btrfs: don't abort raid56 data scrub on first uncorrectable sector

In commit 1009254bf22a ("btrfs: scrub: use scrub_stripe to implement
RAID56 P/Q scrub") a new function scrub_raid56_parity_stripe() was
introduced to recalculate the parity stripe after correcting any
correctable errors.  When it detected unrepaired sectors in a data
stripe, it returned -EIO, which propagated up the stack through
scrub_stripe, scrub_chunk, and scrub_enumerate_chunks, where an error
makes the scrub break out of its loop early and return the error to
userspace.

That was a regression:  uncorrectable data blocks are one of the
expected possible events that occur during a scrub, so scrub should
continue until it has counted and reported all of the uncorrectable
stripes in the filesystem.  Perhaps more importantly, scrub should fix
up any correctable errors that might exist in other stripes beyond the
first uncorrectable stripe.  Errors from this function are only
appropriate when scrub cannot do its job at all:  unable to read csums,
unable to map an extent's data blocks, or out of memory.

On current kernels the bug has changed shape:  at the point where
unrepaired sectors are detected, ret no longer holds -EIO.  It holds
the leftover return value of the last scrub_find_fill_first_stripe()
call from the loop that populates the data stripes, which is 0, or 1
when the last data stripe of the full stripe contains no extents.
When the stale 1 leaks out, the caller treats any nonzero value as an
error ("if (ret) goto out;" in scrub_stripe()) and aborts the whole
scrub with a meaningless positive return value; when it is 0, the
scrub continues only by accident.

scrub_raid56_parity_stripe() has successfully completed its task as
soon as it has performed its data correction and verification steps.
Return 0 explicitly at that point, regardless of the outcome of the
verification.  The P/Q update step is still skipped for the affected
full stripe, so no garbage is written back to the devices.  Detected
errors are reported to the user via device stats and dmesg messages,
not via the return code of the scrub ioctl.

Fixes: 1009254bf22a ("btrfs: scrub: use scrub_stripe to implement RAID56 P/Q scrub")
Signed-off-by: Zygo Blaxell <ce3g8jdj@umail.furryterror.org>

diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index c09d4213ad8915d0227eeb34d9ed6b3eff20bdc8..e6377c7a360200f80d6799ed5a503d9856045d58 100644 (file)
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -2248,7 +2248,7 @@ static int scrub_raid56_parity_stripe(struct scrub_ctx *sctx,
 "scrub: unrepaired sectors detected, full stripe %llu data stripe %u errors %*pbl",
                                  full_stripe_start, i, stripe->nr_sectors,
                                  &error);
-                       return ret;
+                       return 0;
                }
                bitmap_or(&extent_bitmap, &extent_bitmap, &has_extent,
                          stripe->nr_sectors);