Re: [PATCH] nfs: don't extend writes to cover entire page if pagecache is invalid

[Jeff Layton <jlayton@redhat.com>]

On Mon, 10 Dec 2012 16:28:47 +0000
"Myklebust, Trond" <Trond.Myklebust@netapp.com> wrote:

> On Mon, 2012-12-10 at 11:05 -0500, Jeff Layton wrote:
> > On Mon, 10 Dec 2012 14:42:23 +0000
> > "Myklebust, Trond" <Trond.Myklebust@netapp.com> wrote:
> > 
> > > On Mon, 2012-12-10 at 09:25 -0500, Jeff Layton wrote:
> > > > Jian reported that the following sequence would leave "testfile" with
> > > > corrupt data:
> > > > 
> > > >     # mount localhost:/export /mnt/nfs/ -o vers=3
> > > >     # echo abc > /mnt/nfs/testfile; echo def >> /export/testfile; echo ghi >> /mnt/nfs/testfile
> > > >     # cat -v /export/testfile
> > > >     abc
> > > >     ^@^@^@^@ghi
> > > > 
> > > > While there's no locking involved here, the operations are serialized,
> > > > so CTO should prevent corruption.
> > > > 
> > > > The first write to the file is fine and writes 4 bytes. The file is then
> > > > extended on the server. When it's reopened a GETATTR is issued and the
> > > > size change is noticed. This causes NFS_INO_INVALID_DATA to be set on
> > > > the file. Because the file is opened for write only,
> > > > nfs_want_read_modify_write() returns 0 to nfs_write_begin().
> > > > nfs_updatepage then calls nfs_write_pageuptodate() to see if it should
> > > > extend the nfs_page to cover the whole page. NFS_INO_INVALID_DATA is
> > > > still set on the file at that point, but that flag is ignored and
> > > > nfs_pageuptodate erroneously extends the write to cover the whole page,
> > > > with the write done on the server side filled in with zeroes.
> > > > 
> > > > This patch just has that function check for NFS_INO_INVALID_DATA in
> > > > addition to NFS_INO_REVAL_PAGECACHE. This fixes the bug, but looking
> > > > over the code, I wonder if we might have a similar bug in
> > > > nfs_revalidate_size(). The difference between those two flags is very
> > > > subtle, so it seems like we ought to be checking for
> > > > NFS_INO_INVALID_DATA in most of the places that we look for
> > > > NFS_INO_REVAL_PAGECACHE.
> > > > 
> > > > I believe this is regression introduced by commit 8d197a568. The code
> > > > did check for NFS_INO_INVALID_DATA prior to that patch.
> > > > 
> > > > Original bug report is here:
> > > > 
> > > >     https://bugzilla.redhat.com/show_bug.cgi?id=885743
> > > 
> > > Hi Jeff,
> > > 
> > > The point of NFS_INO_REVAL_PAGECACHE is to be able to distinguish
> > > between situations where the file size or change attribute is in doubt
> > > (and so the page cache validity is in doubt), and situations where other
> > > attributes are in doubt (such as ACCESS caches, file owner, nlinks,...).
> > > This is why we're only checking for NFS_INO_REVAL_PAGECACHE and not
> > > NFS_INO_INVALID_DATA in the functions you mention above.
> > > 
> > 
> > Ok, now I'm really confused... I thought that:
> > 
> > NFS_INO_INVALID_DATA == pagecache data was known to be wrong
> > NFS_INO_REVAL_PAGECACHE == pagecache data is questionable
> 
> There is no "known to be wrong" state. Both mean "needs revalidation".
> 
> > ...but if I understand what you're saying above, the "data" that
> > NFS_INO_INVALID_DATA refers to is in peripheral caches, like the ACCESS
> > cache? I guess then I don't quite understand what NFS_INO_INVALID_ATTR
> > and NFS_INO_INVALID_ACCESS are for. Aren't they supposed to indicate that
> > the attrs and access caches are invalid?
> 
> NFS_INO_INVALID_ATTR basically means that some aspect of the file
> metadata needs revalidation, and so a call to nfs_revalidate_inode()
> will always result in a GETATTR rpc call.
> 
> NFS_INO_INVALID_ACCESS means that the file access cache needs
> revalidation. A call to nfs_do_access() will always result in an ACCESS
> rpc call.
> 
> NFS_INO_REVAL_PAGECACHE means that the file page cache data needs
> revalidation. A call to nfs_revalidate_file_size() or
> nfs_revalidate_mapping() will result in a revalidation of the page
> cache. In practice that means a GETATTR rpc call, but that may change if
> future versions of NFS allow for different methods of revalidating the
> page cache; consider, for instance, the byte-range delegation RFC that
> Bruce and I published a few years back.
> 

