[PATCH v3 7/7] btrfs: use larger blocksize for data extent scrub

[Qu Wenruo <wqu@suse.com>]

[PROBLEM]
The existing scrub code for data extents always limit the blocksize to
sectorsize.

This causes quite some extra scrub_block being allocated:
(there is a data extent at logical bytenr 298844160, length 64KiB)

 alloc_scrub_block: new block: logical=298844160 physical=298844160 mirror=1
 alloc_scrub_block: new block: logical=298848256 physical=298848256 mirror=1
 alloc_scrub_block: new block: logical=298852352 physical=298852352 mirror=1
 alloc_scrub_block: new block: logical=298856448 physical=298856448 mirror=1
 alloc_scrub_block: new block: logical=298860544 physical=298860544 mirror=1
 alloc_scrub_block: new block: logical=298864640 physical=298864640 mirror=1
 alloc_scrub_block: new block: logical=298868736 physical=298868736 mirror=1
 alloc_scrub_block: new block: logical=298872832 physical=298872832 mirror=1
 alloc_scrub_block: new block: logical=298876928 physical=298876928 mirror=1
 alloc_scrub_block: new block: logical=298881024 physical=298881024 mirror=1
 alloc_scrub_block: new block: logical=298885120 physical=298885120 mirror=1
 alloc_scrub_block: new block: logical=298889216 physical=298889216 mirror=1
 alloc_scrub_block: new block: logical=298893312 physical=298893312 mirror=1
 alloc_scrub_block: new block: logical=298897408 physical=298897408 mirror=1
 alloc_scrub_block: new block: logical=298901504 physical=298901504 mirror=1
 alloc_scrub_block: new block: logical=298905600 physical=298905600 mirror=1
 ...
 scrub_block_put: free block: logical=298844160 physical=298844160 len=4096 mirror=1
 scrub_block_put: free block: logical=298848256 physical=298848256 len=4096 mirror=1
 scrub_block_put: free block: logical=298852352 physical=298852352 len=4096 mirror=1
 scrub_block_put: free block: logical=298856448 physical=298856448 len=4096 mirror=1
 scrub_block_put: free block: logical=298860544 physical=298860544 len=4096 mirror=1
 scrub_block_put: free block: logical=298864640 physical=298864640 len=4096 mirror=1
 scrub_block_put: free block: logical=298868736 physical=298868736 len=4096 mirror=1
 scrub_block_put: free block: logical=298872832 physical=298872832 len=4096 mirror=1
 scrub_block_put: free block: logical=298876928 physical=298876928 len=4096 mirror=1
 scrub_block_put: free block: logical=298881024 physical=298881024 len=4096 mirror=1
 scrub_block_put: free block: logical=298885120 physical=298885120 len=4096 mirror=1
 scrub_block_put: free block: logical=298889216 physical=298889216 len=4096 mirror=1
 scrub_block_put: free block: logical=298893312 physical=298893312 len=4096 mirror=1
 scrub_block_put: free block: logical=298897408 physical=298897408 len=4096 mirror=1
 scrub_block_put: free block: logical=298901504 physical=298901504 len=4096 mirror=1
 scrub_block_put: free block: logical=298905600 physical=298905600 len=4096 mirror=1

This behavior will waste a lot of memory, especially after we have move
quite some members from scrub_sector to scrub_block.

[FIX]
To reduce the allocation of scrub_block, and reduce memory usage, this
patch will use BTRFS_STRIPE_LEN instead of sectorsize as the blocksize
to scrub data extents.

This results only one scrub_block to be allocated for above data extent:
 alloc_scrub_block: new block: logical=298844160 physical=298844160 mirror=1
 scrub_block_put: free block: logical=298844160 physical=298844160 len=65536 mirror=1

This would greatly reduce the memory usage (even it's just transient)
for larger data extents scrub.

For above example, the memory usage would be:

Old: num_sectors * (sizeof(scrub_block) + sizeof(scrub_sector))
     16          * (408                  + 96) = 8065

New: sizeof(scrub_block) + num_sectors * sizeof(scrub_sector)
     408                 + 16          * 96 = 1944

A good reduction of 75.9%.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 fs/btrfs/scrub.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
index 6c5be7cf00d0..ff251b7fbd47 100644
--- a/fs/btrfs/scrub.c
+++ b/fs/btrfs/scrub.c
@@ -2671,11 +2671,17 @@ static int scrub_extent(struct scrub_ctx *sctx, struct map_lookup *map,
 	u8 csum[BTRFS_CSUM_SIZE];
 	u32 blocksize;
 
+	/*
+	 * Block size determines how many scrub_block will be allocated.
+	 * Here we use BTRFS_STRIPE_LEN (64KiB) as default limit, so we
+	 * won't allocate too many scrub_block, while still won't cause
+	 * too large bios for large extents.
+	 */
 	if (flags & BTRFS_EXTENT_FLAG_DATA) {
 		if (map->type & BTRFS_BLOCK_GROUP_RAID56_MASK)
 			blocksize = map->stripe_len;
 		else
-			blocksize = sctx->fs_info->sectorsize;
+			blocksize = BTRFS_STRIPE_LEN;
 		spin_lock(&sctx->stat_lock);
 		sctx->stat.data_extents_scrubbed++;
 		sctx->stat.data_bytes_scrubbed += len;
-- 
2.37.0