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From:   Jesper Dangaard Brouer <jbrouer@redhat.com>
Subject: Re: [PATCH v3] mm/page_alloc: Further fix __alloc_pages_bulk() return
 value
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Chuck Lever <chuck.lever@oracle.com>, linux-nfs@vger.kernel.org,
        linux-mm@kvack.org, brouer@redhat.com
References: <162497449506.16614.7781489905877008435.stgit@klimt.1015granger.net>
 <be9186d9-1e8e-4d99-ab0f-84c0518025c5@redhat.com>
 <20210630065855.GH3840@techsingularity.net>
Message-ID: <543ef038-d50c-f035-bd9d-ae3140ca0e33@redhat.com>
Date:   Wed, 30 Jun 2021 13:22:24 +0200
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On 30/06/2021 08.58, Mel Gorman wrote:
> On Tue, Jun 29, 2021 at 06:01:12PM +0200, Jesper Dangaard Brouer wrote:
>> On 29/06/2021 15.48, Chuck Lever wrote:
>>
>>> The call site in __page_pool_alloc_pages_slow() also seems to be
>>> confused on this matter. It should be attended to by someone who
>>> is familiar with that code.
>> I don't think we need a fix for __page_pool_alloc_pages_slow(), as the array
>> is guaranteed to be empty.
>>
>> But a fix would look like this:
>>
>> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
>> index c137ce308c27..1b04538a3da3 100644
>> --- a/net/core/page_pool.c
>> +++ b/net/core/page_pool.c
>> @@ -245,22 +245,23 @@ static struct page
>> *__page_pool_alloc_pages_slow(struct page_pool *pool,
>>          if (unlikely(pp_order))
>>                  return __page_pool_alloc_page_order(pool, gfp);
>>
>>          /* Unnecessary as alloc cache is empty, but guarantees zero count */
>> -       if (unlikely(pool->alloc.count > 0))
>> +       if (unlikely(pool->alloc.count > 0))   // ^^^^^^^^^^^^^^^^^^^^^^
>>                  return pool->alloc.cache[--pool->alloc.count];
>>
>>          /* Mark empty alloc.cache slots "empty" for alloc_pages_bulk_array
>> */
>>          memset(&pool->alloc.cache, 0, sizeof(void *) * bulk);
>>
>> +       /* bulk API ret value also count existing pages, but array is empty
>> */
>>          nr_pages = alloc_pages_bulk_array(gfp, bulk, pool->alloc.cache);
>>          if (unlikely(!nr_pages))
>>                  return NULL;
>>
>>          /* Pages have been filled into alloc.cache array, but count is zero
>> and
>>           * page element have not been (possibly) DMA mapped.
>>           */
>> -       for (i = 0; i < nr_pages; i++) {
>> +       for (i = pool->alloc.count; i < nr_pages; i++) {
> That last part would break as the loop is updating pool->alloc_count.
The last part "i = pool->alloc.count" probably is a bad idea.
> Just setting pool->alloc_count = nr_pages would break if PP_FLAG_DMA_MAP
> was set and page_pool_dma_map failed. Right?

Yes, this loop is calling page_pool_dma_map(), and if that fails we 
don't advance pool->alloc_count.

--Jesper