Ok, thanks. Another question though -- what does NFS_INO_INVALID_DATA
mean, and what distinguishes it from NFS_INO_REVAL_PAGECACHE?

> > > So the fix here should be to set NFS_INO_REVAL_PAGECACHE when the change
> > > attribute and/or size change is noticed. The only function I can see
> > > that appears to get this wrong is nfs_wcc_update_inode(). Does fixing
> > > that lay the bug to rest?
> > > 
> > 
> > No, that doesn't fix it. There's another place that misses setting that
> > flag too in nfs_update_inode() where the size changes. I added it there
> > too and that also didn't fix it. Here's why (I think):
> > 
> > When the size change is noticed via the CTO GETATTR call, then my
> > patched kernel now sets NFS_INO_REVAL_PAGECACHE in nfs_update_inode.
> > Now cache_validity is set to 0x2a. The kernel then makes an ACCESS call
> > that also ultimately ends up in nfs_update_inode.
> > 
> > nfs_update_inode saves off the cache_validity flags and clears all of
> > them except for NFS_INO_INVALID_DATA. As best I can tell, the
> > saved_cache_validity is then ending up discarded and on exit the
> > cache_validity is ending up set to just NFS_INO_INVALID_DATA.
> > 
> > It seems like we ought to be restoring the save_cache_validity flags
> > before exiting that function, but I confess that I don't quite grasp
> > the logic in nfs_update_inode. Thoughts?
> 
> If the ACCESS call isn't revalidating the file size, then it should not
> be clearing NFS_INO_REVAL_PAGECACHE; it does this by restoring that bit
> from the "save_cache_validity" variable and storing it in "invalid". As
> far as I can tell, that code is correct in upstream.
> 

FWIW, I'm working on a 3.7-rc8 kernel here...

The ACCESS call ends up with an fattr that has NFS_ATTR_FATTR_SIZE set,
it just doesn't detect any change in size due to the fact that the CTO
GETATTR call has already updated it.

------------------[snip]------------------------
        /* Check if our cached file size is stale */
        if (fattr->valid & NFS_ATTR_FATTR_SIZE) {
                new_isize = nfs_size_to_loff_t(fattr->size);
                cur_isize = i_size_read(inode);
                if (new_isize != cur_isize) {
                        /* Do we perhaps have any outstanding writes, or has
                         * the file grown beyond our last write? */
                        if ((nfsi->npages == 0 && !test_bit(NFS_INO_LAYOUTCOMMIT, &nfsi->flags)) ||
                             new_isize > cur_isize) {
                                i_size_write(inode, new_isize);
                                invalid |= NFS_INO_INVALID_ATTR|NFS_INO_INVALID_DATA;
                        }
                        dprintk("NFS: isize change on server for file %s/%ld "
                                        "(%Ld to %Ld)\n",
                                        inode->i_sb->s_id,
                                        inode->i_ino,
                                        (long long)cur_isize,
                                        (long long)new_isize);
                }
        } else
                invalid |= save_cache_validity & (NFS_INO_INVALID_ATTR
                                | NFS_INO_REVAL_PAGECACHE
                                | NFS_INO_REVAL_FORCED);
------------------[snip]------------------------

So here, we only restore set the REVAL_PAGECACHE flag in invalid if the
size changed, or there was no size attribute present in the response.
The size hasn't changed since the GETATTR call, so REVAL_PAGECACHE is
not restored.

-- 
Jeff Layton <jlayton@redhat.com>