netdev - Re: [PATCH net-next] net: Don't return pfmemalloc pages to the page pool.

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Date:   Fri, 21 Dec 2018 02:13:13 +0000
From:   Saeed Mahameed <saeedm@...lanox.com>
To:     "jonathan.lemon@...il.com" <jonathan.lemon@...il.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "brouer@...hat.com" <brouer@...hat.com>
Subject: Re: [PATCH net-next] net: Don't return pfmemalloc pages to the page
 pool.

On Thu, 2018-12-20 at 16:56 -0800, Jonathan Lemon wrote:
> On 20 Dec 2018, at 15:41, Saeed Mahameed wrote:
> 
> > On Thu, 2018-12-20 at 14:11 -0800, Jonathan Lemon wrote:
> > > (Resending due to mailer issues)
> > > 
> > > On 20 Dec 2018, at 5:03, Jesper Dangaard Brouer wrote:
> > > 
> > > > On Wed, 19 Dec 2018 12:06:51 -0800
> > > > Jonathan Lemon <jonathan.lemon@...il.com> wrote:
> > > > 
> > > > > Return pfmemalloc pages back to the page allocator, instead
> > > > > of
> > > > > holding them in the page pool.
> > > > 
> > > > Have you experience this issue in practice or is it theory?
> > > 
> > > We're seeing the mlx5 driver use pfmemalloc pages with 4.11, and
> > > then
> > > return them
> > > back to the page allocator.  (it's triggering the
> > > mlx5e_page_is_reserved() test).
> > > The page pool code isn't in production use, but the code paths
> > > appear
> > > identical.
> > > 
> > > 
> > > > >  While here, also use the __page_pool_return_page() API.
> > > > 
> > > > Don't combine several unrelated changed in one patch.
> > > 
> > > Okay - will send as 2 separate patches
> > > 
> > > 
> > > > > diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> > > > > index 43a932cb609b..091007ff14a3 100644
> > > > > --- a/net/core/page_pool.c
> > > > > +++ b/net/core/page_pool.c
> > > > > @@ -233,7 +233,7 @@ void __page_pool_put_page(struct
> > > > > page_pool
> > > > > *pool,
> > > > >  	 *
> > > > >  	 * refcnt == 1 means page_pool owns page, and can
> > > > > recycle it.
> > > > >  	 */
> > > > > -	if (likely(page_ref_count(page) == 1)) {
> > > > > +	if (likely(page_ref_count(page) == 1 &&
> > > > > !page_is_pfmemalloc(page)))
> > > > > {
> > 
> > I think this is wrong, if refcount is 1, then this page belongs to
> > pagepool and you must enter this statement's true block, and test
> > page_is_pfmemalloc inside (mark it unlikely),
> > to return a pfmemalloc page, you need to call
> > __page_pool_return_page()
> > to dma_unmap and other cleanups if required.
> 
> If the page belongs to the pool, but it isn't recyclable, the
> fallback
> path is "__page_pool_return_page()".  If it doesn't belong to the
> pool,
> it goes to the non-XDP mode, which is also __page_pool_return_page().
> 

non-XDP mode doesn't explicitly call __page_pool_return_page() but it
does exactly what __page_pool_return_page() is doing now, which is:
	__page_pool_clean_page(pool, page);
	put_page(page);

your code is logically correct for now, but semantically speaking
__page_pool_return_page might change in the future to have special
handling of pages that actually belong only to the pagepool
(refcount==1), so better be safe and handle such pages in the pagepool
path and not in the non-XDP path

all you need is:
    if(unlikely(!page_is_pfmemalloc(page))
           __page_pool_return_page()

early inside if (likely(page_ref_count(page) == 1)) pagepool recycling
block; I don't think this will affect performance.

> Does your dislike stem from the fact that the "non-XDP" mode is taken
> for the "refcount=1, pfmemalloc=T" case?