Re: [PATCH 1/4] fs: fix data invalidation in the cleancache during direct IO

[Ross Zwisler <ross.zwisler@linux.intel.com>]

On Thu, Apr 20, 2017 at 04:44:31PM +0200, Jan Kara wrote:
> On Thu 20-04-17 16:35:10, Jan Kara wrote:
> > On Wed 19-04-17 13:28:36, Ross Zwisler wrote:
> > > On Wed, Apr 19, 2017 at 06:11:31PM +0300, Andrey Ryabinin wrote:
> > > > On 04/18/2017 10:38 PM, Ross Zwisler wrote:
> > > > > On Fri, Apr 14, 2017 at 05:07:50PM +0300, Andrey Ryabinin wrote:
> > > > >> Some direct write fs hooks call invalidate_inode_pages2[_range]()
> > > > >> conditionally iff mapping->nrpages is not zero. If page cache is empty,
> > > > >> buffered read following after direct IO write would get stale data from
> > > > >> the cleancache.
> > > > >>
> > > > >> Also it doesn't feel right to check only for ->nrpages because
> > > > >> invalidate_inode_pages2[_range] invalidates exceptional entries as well.
> > > > >>
> > > > >> Fix this by calling invalidate_inode_pages2[_range]() regardless of nrpages
> > > > >> state.
> > > > >>
> > > > >> Fixes: c515e1fd361c ("mm/fs: add hooks to support cleancache")
> > > > >> Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
> > > > >> ---
> > > > > <>
> > > > >> diff --git a/fs/dax.c b/fs/dax.c
> > > > >> index 2e382fe..1e8cca0 100644
> > > > >> --- a/fs/dax.c
> > > > >> +++ b/fs/dax.c
> > > > >> @@ -1047,7 +1047,7 @@ dax_iomap_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> > > > >>  	 * into page tables. We have to tear down these mappings so that data
> > > > >>  	 * written by write(2) is visible in mmap.
> > > > >>  	 */
> > > > >> -	if ((iomap->flags & IOMAP_F_NEW) && inode->i_mapping->nrpages) {
> > > > >> +	if ((iomap->flags & IOMAP_F_NEW)) {
> > > > >>  		invalidate_inode_pages2_range(inode->i_mapping,
> > > > >>  					      pos >> PAGE_SHIFT,
> > > > >>  					      (end - 1) >> PAGE_SHIFT);
> > > > > 
> > > > > tl;dr: I think the old code is correct, and that you don't need this change.
> > > > > 
> > > > > This should be harmless, but could slow us down a little if we keep
> > > > > calling invalidate_inode_pages2_range() without really needing to.  Really for
> > > > > DAX I think we need to call invalidate_inode_page2_range() only if we have
> > > > > zero pages mapped over the place where we are doing I/O, which is why we check
> > > > > nrpages.
> > > > > 
> > > > 
> > > > Check for ->nrpages only looks strange, because invalidate_inode_pages2_range() also
> > > > invalidates exceptional radix tree entries. Is that correct that we invalidate
> > > > exceptional entries only if ->nrpages > 0 and skip invalidation otherwise?
> > > 
> > > For DAX we only invalidate clean DAX exceptional entries so that we can keep
> > > dirty entries around for writeback, but yes you're correct that we only do the
> > > invalidation if nrpages > 0.  And yes, it does seem a bit weird. :)
> > 
> > Actually in this place the nrpages check is deliberate since there should
> > only be hole pages or nothing in the invalidated range - see the comment
> > before the if. But thinking more about it this assumption actually is not
> > right in presence of zero PMD entries in the radix tree. So this change
> > actually also fixes a possible bug for DAX but we should do it as a
> > separate patch with a proper changelog.
> 
> Something like the attached patch. Ross?

Yep, great catch, this is a real issue.  The attached patch isn't sufficient,
though, because invalidate_inode_pages2_range() for DAX exceptional entries
only wipes out the radix tree entry, and doesn't call unmap_mapping_range() as
it does in the case of real pages.

I'm working on a fix and an associated xfstest test.

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