Date: Tue, 18 Apr 2017 13:38:08 -0600
From: Ross Zwisler <ross.zwisler@linux.intel.com>
To: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Eric Van Hensbergen <ericvh@gmail.com>,
        Ron Minnich <rminnich@sandia.gov>,
        Latchesar Ionkov <lucho@ionkov.net>,
        Steve French <sfrench@samba.org>,
        Matthew Wilcox <mawilcox@microsoft.com>,
        Ross Zwisler <ross.zwisler@linux.intel.com>,
        Trond Myklebust <trond.myklebust@primarydata.com>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jan Kara <jack@suse.cz>, Jens Axboe <axboe@kernel.dk>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Alexey Kuznetsov <kuznet@virtuozzo.com>,
        Christoph Hellwig <hch@lst.de>,
        v9fs-developer@lists.sourceforge.net, linux-kernel@vger.kernel.org,
        linux-cifs@vger.kernel.org, samba-technical@lists.samba.org,
        linux-nfs@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH 1/4] fs: fix data invalidation in the cleancache during
 direct IO
Message-ID: <20170418193808.GA16667@linux.intel.com>
References: <20170414140753.16108-1-aryabinin@virtuozzo.com>
 <20170414140753.16108-2-aryabinin@virtuozzo.com>
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Content-Type: text/plain; charset=us-ascii
In-Reply-To: <20170414140753.16108-2-aryabinin@virtuozzo.com>
Sender: linux-nfs-owner@vger.kernel.org

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.

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.