Re: Another proposal for DAX fault locking

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

On Wed, Feb 10, 2016 at 11:39:43PM +0100, Cedric Blancher wrote:
> AFAIK Solaris 11 uses a sparse tree instead of a array. Solves the
> scalability problem AND deals with variable page size.

Right - seems like tying the radix tree into the locking instead of using an
array would have these same benefits.

> On 10 February 2016 at 23:09, Dave Chinner <david@fromorbit.com> wrote:
> > On Wed, Feb 10, 2016 at 11:32:49AM +0100, Jan Kara wrote:
> >> On Tue 09-02-16 10:18:53, Dan Williams wrote:
> >> > On Tue, Feb 9, 2016 at 9:24 AM, Jan Kara <jack@suse.cz> wrote:
> >> > > Hello,
> >> > >
> >> > > I was thinking about current issues with DAX fault locking [1] (data
> >> > > corruption due to racing faults allocating blocks) and also races which
> >> > > currently don't allow us to clear dirty tags in the radix tree due to races
> >> > > between faults and cache flushing [2]. Both of these exist because we don't
> >> > > have an equivalent of page lock available for DAX. While we have a
> >> > > reasonable solution available for problem [1], so far I'm not aware of a
> >> > > decent solution for [2]. After briefly discussing the issue with Mel he had
> >> > > a bright idea that we could used hashed locks to deal with [2] (and I think
> >> > > we can solve [1] with them as well). So my proposal looks as follows:
> >> > >
> >> > > DAX will have an array of mutexes (the array can be made per device but
> >> > > initially a global one should be OK). We will use mutexes in the array as a
> >> > > replacement for page lock - we will use hashfn(mapping, index) to get
> >> > > particular mutex protecting our offset in the mapping. On fault / page
> >> > > mkwrite, we'll grab the mutex similarly to page lock and release it once we
> >> > > are done updating page tables. This deals with races in [1]. When flushing
> >> > > caches we grab the mutex before clearing writeable bit in page tables
> >> > > and clearing dirty bit in the radix tree and drop it after we have flushed
> >> > > caches for the pfn. This deals with races in [2].
> >> > >
> >> > > Thoughts?
> >> > >
> >> >
> >> > I like the fact that this makes the locking explicit and
> >> > straightforward rather than something more tricky.  Can we make the
> >> > hashfn pfn based?  I'm thinking we could later reuse this as part of
> >> > the solution for eliminating the need to allocate struct page, and we
> >> > don't have the 'mapping' available in all paths...
> >>
> >> So Mel originally suggested to use pfn for hashing as well. My concern with
> >> using pfn is that e.g. if you want to fill a hole, you don't have a pfn to
> >> lock. What you really need to protect is a logical offset in the file to
> >> serialize allocation of underlying blocks, its mapping into page tables,
> >> and flushing the blocks out of caches. So using inode/mapping and offset
> >> for the hashing is easier (it isn't obvious to me we can fix hole filling
> >> races with pfn-based locking).
> >
> > So how does that file+offset hash work when trying to lock different
> > ranges?  file+offset hashing to determine the lock to use only works
> > if we are dealing with fixed size ranges that the locks affect.
> > e.g. offset has 4k granularity for a single page faults, but we also
> > need to handle 2MB granularity for huge page faults, and IIRC 1GB
> > granularity for giant page faults...
> >
> > What's the plan here?
> >
> > Cheers,
> >
> > Dave.
> > --
> > Dave Chinner
> > david@fromorbit.com
> > --
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> 
> 
> 
> -- 
> Cedric Blancher <cedric.blancher@gmail.com>
> Institute Pasteur

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