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

[Kirill A. Shutemov]

Steven Whitehouse wrote:
> 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.

CC: +linux-fsdevel@

So in this case node A will see the file like it was never touched by
node B. It's okay, if new i_size will eventually reach node A.

Is ->readpage() the only way to get i_size updated on node A or it will be
eventually updated without it?

If it's the only way, we need add a explicit way to initiate i_size sync
between nodes on read. Probably, distributed filesystems should provide own
->aio_read() which deal i_size as the filesystem need.

> > > 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. 

That i_size recheck was invented to cover different use case: read vs.
truncate race. Userspace should not see truncate-caused zeros in buffer.
It's not to prevent file extending vs. read() race. This usually harmless:
data is consistent.

-- 
 Kirill A. Shutemov
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