linux-kernel - Re: [PATCH] tmpfs: don't undo fallocate past its last page

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Date:	Mon, 16 May 2016 13:59:10 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Anthony Romano <anthony.romano@...eos.com>, hughd@...gle.com
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tmpfs: don't undo fallocate past its last page

On 05/08/2016 03:16 PM, Anthony Romano wrote:
> When fallocate is interrupted it will undo a range that extends one byte
> past its range of allocated pages. This can corrupt an in-use page by
> zeroing out its first byte. Instead, undo using the inclusive byte range.

Huh, good catch. So why is shmem_undo_range() adding +1 to the value in 
the first place? The only other caller is shmem_truncate_range() and all 
*its* callers do subtract 1 to avoid the same issue. So a nicer fix 
would be to remove all this +1/-1 madness. Or is there some subtle 
corner case I'm missing?

> Signed-off-by: Anthony Romano <anthony.romano@...eos.com>

Looks like a stable candidate patch. Can you point out the commit that 
introduced the bug, for the Fixes: tag?

Thanks,
Vlastimil

> ---
>   mm/shmem.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 719bd6b..f0f9405 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2238,7 +2238,7 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
>   			/* Remove the !PageUptodate pages we added */
>   			shmem_undo_range(inode,
>   				(loff_t)start << PAGE_SHIFT,
> -				(loff_t)index << PAGE_SHIFT, true);
> +				((loff_t)index << PAGE_SHIFT) - 1, true);
>   			goto undone;
>   		}
>
>