linux-kernel - Re: [PATCH v2] fix judgment error in shmem_is

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Message-ID: <20210909061732.2zalspiyfoevd7ea@box.shutemov.name>
Date:   Thu, 9 Sep 2021 09:17:32 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Liu Yuntao <liuyuntao10@...wei.com>
Cc:     akpm@...ux-foundation.org, hughd@...gle.com,
        kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, liusirui@...wei.com, windspectator@...il.com,
        wuxu.wu@...wei.com
Subject: Re: [PATCH v2] fix judgment error in shmem_is_huge()

On Thu, Sep 09, 2021 at 11:20:07AM +0800, Liu Yuntao wrote:
> In the case of SHMEM_HUGE_WITHIN_SIZE, the page index is not rounded
> up correctly. When the page index points to the first page in a huge
> page, round_up() cannot bring it to the end of the huge page, but
> to the end of the previous one.
> 
> an example:
> HPAGE_PMD_NR on my machine is 512(2 MB huge page size).
> After allcoating a 3000 KB buffer, I access it at location 2050 KB.
> In shmem_is_huge(), the corresponding index happens to be 512.
> After rounded up by HPAGE_PMD_NR, it will still be 512 which is
> smaller than i_size, and shmem_is_huge() will return true.
> As a result, my buffer takes an additional huge page, and that
> shouldn't happen when shmem_enabled is set to within_size.
> 
> Fixes: f3f0e1d2150b2b ("khugepaged: add support of collapse for tmpfs/shmem pages")
> Signed-off-by: Liu Yuntao <liuyuntao10@...wei.com>

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>


-- 
 Kirill A. Shutemov