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Message-ID: <20190105164609.3793fc02@redhat.com> Date: Sat, 5 Jan 2019 16:46:09 +0100 From: Jesper Dangaard Brouer <brouer@...hat.com> To: Jonathan Lemon <jonathan.lemon@...il.com> Cc: brouer@...hat.com, <netdev@...r.kernel.org>, Alexander Duyck <alexander.duyck@...il.com>, Alexei Starovoitov <alexei.starovoitov@...il.com>, kernel-team@...com, Jes Sorensen <jsorensen@...com>, Saeed Mahameed <saeedm@...lanox.com>, David Miller <davem@...emloft.net> Subject: Re: [PATCH 1/2 net-next] net: Don't return pfmemalloc pages to the page pool. On Thu, 20 Dec 2018 14:21:32 -0800 Jonathan Lemon <jonathan.lemon@...il.com> wrote: > Return pfmemalloc pages back to the page allocator, instead of holding them > in the page pool. > > Signed-off-by: Jonathan Lemon <jonathan.lemon@...il.com> > --- > net/core/page_pool.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/net/core/page_pool.c b/net/core/page_pool.c > index 43a932cb609b..364b893be66f 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 took at closer look at the page_pool issue recycling pages from emergency reserve (pfmemalloc), and it actually cannot happen, because page_pool does not use the __GFP_MEMALLOC gfp_t flag. Thus, page_pool are not allowed to get pages from the emergency reserve in the first place (unless ksoftirqd current->flags have PF_MEMALLOC, which I don't think it have). See: page_pool_dev_alloc_pages() compared to __dev_alloc_pages(). The doc for: /* %__GFP_MEMALLOC allows access to all memory. This should only be used when * the caller guarantees the allocation will allow more memory to be freed * very shortly e.g. process exiting or swapping. Users either should * be the MM or co-ordinating closely with the VM (e.g. swap over NFS). */ With that desc, I don't understand why we actually allow dev_alloc_pages() to get emergency reserve (pfmemalloc) pages, as we store these in an RX-ring queue (usual size 512-1024) that isn't used until N-packets later... even if used as a signal to network stack, to free other resources, this happens at a later point-in-time, not "very shortly". -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
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