btrfs: scrub: move scrub_remap_extent() call into scrub_extent()

[SUSPICIOUS CODE]
When refactoring scrub code, I noticed 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_remap_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_remap_extent() for regular replace.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index 9767d77..e7b0323 100644 (file)
--- 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 work_struct *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 *sector);
 static void scrub_wr_submit(struct scrub_ctx *sctx);
@@ -2187,7 +2187,7 @@ 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;
        }
@@ -2510,8 +2510,11 @@ 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;
@@ -2539,6 +2542,18 @@ 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;
@@ -2549,15 +2564,15 @@ 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;
 }
@@ -3244,14 +3259,11 @@ 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) ||
@@ -3307,11 +3319,7 @@ 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,
@@ -3331,10 +3339,10 @@ 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;
@@ -4342,11 +4350,11 @@ 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;