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From:   Maurizio Lombardi <mlombard@redhat.com>
Date:   Mon, 11 Jul 2022 18:17:38 +0200
Message-ID: <CAFL455nwqqrviZranVvVgRapSF_Na3vwR4NYM+=Hqbvt3+fJeA@mail.gmail.com>
Subject: Re: [PATCH] mm: prevent page_frag_alloc() from corrupting the memory
To:     Alexander Duyck <alexander.duyck@gmail.com>
Cc:     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>, Chen Lin <chen45464546@163.com>
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po 11. 7. 2022 v 17:34 odes=C3=ADlatel 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=3D3 cache, the normal page_frag_alloc() behaviour w=
ill
therefore be restored.

Maurizio