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Date:   Thu, 14 Feb 2019 16:13:14 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Alexander Duyck <alexander.duyck@...il.com>
Cc:     linux-mm <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Pavel Tatashin <pavel.tatashin@...rosoft.com>,
        Oscar Salvador <osalvador@...e.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Aaron Lu <aaron.lu@...el.com>, Netdev <netdev@...r.kernel.org>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>
Subject: Re: [PATCH] mm: page_alloc: fix ref bias in page_frag_alloc() for
 1-byte allocs

On Wed, Feb 13, 2019 at 11:42 PM Alexander Duyck
<alexander.duyck@...il.com> wrote:
> On Wed, Feb 13, 2019 at 12:42 PM Jann Horn <jannh@...gle.com> wrote:
> > The basic idea behind ->pagecnt_bias is: If we pre-allocate the maximum
> > number of references that we might need to create in the fastpath later,
> > the bump-allocation fastpath only has to modify the non-atomic bias value
> > that tracks the number of extra references we hold instead of the atomic
> > refcount. The maximum number of allocations we can serve (under the
> > assumption that no allocation is made with size 0) is nc->size, so that's
> > the bias used.
> >
> > However, even when all memory in the allocation has been given away, a
> > reference to the page is still held; and in the `offset < 0` slowpath, the
> > page may be reused if everyone else has dropped their references.
> > This means that the necessary number of references is actually
> > `nc->size+1`.
> >
> > Luckily, from a quick grep, it looks like the only path that can call
> > page_frag_alloc(fragsz=1) is TAP with the IFF_NAPI_FRAGS flag, which
> > requires CAP_NET_ADMIN in the init namespace and is only intended to be
> > used for kernel testing and fuzzing.
>
> Actually that has me somewhat concerned. I wouldn't be surprised if
> most drivers expect the netdev_alloc_frags call to at least output an
> SKB_DATA_ALIGN sized value.
>
> We probably should update __netdev_alloc_frag and __napi_alloc_frag so
> that they will pass fragsz through SKB_DATA_ALIGN.

Do you want to do a separate patch for that? I'd like to not mix
logically separate changes in a single patch, and I also don't have a
good understanding of the alignment concerns here.

> > To test for this issue, put a `WARN_ON(page_ref_count(page) == 0)` in the
> > `offset < 0` path, below the virt_to_page() call, and then repeatedly call
> > writev() on a TAP device with IFF_TAP|IFF_NO_PI|IFF_NAPI_FRAGS|IFF_NAPI,
> > with a vector consisting of 15 elements containing 1 byte each.
> >
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Jann Horn <jannh@...gle.com>
> > ---
> >  mm/page_alloc.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 35fdde041f5c..46285d28e43b 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -4675,11 +4675,11 @@ void *page_frag_alloc(struct page_frag_cache *nc,
> >                 /* Even if we own the page, we do not use atomic_set().
> >                  * This would break get_page_unless_zero() users.
> >                  */
> > -               page_ref_add(page, size - 1);
> > +               page_ref_add(page, size);
> >
> >                 /* reset page count bias and offset to start of new frag */
> >                 nc->pfmemalloc = page_is_pfmemalloc(page);
> > -               nc->pagecnt_bias = size;
> > +               nc->pagecnt_bias = size + 1;
> >                 nc->offset = size;
> >         }
> >
> > @@ -4695,10 +4695,10 @@ void *page_frag_alloc(struct page_frag_cache *nc,
> >                 size = nc->size;
> >  #endif
> >                 /* OK, page count is 0, we can safely set it */
> > -               set_page_count(page, size);
> > +               set_page_count(page, size + 1);
> >
> >                 /* reset page count bias and offset to start of new frag */
> > -               nc->pagecnt_bias = size;
> > +               nc->pagecnt_bias = size + 1;
> >                 offset = size - fragsz;
> >         }
>
> If we already have to add a constant it might be better to just use
> PAGE_FRAG_CACHE_MAX_SIZE + 1 in all these spots where you are having
> to use "size + 1" instead of "size". That way we can avoid having to
> add a constant to a register value and then program that value.
> instead we can just assign the constant value right from the start.

I doubt that these few instructions make a difference, but sure, I can
send a v2 with that changed.

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