Re: [PATCH] VFS readahead bug in 2.6.8-rc[1-3]

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

Phillip Lougher wrote:
> Nick Piggin wrote:
> 
>> Ram Pai wrote:
>>
>>>
>>> there is a check in __do_page_cache_readahead()  that validates this.
>>> But it is still not guaranteed to work correctly against races.
>>> The filesystem has to handle such out-of-bound requests gracefully.
>>>
>>> However with Nick's fix in do_generic_mapping_read() the filesystem 
>>> is gauranteed to be called with out-of-bound index, if the file size 
>>> is a multiple of 4k. Without the fix, the filesystem might get
>>> called with out-of-bound index only in racy conditions.
>>>
>>
>> How's this?
>>
> 
> It doesn't work.  It correctly handles the case where *ppos is equal
> to i_size on entry to the function (and this does work for files 0, 4k
> and n * 4k in length), but it doesn't handle readahead inside the for
> loop.  The check needs to be in the for loop.
> 
> 

I don't quite follow. What is i_size, *ppos, and desc->count
required for your problem to trigger?