On Wed, 23 Mar 2011 12:27:31 +0300
Pavel Shilovsky <piastry@xxxxxxxxxxx> wrote:
> 2011/3/23 Steve French <smfrench@xxxxxxxxx>:
> > Pavel,
> > Jeff and I talked about invalidate_mapping and its uses this
> > afternoon. ÂA few comments:
> > 1) Â Take a look at what happens on filemap_fdatawait call in
> > invalidate_mapping to see if it makes sense to return errors through
> > back to some of the callers of invalidate_mapping, in particular,
> > strict_fsync. ÂIf we can't write out the file data (ENOSPC, or host
> > down etc.), we want to make sure that the return code gets sent back
> > on any calls that can reasonably expect such an error.
>
> vfs_fsync_range has already done filemap_fdatawait call - so, in this
> case there is no need to do it again in cifs_invalidate_mapping. The
> only reason for calling this is to invalidate_inode_pages2 but any
> error there shouldn't affect fsync behavior, as I think.
>
> > 2) If invalidate_inode_pages2 fails (e.g. with EBUSY, because one of
> > the pages couldn't be freed from the mapping because it just got
> > redirtied right after we flushed it the line before) we set the
> > mapping to invalid but don't check
> > Â Âcifs_i->invalid_mapping
> > in many places. ÂShould we add checks for cifs_i->invalid_mapping in
> > more places?
>
> The one place where we should add such a check is read call, but it
> needs cifs_revalidate_file instead (that I am going to provide next)
> before generic_file_aio_read. In this case cifs_revalidate will check
> for invalid_mapping and needs to return a error if we could not
> invalidate all inode pages in cifs_invalidate_mapping (because it
> returns wrong data to the read call). But as you noticed lseek
> shouldn't think about this error and it uses cifs_revalidate_file too.
>
> So, we may add extra check for -EBUSY error code in callers of
> cifs_revalidate_{dentry,file} and cifs_invaliadate_mapping and
> separate them into two groups:
> 1) that aware about -EBUSY error code and return a error the it's caller.
> these are: cifs_d_revalidate, cifs_file_aio_read (future
> implementation), cifs_file_strict_mmap.
> 2) that doesn't aware about it and return ok in this case.
> these are: cifs_getattrs, cifs_lseek, cifs_fsync.
>
> Thoughts?
>
The main reason I think we need to reconsider that error is that I
spent several months tracking down a rather nasty data corruption bug
relating to mmap on NFS in RHEL5 a few years ago:
https://bugzilla.redhat.com/show_bug.cgi?id=435291
Part of the problem there was that the NFS client ignored the return
code from invalidate_inode_pages2. The other part of the problem was a
lack of synchronization between mmap calls and the page fault handler.
Needless to say, this was not a fun problem to track down. I think you
need to be very careful about ignoring errors from
invalidate_inode_pages2 and think carefully about what a failure there
means for all cases.
--
Jeff Layton <jlayton@xxxxxxxxxx>
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