Re: [PATCH 0/6] xfs: reflink fixes

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Sat, Jan 20, 2018 at 09:33:49PM -0800, Darrick J. Wong wrote:
> Hi all,
> 
> Running generic/232 with quotas and reflink demonstrated that there was
> something wrong with the way we did quota accounting -- on an otherwise
> idle system, fs-wide du block count numbers didn't match the quota
> reports.  I started digging into why the quota accounting was wrong, and
> the following are the results of my bug hunt.

Well, I wasn't expecting 4.15 to be delayed again, but I guess it has.
To disambiguate: this series is intended for 4.16, even though I already
sent out the AGFL series tagged for 4.17.

--D

> The first patch teaches the reflink code to break layout leases before
> commencing the block remapping work.  This time we avoid the "looping
> trying to get a lock" that Christoph complained about, in favor of
> dropping both locks and retrying if we can't cleanly break the layouts
> without waiting.
> 
> The second patch changes the source file locking (if src != dest) during
> a reflink operation to take the shared locks when possible.  The only
> thing changing in the source file is the setting of the reflink iflag,
> for which we will still take ILOCK_EXCL.  The net result of this is
> less lock contention during fsstress and a 30% lower runtime, not that
> anyone cares about fsstress benchmarking. :)
> 
> Patch three ensure that we attach dquots to inodes before we start
> reflinking their blocks.  This could lead to quota undercharging; an
> fstest to check this will be sent separately.
> 
> Patch four reorganizes the copy on write quota updating code to reflect
> how the CoW fork works now.  In short, the CoW fork is entirely in
> memory, so we can only use the in-memory quota reservation counters for
> all CoW blocks; the accounting only becomes permanent if we remap an
> extent into the data fork.
> 
> Patch five creates a separate i_cow_blocks counter to track all the CoW
> blocks assigned to a file, which makes changing a file's uid/gid/prjid
> easier, makes reporting cow blocks via stat easy, and enables various
> cleanups.
> 
> Patch six fixes a serious potential corruption problem with the cow
> extent allocation -- when we allocate into the CoW fork with the cow
> extent size hint set, the allocator enlarges the allocation request to
> try to hit alignment goals.  However, if the allocated extent does not
> actually fulfill any of the requested range, we send a garbage
> zero-length extent back to the iomap code (which also doesn't notice),
> and the write lands at the startblock of the garbage extent.  The fix is
> to detect that we didn't fill the entire requested range and fix up the
> returned mapping so that we always fill the first block of the
> requested allocation.
> 
> --D
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