Re: [PATCH][RFC] truncate vs add_to_page_cache race

[Nick Piggin <nickpiggin@yahoo.com.au>]

Nick Piggin wrote:
> Nick Piggin wrote:
> 
>> Andrew Morton wrote:
>>
>>> Nick Piggin <nickpiggin@yahoo.com.au> wrote:
>>
>>
>>
>>>> OK, I made a debug patch to printk and schedule_timeout in this
>>>> race window so I can easily truncate the file. When this happens,
>>>> it turns out that the readpage thinks it is reading a hole and
>>>> fills the page with zeros -> invalid result?
>>>
>>>
>>>
>>>
>>> A zero-filled pagecache page outside i_size is OK.
>>>
>>
>> Yes. But in this case the zero filled page actually gets
>> read by read(2).
>>
>> In any case, I think my patch won't close the race completely.
>>
> 
> This following patch should be right.
> 
> It causes the zeros to not get copied back unless i_size
> gets extended again.
> 

However, it causes the fast path reading off the end of a file
to always go into ->readpage and copy the non-existant page of
zeros. This could be fixed no problem, but I'll shut up and let
others comment in case I'm making a fool of myself :)