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From:   Maurizio Lombardi <mlombard@redhat.com>
Date:   Fri, 8 Jul 2022 10:06:53 +0200
Message-ID: <CAFL455=ZcU_fyM9kiuZUJeVmRv9Jx_FmURcweCrTXheRoKkSqg@mail.gmail.com>
Subject: Re: Re: [PATCH v2] mm: page_frag: Warn_on when frag_alloc size is
 bigger than PAGE_SIZE
To:     =?UTF-8?B?5oSa5qCR?= <chen45464546@163.com>
Cc:     Alexander Duyck <alexander.duyck@gmail.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Netdev <netdev@vger.kernel.org>
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st 1. 6. 2022 v 14:49 odes=C3=ADlatel =E6=84=9A=E6=A0=91 <chen45464546@163.=
com> napsal:
> Can we just add code to the relatively slow path to capture the mistake
> before it lead to memory corruption?
> Like:
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e6f211d..ac60a97 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5580,6 +5580,7 @@ void *page_frag_alloc_align(struct page_frag_cache =
*nc,
>                 /* reset page count bias and offset to start of new frag =
*/
>                 nc->pagecnt_bias =3D PAGE_FRAG_CACHE_MAX_SIZE + 1;
>                 offset =3D size - fragsz;
> +               BUG_ON(offset < 0);
>         }
>

Personally, I'm not really convinced this is the best solution.
The next time a driver abuses  the page_frag_alloc() interface, the
bug may go unnoticed for a long time...
until a server in production runs into OOM and crashes because it hits
the BUG_ON().

And why should the kernel panic? It's perfectly able to handle this
condition by failing
the allocation and returning NULL, and printing a warning maybe.

Maurizio