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

[Anthony Romano <anthony.romano@coreos.com>]

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The code for shmem_undo_range is very similar to truncate_inode_pages_range
so I assume that's why it's using an inclusive range.

It appears the bug was introduced in
1635f6a74152f1dcd1b888231609d64875f0a81a

On Mon, May 16, 2016 at 4:59 AM, Vlastimil Babka <vbabka@suse.cz> wrote:

> 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@coreos.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;
>>                 }
>>
>>
>>
>

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