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

[Ross Zwisler]

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@xxxxxxxxxxxxx>
> ---
<>
> 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.

Is DAX even allowed to be used at the same time as cleancache?  From a brief
look at Documentation/vm/cleancache.txt, it seems like these two features are
incompatible.  With DAX we already are avoiding the page cache completely.

Anyway, I don't see how this change in DAX can save us from a data corruption
(which is what you're seeing, right?), and I think it could slow us down, so
I'd prefer to leave things as they are.