Btrfs: clear the extent uptodate bits during parent transid failures
PATCH 1/1 Btrfs: clear the extent uptodate bits during parent transid failures
Normally I just toss patches into git, but this one is pretty subtle and I wanted to send it around for extra review. QA at Oracle did a test where they unplugged one drive of a btrfs raid1 mirror for a while and then plugged it back in.
The end result is that we have a whole bunch of out-of-date blocks on the bad mirror. The btrfs parent transid pointers are supposed to detect these bad blocks and then we're supposed to read from the good copy instead.
The good news is we did detect the bad blocks. The bad news is we didn't jump over to the good mirror instead. This patch explains why:
Author: Chris Mason <[email protected]> Date: Wed Feb 22 12:36:24 2012 -0500
Btrfs: clear the extent uptodate bits during parent transid failures
If btrfs reads a block and finds a parent transid mismatch, it clears the uptodate flags on the extent buffer, and the pages inside it. But we only clear the uptodate bits in the state tree if the block straddles more than one page.
This is from an old optimization from to reduce contention on the extent state tree. But it is buggy because the code that retries a read from a different copy of the block is going to find the uptodate state bits set and skip the IO.
The end result of the bug is that we'll never actually read the good copy (if there is one).
The fix here is to always clear the uptodate state bits, which is safe because this code is only called when the parent transid fails.
fs/btrfs/extent_io.c +3 -4
@@ -3852,10 +3852,9 @@ int clear_extent_buffer_uptodate(struct extent_io_tree *tree, num_pages =num_extent_pages (eb->start, eb->len);clear_bit (EXTENT_BUFFER_UPTODATE , &eb->bflags); -if (eb_straddles_pages (eb)) { -clear_extent_uptodate (tree, eb->start, eb->start + eb->len -1 , - cached_state,GFP_NOFS ); - } +clear_extent_uptodate (tree, eb->start, eb->start + eb->len -1 , + cached_state,GFP_NOFS ); +for (i =0 ; i < num_pages; i++) { page =extent_buffer_page (eb, i);if (page)
Quoted 28 lines · Chris Mason
Normally I just toss patches into git, but this one is pretty subtle and I wanted to send it around for extra review. QA at Oracle did a test where they unplugged one drive of a btrfs raid1 mirror for a while and then plugged it back in.
The end result is that we have a whole bunch of out-of-date blocks on the bad mirror. The btrfs parent transid pointers are supposed to detect these bad blocks and then we're supposed to read from the good copy instead.
The good news is we did detect the bad blocks. The bad news is we didn't jump over to the good mirror instead. This patch explains why:
Author: Chris Mason <[email protected]> Date: Wed Feb 22 12:36:24 2012 -0500Btrfs: clear the extent uptodate bits during parent transid failures
If btrfs reads a block and finds a parent transid mismatch, it clears the uptodate flags on the extent buffer, and the pages inside it. But we only clear the uptodate bits in the state tree if the block straddles more than one page.
This is from an old optimization from to reduce contention on the extent state tree. But it is buggy because the code that retries a read from a different copy of the block is going to find the uptodate state bits set and skip the IO.
Quoted 6 lines
The end result of the bug is that we'll never actually read the good copy (if there is one).
The fix here is to always clear the uptodate state bits, which is safe because this code is only called when the parent transid fails.
or we can be safer:
fs/btrfs/extent_io.c +5 -1
@@ -3859,8 +3859,12 @@ int clear_extent_buffer_uptodate(struct extent_io_tree *tree, }for (i =0 ; i < num_pages; i++) { page =extent_buffer_page (eb, i); -if (page) +if (page) { +u64 start = (u64 )page->index <<PAGE_CACHE_SHIFT ; +u64 end = start +PAGE_CACHE_SIZE -1 ; +ClearPageUptodate (page); +clear_extent_uptodate (tree, start, end,NULL ,GFP_NOFS ); }return 0 ; }
Quoted 25 lines
Signed-off-by Chris Mason <[email protected]>fs/btrfs/extent_io.c +3
-5@@ -3852,10 +3852,9 @@ int clear_extent_buffer_uptodate(struct extent_io_tree *tree, num_pages =num_extent_pages (eb->start, eb->len);clear_bit (EXTENT_BUFFER_UPTODATE , &eb->bflags); -if (eb_straddles_pages (eb)) { -clear_extent_uptodate (tree, eb->start, eb->start + eb->len -1 , - cached_state,GFP_NOFS ); - } +clear_extent_uptodate (tree, eb->start, eb->start + eb->len -1 , + cached_state,GFP_NOFS ); +for (i =0 ; i < num_pages; i++) { page =extent_buffer_page (eb, i);if (page) --To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to [email protected]
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Quoted 29 lines · Liu Bo
Quoted 28 lines · Chris Mason
Normally I just toss patches into git, but this one is pretty subtle and I wanted to send it around for extra review. QA at Oracle did a test where they unplugged one drive of a btrfs raid1 mirror for a while and then plugged it back in.
The end result is that we have a whole bunch of out-of-date blocks on the bad mirror. The btrfs parent transid pointers are supposed to detect these bad blocks and then we're supposed to read from the good copy instead.
The good news is we did detect the bad blocks. The bad news is we didn't jump over to the good mirror instead. This patch explains why:
Author: Chris Mason <[email protected]> Date: Wed Feb 22 12:36:24 2012 -0500Btrfs: clear the extent uptodate bits during parent transid failures
If btrfs reads a block and finds a parent transid mismatch, it clears the uptodate flags on the extent buffer, and the pages inside it. But we only clear the uptodate bits in the state tree if the block straddles more than one page.
This is from an old optimization from to reduce contention on the extent state tree. But it is buggy because the code that retries a read from a different copy of the block is going to find the uptodate state bits set and skip the IO.
Quoted 8 lines
Quoted 6 lines
The end result of the bug is that we'll never actually read the good copy (if there is one).
The fix here is to always clear the uptodate state bits, which is safe because this code is only called when the parent transid fails.
Reviewed-by Liu Bo <[email protected]>
Thanks!
Quoted 11 lines
or we can be safer:
fs/btrfs/extent_io.c +0
-0@@ -3859,8 +3859,12 @@ int clear_extent_buffer_uptodate(struct extent_io_tree *tree, }for (i =0 ; i < num_pages; i++) { page =extent_buffer_page (eb, i);
fs/btrfs/extent_io.c int clear_extent_buffer_uptodate(struct extent_io_tree *tree, · line 3870
Quoted 10 lines
fs/btrfs/extent_io.c +5
-1-if (page) +if (page) { +u64 start = (u64 )page->index <<PAGE_CACHE_SHIFT ; +u64 end = start +PAGE_CACHE_SIZE -1 ; +ClearPageUptodate (page); +clear_extent_uptodate (tree, start, end,NULL ,GFP_NOFS ); }return 0 ; }
Hmmm, I'm not sure this is safer. Our readpage trusts the extent uptodate bits unconditionally, so we should really clear them unconditionally as well.
-chris