Re: [PATCH 001 of 2] Fix read/truncate race.

Jens Axboe
2007-06-07T07:21:08Z

On Thu, Jun 07 2007, NeilBrown wrote:
>
> do_generic_mapping_read currently samples the i_size at the start
> and doesn't do so again unless it needs to call ->readpage to load
> a page. After ->readpage it has to re-sample i_size as a truncate
> may have caused that page to be filled with zeros, and the read()
> call should not see these.
>
> However there are other activities that might cause ->readpage to be
> called on a page between the time that do_generic_mapping_read
> samples i_size and when it finds that it has an uptodate page. These
> include at least read-ahead and possibly another thread performing a
> read.
>
> So do_generic_mapping_read must sample i_size *after* it has an
> uptodate page. Thus the current sampling at the start and after a read
> can be replaced with a sampling before the copy-out.
>
> The same change applied to __generic_file_splice_read.
>
> Note that this fixes any race with truncate_complete_page, but does
> not fix a possible race with truncate_partial_page. If a partial
> truncate happens after do_generic_mapping_read samples i_size and
> before the copy_out, the nuls that truncate_partial_page place in the
> page could be copied out incorrectly.
>
> I think the best fix for that is to *not* zero out parts of the page
> in truncate_partial_page, but rather to zero out the tail of a page
> when increasing i_size.

Thanks Neil, if you don't mind I'll steal the splice bits and add them
to the splice branch, as this particular patch conflicts with other
isize fixes that Hugh and I have been working on.

--
Jens Axboe