Re: [PATCH] mm, fs: avoid page allocation beyond i_size on read

[Steven Whitehouse]

Hi,

On Wed, 2013-08-21 at 13:58 -0700, Andrew Morton wrote:
> On Wed, 21 Aug 2013 17:42:12 +0100 Steven Whitehouse <swhiteho@xxxxxxxxxx> wrote:
> 
> > > I don't think the change is harmful. The worst case scenario is race with
> > > write or truncate, but it's valid to return EOF in this case.
> > > 
> > > What scenario do you have in mind?
> > > 
> > 
> > 1. File open on node A
> > 2. Someone updates it on node B by extending the file
> > 3. Someone reads the file on node A beyond end of original file size,
> > but within end of new file size as updated by node B. Without the patch
> > this works, with it, it will fail. The reason being the i_size would not
> > be up to date until after readpage(s) has been called.
> > 
> > I think this is likely to be an issue for any distributed fs using
> > do_generic_file_read(), although it would certainly affect GFS2, since
> > the locking is done at page cache level,
> 
> Boy, that's rather subtle.  I'm surprised that the generic filemap.c
> stuff works at all in that sort of scenario.
> 
> Can we put the i_size check down in the no_cached_page block?  afaict
> that will solve the problem without breaking GFS2 and is more
> efficient?
> 

Well I think is even more subtle, since it relies on ->readpages
updating the file size, even if it has failed to actually read the
required pages :-) Having said that, we do rely on ->readpages updating
the inode size elsewhere in this function, as per the block comment
immediately following the page_ok label. 

This should work for GFS2 though, and I did check OCFS2 and I think it
should work for them too,

Steve.



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