linux-kernel - Re: [PATCH V3] mm: prevent page_frag_alloc() from corrupting the memory

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Date:   Mon, 18 Jul 2022 15:50:41 +0200
From:   Maurizio Lombardi <mlombard@...hat.com>
To:     Chen Lin <chen45464546@....com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH V3] mm: prevent page_frag_alloc() from corrupting the memory

po 18. 7. 2022 v 15:14 odesílatel Chen Lin <chen45464546@....com> napsal:
> ----------------------------------------
> If we can accept adding a branch to this process, why not add it at the beginning like below?
> The below changes are also more in line with the definition of "page fragment",
> which i mean the above changes may make the allocation of more than one page successful.
>
> index 7a28f7d..9d09ea5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5551,6 +5551,8 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
>
>         offset = nc->offset - fragsz;
>         if (unlikely(offset < 0)) {
> +               if (unlikely(fragsz > PAGE_SIZE))
> +                       return NULL;
>                 page = virt_to_page(nc->va);
>
>                 if (!page_ref_sub_and_test(page, nc->pagecnt_bias))
>

This will make *all* page_frag_alloc() calls with fragsz > PAGE_SIZE
fail, so it will
basically break all those drivers that are relying on the current behaviour.
With my patch it will return NULL only if the cache isn't big enough.

Maurizio.