Re: [PATCH 1/2] xfs: force writes to delalloc regions to unwritten

Brian Foster
2020-05-07T10:32:53Z

On Mon, Feb 03, 2020 at 12:14:45PM -0800, Darrick J. Wong wrote:
> On Mon, Jan 20, 2020 at 07:49:25AM +1100, Dave Chinner wrote:
> > On Wed, Jan 15, 2020 at 10:15:50PM -0800, Darrick J. Wong wrote:
> > > From: Darrick J. Wong <darrick.wong@oracle.com>
> > >
> > > When writing to a delalloc region in the data fork, commit the new
> > > allocations (of the da reservation) as unwritten so that the mappings
> > > are only marked written once writeback completes successfully. This
> > > fixes the problem of stale data exposure if the system goes down during
> > > targeted writeback of a specific region of a file, as tested by
> > > generic/042.
> > >
> > > Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> > > ---
> > > fs/xfs/libxfs/xfs_bmap.c | 28 +++++++++++++++++-----------
> > > 1 file changed, 17 insertions(+), 11 deletions(-)
> > >
> > >
> > > diff --git a/fs/xfs/libxfs/xfs_bmap.c b/fs/xfs/libxfs/xfs_bmap.c
> > > index 4544732d09a5..220ea1dc67ab 100644
> > > --- a/fs/xfs/libxfs/xfs_bmap.c
> > > +++ b/fs/xfs/libxfs/xfs_bmap.c
> > > @@ -4190,17 +4190,7 @@ xfs_bmapi_allocate(
> > > bma->got.br_blockcount = bma->length;
> > > bma->got.br_state = XFS_EXT_NORM;
> > >
> > > - /*
> > > - * In the data fork, a wasdelay extent has been initialized, so
> > > - * shouldn't be flagged as unwritten.
> > > - *
> > > - * For the cow fork, however, we convert delalloc reservations
> > > - * (extents allocated for speculative preallocation) to
> > > - * allocated unwritten extents, and only convert the unwritten
> > > - * extents to real extents when we're about to write the data.
> > > - */
> > > - if ((!bma->wasdel || (bma->flags & XFS_BMAPI_COWFORK)) &&
> > > - (bma->flags & XFS_BMAPI_PREALLOC))
> > > + if (bma->flags & XFS_BMAPI_PREALLOC)
> > > bma->got.br_state = XFS_EXT_UNWRITTEN;
> > >
> > > if (bma->wasdel)
> > > @@ -4608,8 +4598,24 @@ xfs_bmapi_convert_delalloc(
> > > bma.offset = bma.got.br_startoff;
> > > bma.length = max_t(xfs_filblks_t, bma.got.br_blockcount, MAXEXTLEN);
> > > bma.minleft = xfs_bmapi_minleft(tp, ip, whichfork);
> > > +
> > > + /*
> > > + * When we're converting the delalloc reservations backing dirty pages
> > > + * in the page cache, we must be careful about how we create the new
> > > + * extents:
> > > + *
> > > + * New CoW fork extents are created unwritten, turned into real extents
> > > + * when we're about to write the data to disk, and mapped into the data
> > > + * fork after the write finishes. End of story.
> > > + *
> > > + * New data fork extents must be mapped in as unwritten and converted
> > > + * to real extents after the write succeeds to avoid exposing stale
> > > + * disk contents if we crash.
> > > + */
> > > if (whichfork == XFS_COW_FORK)
> > > bma.flags = XFS_BMAPI_COWFORK | XFS_BMAPI_PREALLOC;
> > > + else
> > > + bma.flags = XFS_BMAPI_PREALLOC;
> >
> > bma.flags = XFS_BMAPI_PREALLOC;
> > if (whichfork == XFS_COW_FORK)
> > bma.flags |= XFS_BMAPI_COWFORK;
> >
> > However, I'm still not convinced that this is the right/best
> > solution to the problem. It is the easiest, yes, but the down side
> > on fast/high iops storage and/or under low memory conditions has
> > potential to be extremely significant.
> >
> > I suspect that heavy users of buffered O_DSYNC writes into sparse
> > files are going to notice this the most - there are databases out
> > there that work this way. And I suspect that most of the workloads
> > that use buffered O_DSYNC IO heavily won't see this change for years
> > as enterprise upgrade cycles are notoriously slow.
> >
> > IOWs, all I see this change doing is kicking the can down the road
> > and guaranteeing that we'll still have to solve this stale data
> > exposure problem more efficiently in the future. And instead of
> > doing it now when we have the time and freedom to do the work, it
> > will have to be done urgently under high priority escalation
> > pressures...
>
> FWIW I'm *already* under urgent high priority GA blocker escalation
> pressure, which is why this came up again.
>
> Granted it did take 12 days of losing the battle with the distro folks
> that this really isn't a release blocker (but teh sekuritehs!!) but...oh
> right, I forgot that xfs actually /does/ crash more than once per day in
> our environment.
>
> I guess *we* will find out how much performance really disappears if you
> do it this way. :P
>

Sorry for resurrecting an old thread here, but I was thinking about this
problem a bit and realized I didn't have a great handle on the concerns
with using unwritten extents for delalloc writeback. Dave calls out the
O_DSYNC buffered writes into sparse files case above. I don't see any
numbers posted here so I ran some quick tests using a large ramdisk to
get low latency I/O.

I only seem to require a couple threads to max out single file, random
4k dsync buffered write iops in this particular setup. I see ~30.6k iops
from a baseline 5.7.0-rc1 kernel and that drops to ~25.7k iops when
using unwritten extents for delalloc conversion. However, note that the
same workload through single threaded aio+dio (qd 32) runs at ~63.7k
iops. That's already using unwritten extents for dio so it's unaffected
by this patch. Also note that using a 10MB extent size hint puts the
dsync buffered write case at ~27k iops (again for both kernels because
we're already using unwritten extents in that case as well).

For reference, full file preallocation (i.e. no allocs, unwritten
extents) runs at ~27k iops for the buffered write case and ~87k iops for
aio+dio. The overwrite (no unwritten, no alloc) case gets to ~250k iops
with the same couple dsync buffered write threads and close to 300k iops
with single threaded aio+dio (which I think is maxing out my memory
bandwidth).

Altogether, this has me wondering whether it's really worth the
complexity of trying to avoid the overhead of unwritten extents for
delalloc conversion. There is a noticeable hit, but it's an already slow
path compared to async I/O mechanisms. Further, it's a workload that
typically comes with a recommendation to use extent size hints to avoid
fragmentation issues and minimize allocation overhead, and that feature
already bypasses delalloc extents in favor of unwritten extents.
Thoughts? Suggestions for other tests?

Brian

> --D
>
> > Cheers,
> >
> > Dave.
> > --
> > Dave Chinner
> > david@fromorbit.com
>