xfs: store in-progress CoW allocations in the refcount btree
authorDarrick J. Wong <darrick.wong@oracle.com>
Mon, 3 Oct 2016 16:11:39 +0000 (09:11 -0700)
committerDarrick J. Wong <darrick.wong@oracle.com>
Wed, 5 Oct 2016 23:26:05 +0000 (16:26 -0700)
commit174edb0e46e520230791a1a894397b7c824cefc4
treec15257ff7f762c89bc3ddebe5bba23538a57cbcf
parent5e7e605c4d1b5229c7df8a71798df00a4300428b
xfs: store in-progress CoW allocations in the refcount btree

Due to the way the CoW algorithm in XFS works, there's an interval
during which blocks allocated to handle a CoW can be lost -- if the FS
goes down after the blocks are allocated but before the block
remapping takes place.  This is exacerbated by the cowextsz hint --
allocated reservations can sit around for a while, waiting to get
used.

Since the refcount btree doesn't normally store records with refcount
of 1, we can use it to record these in-progress extents.  In-progress
blocks cannot be shared because they're not user-visible, so there
shouldn't be any conflicts with other programs.  This is a better
solution than holding EFIs during writeback because (a) EFIs can't be
relogged currently, (b) even if they could, EFIs are bound by
available log space, which puts an unnecessary upper bound on how much
CoW we can have in flight, and (c) we already have a mechanism to
track blocks.

At mount time, read the refcount records and free anything we find
with a refcount of 1 because those were in-progress when the FS went
down.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
fs/xfs/libxfs/xfs_bmap.c
fs/xfs/libxfs/xfs_format.h
fs/xfs/libxfs/xfs_refcount.c
fs/xfs/libxfs/xfs_refcount.h
fs/xfs/xfs_mount.c
fs/xfs/xfs_refcount_item.c
fs/xfs/xfs_reflink.c
fs/xfs/xfs_reflink.h
fs/xfs/xfs_super.c
fs/xfs/xfs_trace.h