[PATCH v4 9/9] btrfs: move scrub_remap_extent() call into scrub_extent() with more comments

Qu Wenruo
2022-03-11T07:39:22Z

[SUSPICIOUS CODE]
When refactoring scrub code, I notice a very strange behavior around
scrub_remap_extent():

if (sctx->is_dev_replace)
scrub_remap_extent(fs_info, cur_logical, scrub_len,
&cur_physical, &target_dev, &cur_mirror);

As replace target is a 1:1 copy of the source device, thus physical
offset inside the target should be the same as physical inside source,
thus this remap call makes no sense to me.

[REAL FUNCTIONALITY]
After more investigation, the function name scrub_remape_extent()
doesn't tell anything of the truth, nor does its if () condition.

The real story behind this function is that, for scrub_pages() we never
expect missing device, even for replacing missing device.

What scrub_remap_extent() is really doing is to find a live mirror, and
make later scrub_pages() to read data from the good copy, other than
from the missing device and increase error counters unnecessarily.

[IMPROVEMENT]
We have no need to bother scrub_remap_extent() in scrub_simple_mirror()
at all, we only need to call it before we call scrub_pages().

And rename the function to scrub_find_live_copy(), add extra comments on
them.

By this we can remove one parameter from scrub_extent(), and reduce the
unnecessary calls to scrub_remape_extent() for regular replace.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
fs/btrfs/scrub.c | 62 +++++++++++++++++++++++++++---------------------
1 file changed, 35 insertions(+), 27 deletions(-)

diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index 6f1adcf565bd..35fcb7f68185 100644
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -233,11 +233,11 @@ static int scrub_sectors(struct scrub_ctx *sctx, u64 logical, u32 len,
static void scrub_bio_end_io(struct bio *bio);
static void scrub_bio_end_io_worker(struct btrfs_work *work);
static void scrub_block_complete(struct scrub_block *sblock);
-static void scrub_remap_extent(struct btrfs_fs_info *fs_info,
- u64 extent_logical, u32 extent_len,
- u64 *extent_physical,
- struct btrfs_device **extent_dev,
- int *extent_mirror_num);
+static void scrub_find_good_copy(struct btrfs_fs_info *fs_info,
+ u64 extent_logical, u32 extent_len,
+ u64 *extent_physical,
+ struct btrfs_device **extent_dev,
+ int *extent_mirror_num);
static int scrub_add_sector_to_wr_bio(struct scrub_ctx *sctx,
struct scrub_sector *ssector);
static void scrub_wr_submit(struct scrub_ctx *sctx);
@@ -2213,7 +2213,7 @@ static void scrub_missing_raid56_pages(struct scrub_block *sblock)
* We shouldn't be scrubbing a missing device. Even for dev
* replace, we should only get here for RAID 5/6. We either
* managed to mount something with no mirrors remaining or
- * there's a bug in scrub_remap_extent()/btrfs_map_block().
+ * there's a bug in scrub_find_good_copy()/btrfs_map_block().
*/
goto bioc_out;
}
@@ -2532,8 +2532,11 @@ static int scrub_find_csum(struct scrub_ctx *sctx, u64 logical, u8 *csum)
static int scrub_extent(struct scrub_ctx *sctx, struct map_lookup *map,
u64 logical, u32 len,
u64 physical, struct btrfs_device *dev, u64 flags,
- u64 gen, int mirror_num, u64 physical_for_dev_replace)
+ u64 gen, int mirror_num)
{
+ struct btrfs_device *src_dev = dev;
+ u64 src_physical = physical;
+ int src_mirror = mirror_num;
int ret;
u8 csum[BTRFS_CSUM_SIZE];
u32 blocksize;
@@ -2561,6 +2564,18 @@ static int scrub_extent(struct scrub_ctx *sctx, struct map_lookup *map,
WARN_ON(1);
}

+ /*
+ * For dev-replace case, we can have @dev being a missing device.
+ * Regular scrub will avoid its execution on missing device at all,
+ * as that would trigger tons of read error.
+ *
+ * Reading from missing device will cause read error counts to
+ * increase unnecessarily.
+ * So here we change the read source to a good mirror.
+ */
+ if (sctx->is_dev_replace && !dev->bdev)
+ scrub_find_good_copy(sctx->fs_info, logical, len, &src_physical,
+ &src_dev, &src_mirror);
while (len) {
u32 l = min(len, blocksize);
int have_csum = 0;
@@ -2571,15 +2586,15 @@ static int scrub_extent(struct scrub_ctx *sctx, struct map_lookup *map,
if (have_csum == 0)
++sctx->stat.no_csum;
}
- ret = scrub_sectors(sctx, logical, l, physical, dev, flags, gen,
- mirror_num, have_csum ? csum : NULL,
- physical_for_dev_replace);
+ ret = scrub_sectors(sctx, logical, l, src_physical, src_dev,
+ flags, gen, src_mirror,
+ have_csum ? csum : NULL, physical);
if (ret)
return ret;
len -= l;
logical += l;
physical += l;
- physical_for_dev_replace += l;
+ src_physical += l;
}
return 0;
}
@@ -3269,14 +3284,11 @@ static int scrub_simple_mirror(struct scrub_ctx *sctx,
path.skip_locking = 1;
/* Go through each extent items inside the logical range */
while (cur_logical < logical_end) {
- int cur_mirror = mirror_num;
- struct btrfs_device *target_dev = device;
u64 extent_start;
u64 extent_len;
u64 extent_flags;
u64 extent_gen;
u64 scrub_len;
- u64 cur_physical;

/* Canceled ? */
if (atomic_read(&fs_info->scrub_cancel_req) ||
@@ -3332,11 +3344,7 @@ static int scrub_simple_mirror(struct scrub_ctx *sctx,
scrub_len = min(min(extent_start + extent_len,
logical_end), cur_logical + max_length) -
cur_logical;
- cur_physical = cur_logical - logical_start + physical;

- if (sctx->is_dev_replace)
- scrub_remap_extent(fs_info, cur_logical, scrub_len,
- &cur_physical, &target_dev, &cur_mirror);
if (extent_flags & BTRFS_EXTENT_FLAG_DATA) {
ret = btrfs_lookup_csums_range(csum_root, cur_logical,
cur_logical + scrub_len - 1,
@@ -3356,10 +3364,10 @@ static int scrub_simple_mirror(struct scrub_ctx *sctx,
cur_logical += scrub_len;
continue;
}
- ret = scrub_extent(sctx, map, cur_logical, scrub_len, cur_physical,
- target_dev, extent_flags, extent_gen,
- cur_mirror, cur_logical - logical_start +
- physical);
+ ret = scrub_extent(sctx, map, cur_logical, scrub_len,
+ cur_logical - logical_start + physical,
+ device, extent_flags, extent_gen,
+ mirror_num);
scrub_free_csums(sctx);
if (ret)
break;
@@ -4344,11 +4352,11 @@ int btrfs_scrub_progress(struct btrfs_fs_info *fs_info, u64 devid,
return dev ? (sctx ? 0 : -ENOTCONN) : -ENODEV;
}

-static void scrub_remap_extent(struct btrfs_fs_info *fs_info,
- u64 extent_logical, u32 extent_len,
- u64 *extent_physical,
- struct btrfs_device **extent_dev,
- int *extent_mirror_num)
+static void scrub_find_good_copy(struct btrfs_fs_info *fs_info,
+ u64 extent_logical, u32 extent_len,
+ u64 *extent_physical,
+ struct btrfs_device **extent_dev,
+ int *extent_mirror_num)
{
u64 mapped_length;
struct btrfs_io_context *bioc = NULL;
--
2.35.1