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Date:   Wed, 30 Jun 2021 07:58:55 +0100
From:   Mel Gorman <mgorman@techsingularity.net>
To:     Jesper Dangaard Brouer <jbrouer@redhat.com>
Cc:     Chuck Lever <chuck.lever@oracle.com>, linux-nfs@vger.kernel.org,
        linux-mm@kvack.org, brouer@redhat.com
Subject: Re: [PATCH v3] mm/page_alloc: Further fix __alloc_pages_bulk()
 return value
Message-ID: <20210630065855.GH3840@techsingularity.net>
References: <162497449506.16614.7781489905877008435.stgit@klimt.1015granger.net>
 <be9186d9-1e8e-4d99-ab0f-84c0518025c5@redhat.com>
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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.
Just setting pool->alloc_count = nr_pages would break if PP_FLAG_DMA_MAP
was set and page_pool_dma_map failed. Right?

-- 
Mel Gorman
SUSE Labs