Re: [PATCH] mm: prevent page_frag_alloc() from corrupting the memory

[Maurizio Lombardi <mlombard@redhat.com>]

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po 11. 7. 2022 v 20:23 odesílatel Alexander Duyck <alexander.duyck@gmail.com>
napsal:

> On Mon, Jul 11, 2022 at 9:17 AM Maurizio Lombardi <mlombard@redhat.com>
> wrote:
> >
> > po 11. 7. 2022 v 17:34 odesílatel Alexander Duyck
> > <alexander.duyck@gmail.com> napsal:
> > >
> > > Rather than forcing us to free the page it might be better to move the
> > > lines getting the size and computing the offset to the top of the "if
> > > (unlikely(offset < 0)) {" block. Then instead of freeing the page we
> > > could just return NULL and don't have to change the value of any
> > > fields in the page_frag_cache.
> > >
> > > That way a driver performing bad requests can't force us to start
> > > allocating and freeing pages like mad by repeatedly flushing the
> > > cache.
> > >
> >
> > I understand. On the other hand, if we free the cache page then the
> > next time __page_frag_cache_refill() runs it may be successful
> > at allocating the order=3 cache, the normal page_frag_alloc() behaviour
> will
> > therefore be restored.
>
> That is a big "maybe". My concern is that it will actually make memory
> pressure worse by forcing us to reduce the number of uses for a lower
> order page. One bad actor will have us flushing memory like mad so a
> guy expecting a small fragment may end up allocating 32K pages because
> someone else is trying to allocate them.
>
> I recommend we do not optimize for a case which this code was not
> designed for. Try to optimize for the standard case that most of the
> drivers are using. These drivers that are allocating higher order
> pages worth of memory should really be using alloc_pages. Using this
> to allocate pages over 4K in size is just a waste since they are not
> likely to see page reuse which is what this code expects to see.


Ok, thanks for the review, I will submit a V2 soon.

Maurizio

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