
This iterator also supports huge pages, even though none of the
filesystems have been converted to use them yet."
> +static inline struct page *readahead_page(struct readahead_control *rac)
> +{
> + struct page *page;
> +
> + if (!rac->_nr_pages)
> + return NULL;
Hmmmm.
> +
> + page = xa_load(&rac->mapping->i_pages, rac->_start);
> + VM_BUG_ON_PAGE(!PageLocked(page), page);
> + rac->_batch_count = hpage_nr_pages(page);
So we could have rac->_nr_pages = 2, and then we get an order 2
large page returned, and so rac->_batch_count = 4.
> +
> + return page;
> +}
> +
> +static inline void readahead_next(struct readahead_control *rac)
> +{
> + rac->_nr_pages -= rac->_batch_count;
> + rac->_start += rac->_batch_count;
This results in rac->_nr_pages = -2 (or a huge positive number).
That means that readahead_page() will not terminate when it should,
and potentially will panic if it doesn't find the page that it
thinks should be there at rac->_start + 4...
> +#define readahead_for_each(rac, page) \
> + for (; (page = readahead_page(rac)); readahead_next(rac))
> +
> /* 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 bdc5759000d3..9e430daae42f 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -113,12 +113,11 @@ 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;
>
> blk_start_plug(&plug);
>
> @@ -127,19 +126,13 @@ 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))
> + } else {
> + readahead_for_each(rac, page) {
> aops->readpage(rac->file, page);
> - put_page(page);
> + put_page(page);
> + }
> }
Nice simplification and gets rid of the need for rac->mapping, but I
still find the aops variable weird.
> -out:
> blk_finish_plug(&plug);
> }
>
> @@ -159,6 +152,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
> unsigned long i;
> 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,
[ I do find these unstructured mixes of declarations and
initialisations dense and difficult to read.... ]
> @@ -196,8 +190,14 @@ void __do_page_cache_readahead(struct address_space *mapping,
> page = __page_cache_alloc(gfp_mask);
> if (!page)
> break;
> - page->index = offset;
> - list_add(&page->lru, &page_pool);
> + if (use_list) {
> + page->index = offset;
> + list_add(&page->lru, &page_pool);
> + } else if (add_to_page_cache_lru(page, mapping, offset,
> + gfp_mask) < 0) {
> + put_page(page);
> + goto read;
> + }
Ok, so that's why you put read code at the end of the loop. To turn
the code into spaghetti :/
How much does this simplify down when we get rid of ->readpages and
can restructure the loop? This really seems like you're trying to
flatten two nested loops into one by the use of goto....
Cheers,
Dave.
--
Dave Chinner
[email protected]
On Tue, Feb 18, 2020 at 05:14:59PM +1100, Dave Chinner wrote:
> On Mon, Feb 17, 2020 at 10:45:52AM -0800, Matthew Wilcox wrote:
> > From: "Matthew Wilcox (Oracle)" <[email protected]>
> >
> > At allocation time, put the pages in the cache unless we're using
> > ->readpages. Add the readahead_for_each() iterator for the benefit of
> > the ->readpage fallback. This iterator supports huge pages, even though
> > none of the filesystems to be converted do yet.
>
> This could be better written - took me some time to get my head
> around it and the code.
>
> "When populating the page cache for readahead, mappings that don't
> use ->readpages need to have their pages added to the page cache
> before ->readpage is called. Do this insertion earlier so that the
> pages can be looked up immediately prior to ->readpage calls rather
> than passing them on a linked list. This early insert functionality
> is also required by the upcoming ->readahead method that will
> replace ->readpages.
>
> Optimise and simplify the readpage loop by adding a
> readahead_for_each() iterator to provide the pages we need to read.
> This iterator also supports huge pages, even though none of the
> filesystems have been converted to use them yet."
Thanks, I'll use that.
> > +static inline struct page *readahead_page(struct readahead_control *rac)
> > +{
> > + struct page *page;
> > +
> > + if (!rac->_nr_pages)
> > + return NULL;
>
> Hmmmm.
>
> > +
> > + page = xa_load(&rac->mapping->i_pages, rac->_start);
> > + VM_BUG_ON_PAGE(!PageLocked(page), page);
> > + rac->_batch_count = hpage_nr_pages(page);
>
> So we could have rac->_nr_pages = 2, and then we get an order 2
> large page returned, and so rac->_batch_count = 4.
Well, no, we couldn't. rac->_nr_pages is incremented by 4 when we add
an order-2 page to the readahead. I can put a
BUG_ON(rac->_batch_count > rac->_nr_pages)
in here to be sure to catch any logic error like that.
> > @@ -159,6 +152,7 @@ void __do_page_cache_readahead(struct address_space *mapping,
> > unsigned long i;
> > 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,
>
> [ I do find these unstructured mixes of declarations and
> initialisations dense and difficult to read.... ]
Fair ... although I didn't create this mess, I can tidy it up a bit.
