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.le...@gmail.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.
> > I don't like adding this in the hot-path. Instead we could move
> > this
> > to the page alloc slow-path, and reject allocating pages with
> > pgmemalloc in the first place.
>
I prefer to enhance the above solution, no need to risk pagepool users
going out of memory under emergencies.
> No real objection to that - but then why bother with pfmemalloc? If
> the
> driver can't
> obtain pages for emergency use, then they might as well not exist.
>
> --
> Jonathan
>
