linux-ext4 - Re: [RFC PATCH] dax, ext2, ext4, XFS: fix data corruption race

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Date:	Tue, 26 Jan 2016 09:47:46 -0500
From:	Matthew Wilcox <willy@...ux.intel.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Dave Chinner <david@...morbit.com>,
	Ross Zwisler <ross.zwisler@...ux.intel.com>,
	linux-kernel@...r.kernel.org, Theodore Ts'o <tytso@....edu>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Jan Kara <jack@...e.com>, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-nvdimm@...ts.01.org,
	xfs@....sgi.com
Subject: Re: [RFC PATCH] dax, ext2, ext4, XFS: fix data corruption race

On Tue, Jan 26, 2016 at 02:05:21PM +0100, Jan Kara wrote:
> On Tue 26-01-16 07:48:12, Matthew Wilcox wrote:
> > I *think* that what Dave's proposing (and if he isn't, I'm proposing it
> > for him) is that the filesystem takes its allocation lock shared during
> > the ->fault handler, then in the ->page_mkwrite handler, it knows that an
> > allocation is coming, so it takes its allocation lock in exclusive mode.
> > 
> > So read vs write faults won't be able to race because the allocation lock
> > will prevent it.
> 
> So this is correct and clean design but we will take the lock in exclusive
> mode (and thus hurt scalability) for every write fault, not just for the
> ones allocating blocks. And at the moment we take exclusive lock for write
> faults, there's no more need for having the hole page instantiated - we can
> still do it for simplicity but it's no longer necessary to avoid data
> corruption.

In my mind we take it only for allocating writes, because we also include
the patch to insert PFNs with the writable bit set in the dax_fault
handler if the page fault was for writes.

Although that only works when the *first* fault is a write ... if we
read and page then write the same page, we will indeed take the lock
in exclusive mode.  I think that's fixable too -- in the page_mkwrite
handler, take the lock in exclusive mode only if there's a page in the
radix tree.  I'll take a look at that optimisation after doing the first
couple of steps.
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