> > - page->index = offset;
> > - list_add(&page->lru, &page_pool);
> > + if (use_list) {
> > + page->index = offset;
> > + list_add(&page->lru, &page_pool);
> > + } else if (add_to_page_cache_lru(page, mapping, offset,
> > + gfp_mask) < 0) {
> > + put_page(page);
> > + goto read;
> > + }
>
> Ok, so that's why you put read code at the end of the loop. To turn
> the code into spaghetti :/
>
> How much does this simplify down when we get rid of ->readpages and
> can restructure the loop? This really seems like you're trying to
> flatten two nested loops into one by the use of goto....
I see it as having two failure cases in this loop. One for "page is
already present" (which already existed) and one for "allocated a page,
but failed to add it to the page cache" (which used to be done later).
I didn't want to duplicate the "call read_pages()" code. So I reshuffled
the code rather than add a nested loop. I don't think the nested loop
is easier to read (we'll be at 5 levels of indentation for some statements).
Could do it this way ...
@@ -218,18 +218,17 @@ void page_cache_readahead_limit(struct address_space *mapping,
} else if (add_to_page_cache_lru(page, mapping, offset,
gfp_mask) < 0) {
put_page(page);
- goto read;
+read:
+ if (readahead_count(&rac))
+ read_pages(&rac, &page_pool);
+ rac._nr_pages = 0;
+ rac._start = ++offset;
+ continue;
}
if (i == nr_to_read - lookahead_size)
SetPageReadahead(page);
rac._nr_pages++;
offset++;
- continue;
-read:
- if (readahead_count(&rac))
- read_pages(&rac, &page_pool);
- rac._nr_pages = 0;
- rac._start = ++offset;
}
/*
but I'm not sure that's any better.
On 2/18/20 5:02 PM, Matthew Wilcox wrote:
> On Tue, Feb 18, 2020 at 04:01:43PM -0800, John Hubbard wrote:
>> How about this instead? It uses the "for" loop fully and more naturally,
>> and is easier to read. And it does the same thing:
>>
>> static inline struct page *readahead_page(struct readahead_control *rac)
>> {
>> struct page *page;
>>
>> if (!rac->_nr_pages)
>> return NULL;
>>
>> page = xa_load(&rac->mapping->i_pages, rac->_start);
>> VM_BUG_ON_PAGE(!PageLocked(page), page);
>> rac->_batch_count = hpage_nr_pages(page);
>>
>> return page;
>> }
>>
>> static inline struct page *readahead_next(struct readahead_control *rac)
>> {
>> rac->_nr_pages -= rac->_batch_count;
>> rac->_start += rac->_batch_count;
>>
>> return readahead_page(rac);
>> }
>>
>> #define readahead_for_each(rac, page) \
>> for (page = readahead_page(rac); page != NULL; \
>> page = readahead_page(rac))
>
> I'm assuming you mean 'page = readahead_next(rac)' on that second line.
>
> If you keep reading all the way to the penultimate patch, it won't work
> for iomap ... at least not in the same way.
>
OK, so after an initial look at patch 18's ("iomap: Convert from readpages to
readahead") use of readahead_page() and readahead_next(), I'm not sure what
I'm missing. Seems like it would work...?
thanks,
--
John Hubbard
NVIDIA
On Tue, Feb 18, 2020 at 04:01:43PM -0800, John Hubbard wrote:
> How about this instead? It uses the "for" loop fully and more naturally,
> and is easier to read. And it does the same thing:
>
> static inline struct page *readahead_page(struct readahead_control *rac)
> {
> struct page *page;
>
> if (!rac->_nr_pages)
> return NULL;
>
> page = xa_load(&rac->mapping->i_pages, rac->_start);
> VM_BUG_ON_PAGE(!PageLocked(page), page);
> rac->_batch_count = hpage_nr_pages(page);
>
> return page;
> }
>
> static inline struct page *readahead_next(struct readahead_control *rac)
> {
> rac->_nr_pages -= rac->_batch_count;
> rac->_start += rac->_batch_count;
>
> return readahead_page(rac);
> }
>
> #define readahead_for_each(rac, page) \
> for (page = readahead_page(rac); page != NULL; \
> page = readahead_page(rac))
I'll go you one better ... how about we do this instead:
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;
}
#define readahead_for_each(rac, page) \
while ((page = readahead_page(rac)))
No more readahead_next() to forget to add to filesystems which don't use
the readahead_for_each() iterator. Ahem.
On 2/19/20 7:01 AM, Matthew Wilcox wrote:
> On Wed, Feb 19, 2020 at 06:52:46AM -0800, Christoph Hellwig wrote:
>> On Wed, Feb 19, 2020 at 06:41:17AM -0800, Matthew Wilcox wrote:
>>> #define readahead_for_each(rac, page) \
>>> while ((page = readahead_page(rac)))
>>>
>>> No more readahead_next() to forget to add to filesystems which don't use
>>> the readahead_for_each() iterator. Ahem.
Yes, this looks very clean. And less error-prone, which I definitely
appreciate too. :)
>>
>> And then kill readahead_for_each and open code the above to make it
>> even more obvious?
>
> Makes sense.
>
Great!
thanks,
--
John Hubbard
NVIDIA