linux-kernel - Re: [PATCH linux-next] Fix shmem huge page failed to set F_SEAL

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Date:   Tue, 15 Feb 2022 14:12:36 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     cgel.zte@...il.com
Cc:     hughd@...gle.com, mike.kravetz@...cle.com, kirill@...temov.name,
        songliubraving@...com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, yang.yang29@....com.cn,
        wang.yong12@....com.cn, Zeal Robot <zealci@....com.cn>
Subject: Re: [PATCH linux-next] Fix shmem huge page failed to set
 F_SEAL_WRITE attribute problem

On Tue, 15 Feb 2022 07:37:43 +0000 cgel.zte@...il.com wrote:

> From: wangyong <wang.yong12@....com.cn>
> 
> After enabling tmpfs filesystem to support transparent hugepage with the
> following command:
>  echo always > /sys/kernel/mm/transparent_hugepage/shmem_enabled
> The docker program adds F_SEAL_WRITE through the following command will
> prompt EBUSY.
>  fcntl(5, F_ADD_SEALS, F_SEAL_WRITE)=-1.
> 
> It is found that in memfd_wait_for_pins function, the page_count of
> hugepage is 512 and page_mapcount is 0, which does not meet the
> conditions:
>  page_count(page) - page_mapcount(page) != 1.
> But the page is not busy at this time, therefore, the page_order of
> hugepage should be taken into account in the calculation.

What are the real-world runtime effects of this?

Do we think that this fix (or one similar to it) should be backported
into -stable kernels?

If "yes" then Mike's 5d752600a8c373 ("mm: restructure memfd code") will
get in the way because it moved lots of code around.

But then, that's four years old and perhaps that's far enough back in
time.