Re: Checking page_count(page) in invalidate_complete_page

[Chuck Lever <chuck.lever@oracle.com>]

Nick Piggin wrote:
> Chuck Lever wrote:
>> Invalidate_inode_pages2() is used to remove page cache data that the 
>> client has determined is stale.  The client detects that the file has 
>> changed on the server, and it is not responsible for those changes, by 
>> examining the file's attributes and noticing mtime or size changes. 
>> When such a change is detected, all pages cached for a file are 
>> invalidated, and the page cache is gradually repopulated from the 
>> server as applications access parts of the file.
>>
> 
> If you can guarantee that a pagecache page can never get mapped to a 
> user mapping (eg. perhaps for
> directories and symlinks) and also ensure that you don't dirty it via 
> the filesystem, then you don't
> have to worry about it becoming dirty, so we can skip the checks Andrew 
> has added and maybe add a
> WARN_ON(PageDirty()).

As far as I can tell, for cached non-regular files, they can't be mapped 
or written to, and cannot be targets of direct I/O.

So, I think it's acceptable to add logic in nfs_revalidate_mapping() to 
use invalidate_inode_pages2() for non-regular files (and check the 
return code, of course), as long as we have addressed the page count 
issue somehow.

> Now that won't help you for regular file pages that can be mmapped. For 
> those, we need to ensure
> that the page isn't mapped, and the page will not be dirtied via a 
> get_user_pages user, and you need
> to ensure that it can't get dirtied via the filesystem. The former two 
> will require VM changes of
> the scale that aren't going to get into 2.6.19, but I'm working on them.
> 
> For now, can make do with flushing lru pagevecs, and also testing and 
> retrying in the caller?

I planned to play with this a bit yesterday, but got swept up in some 
higher priority issues.  Will try to get to testing this soonest.

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