linux-kernel - Re: [PATCH v3 2/2] btrfs: allocate page arrays using bulk page allocator

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Date:   Thu, 31 Mar 2022 14:19:07 -0400
From:   Sweet Tea Dorminy <sweettea-kernel@...miny.me>
To:     dsterba@...e.cz, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>,
        Nick Terrell <terrelln@...com>, linux-kernel@...r.kernel.org,
        linux-btrfs@...r.kernel.org, kernel-team@...com,
        Nikolay Borisov <nborisov@...e.com>
Subject: Re: [PATCH v3 2/2] btrfs: allocate page arrays using bulk page
 allocator



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@...miny.me>
>> Reviewed-by: Nikolay Borisov <nborisov@...e.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