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SubjectRe: [PATCH v3 2/2] btrfs: allocate page arrays using bulk page allocator
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On 3/31/22 13:35, David Sterba wrote:
> On Wed, Mar 30, 2022 at 04:11:23PM -0400, Sweet Tea Dorminy wrote:
>> While calling alloc_page() in a loop is an effective way to populate an
>> array of pages, the kernel provides a method to allocate pages in bulk.
>> alloc_pages_bulk_array() populates the NULL slots in a page array, trying to
>> grab more than one page at a time.
>>
>> Unfortunately, it doesn't guarantee allocating all slots in the array,
>> but it's easy to call it in a loop and return an error if no progress
>> occurs. Similar code can be found in xfs/xfs_buf.c:xfs_buf_alloc_pages().
>>
>> Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
>> Reviewed-by: Nikolay Borisov <nborisov@suse.com>
>> ---
>> Changes in v3:
>> - Added a newline after variable declaration
>> Changes in v2:
>> - Moved from ctree.c to extent_io.c
>> ---
>> fs/btrfs/extent_io.c | 24 +++++++++++++++---------
>> 1 file changed, 15 insertions(+), 9 deletions(-)
>>
>> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
>> index ab4c1c4d1b59..b268e47aa2b7 100644
>> --- a/fs/btrfs/extent_io.c
>> +++ b/fs/btrfs/extent_io.c
>> @@ -3144,19 +3144,25 @@ static void end_bio_extent_readpage(struct bio *bio)
>> */
>> int btrfs_alloc_page_array(unsigned long nr_pages, struct page **page_array)
>> {
>> - int i;
>> + long allocated = 0;
>> +
>> + for (;;) {
>> + long last = allocated;
>>
>> - for (i = 0; i < nr_pages; i++) {
>> - struct page *page;
>> + allocated = alloc_pages_bulk_array(GFP_NOFS, nr_pages,
>> + page_array);
>> + if (allocated == nr_pages)
>> + return 0;
>>
>> - if (page_array[i])
>> + if (allocated != last)
>> continue;
>> - page = alloc_page(GFP_NOFS);
>> - if (!page)
>> - return -ENOMEM;
>> - page_array[i] = page;
>> + /*
>> + * During this iteration, no page could be allocated, even
>> + * though alloc_pages_bulk_array() falls back to alloc_page()
>> + * if it could not bulk-allocate. So we must be out of memory.
>> + */
>> + return -ENOMEM;
>> }
>
> I find the way the loop is structured a bit cumbersome so I'd suggest to
> rewrite it as:
>
> int btrfs_alloc_page_array(unsigned int nr_pages, struct page **page_array)
> {
> unsigned int allocated;
>
> for (allocated = 0; allocated < nr_pages;) {
> unsigned int last = allocated;
>
> allocated = alloc_pages_bulk_array(GFP_NOFS, nr_pages, page_array);
>
> /*
> * During this iteration, no page could be allocated, even
> * though alloc_pages_bulk_array() falls back to alloc_page()
> * if it could not bulk-allocate. So we must be out of memory.
> */
> if (allocated == last)
> return -ENOMEM;
> }
> return 0;
> }
Sounds good, I'll amend it that way.

>
> Also in the xfs code there's memalloc_retry_wait() which is supposed to be
> called when repeated memory allocation is retried. What was the reason
> you removed it?

Trying to keep the behavior as close as possible to the existing behavior.

The current behavior of each alloc_page loop is to fail if alloc_page()
fails; in the worst case, alloc_pages_bulk_array() calls alloc_page()
after trying to get a batch, so I figured the worst case is still
basically a loop calling alloc_page() and failing if it ever fails.

Reading up on it, though, arguably the memalloc_retry_wait() should
already be in all the callsites, so maybe I should insert a patch in the
middle that just adds the memalloc_retry_wait() into
btrfs_alloc_page_array()? Since it's an orthogonal fixup to either the
refactoring or the conversion to alloc_pages_bulk_array()?

Thanks!

Sweet Tea

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