On Wed, Nov 14, 2018 at 09:50:20PM -0800, Darrick J. Wong wrote: > On Tue, Nov 13, 2018 at 12:08:19PM -0500, Brian Foster wrote: > > Page writeback indirectly handles shared extents via the existence > > of overlapping COW fork blocks. If COW fork blocks exist, writeback > > always performs the associated copy-on-write regardless if the > > underlying blocks are actually shared. If the blocks are shared, > > then overlapping COW fork blocks must always exist. > > > > fstests shared/010 reproduces a case where a buffered write occurs > > over a shared block without performing the requisite COW fork > > reservation. This ultimately causes writeback to the shared extent > > and data corruption that is detected across md5 checks of the > > filesystem across a mount cycle. > > > > The problem occurs when a buffered write lands over a shared extent > > that crosses an extent size hint boundary and that also happens to > > have a partial COW reservation that doesn't cover the start and end > > blocks of the data fork extent. > > > > For example, a buffered write occurs across the file offset (in FSB > > units) range of [29, 57]. A shared extent exists at blocks [29, 35] > > and COW reservation already exists at blocks [32, 34]. After > > accommodating a COW extent size hint of 32 blocks and the existing > > reservation at offset 32, xfs_reflink_reserve_cow() allocates 32 > > blocks of reservation at offset 0 and returns with COW reservation > > across the range of [0, 34]. The associated data fork extent is > > still [29, 35], however, which isn't fully covered by the COW > > reservation. > > > > This leads to a buffered write at file offset 35 over a shared > > extent without associated COW reservation. Writeback eventually > > kicks in, performs an overwrite of the underlying shared block and > > causes the associated data corruption. > > > > Update xfs_reflink_reserve_cow() to accommodate the fact that a > > delalloc allocation request may not fully cover the extent in the > > data fork. Trim the data fork extent appropriately, just as is done > > for shared extent boundaries and/or existing COW reservations that > > happen to overlap the start of the data fork extent. This prevents > > shared/010 failures due to data corruption on reflink enabled > > filesystems. > > > > Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx> > > --- > > > > This is not fully tested yet beyond verification that it solves the > > problem reproduced by shared/010. I'll be running more tests today, but > > I'm sending sooner for review and testing due to the nature of the > > problem and the fact that it's a fairly isolated change. I'll follow up > > if I discover any resulting regressions.. > > Did you find any regressions? > I ended up having to restart my test run because I was hitting writeback livelocks reported in the other large blocksize series. The testing otherwise finished last night with no regressions. I do see what look like corruption failures on generic/127 and generic/091 with fstests patched with your fsx enhancements, but I see those same failures on for-next so I suspect that is an independent issue. > I ran this through my overnight tests and saw no adverse effects, though > Dave was complaining yesterday about continuing generic/091 corruptions > (which I didn't see with this patch applied...) > > Anyway it looks reasonable to me... > > Reviewed-by: Darrick J. Wong <darrick.wong@xxxxxxxxxx> > Thanks for the review and additional testing. Brian > --D > > > Brian > > > > fs/xfs/xfs_reflink.c | 1 + > > 1 file changed, 1 insertion(+) > > > > diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c > > index ecdb086bc23e..c56bdbfcf7ae 100644 > > --- a/fs/xfs/xfs_reflink.c > > +++ b/fs/xfs/xfs_reflink.c > > @@ -296,6 +296,7 @@ xfs_reflink_reserve_cow( > > if (error) > > return error; > > > > + xfs_trim_extent(imap, got.br_startoff, got.br_blockcount); > > trace_xfs_reflink_cow_alloc(ip, &got); > > return 0; > > } > > -- > > 2.17.2 > >
