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btrfs: fix error handling when in a COW window in run_delalloc_nocow
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When run_delalloc_nocow has cow_start set to a value other than (u64)-1,
it has delayed COW writeback pending behind cur_offset.  When an error
occurs in such a window, the range going back to cow_start and not just
cur_offset needs to be unlocked, but only two error cases handle this
correctly  Move the code to handle unlock the COW range to the common
error handling label and document the logic.

To make things even more complicated, cow_file_range as called by
fallback_to_cow will unlock the range it is operating on when it fails as
well, so we need to reset cow_start right after caling fallback_to_cow
instead of only when it succeeded.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Sterba <dsterba@suse.com>
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Christoph Hellwig authored and David Sterba committed Aug 21, 2023
1 parent 332581b commit 953fa5c
Showing 1 changed file with 12 additions and 10 deletions.
22 changes: 12 additions & 10 deletions fs/btrfs/inode.c
Original file line number Diff line number Diff line change
@@ -2029,11 +2029,8 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
leaf = path->nodes[0];
if (path->slots[0] >= btrfs_header_nritems(leaf)) {
ret = btrfs_next_leaf(root, path);
if (ret < 0) {
if (cow_start != (u64)-1)
cur_offset = cow_start;
if (ret < 0)
goto error;
}
if (ret > 0)
break;
leaf = path->nodes[0];
@@ -2096,13 +2093,10 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,

nocow_args.start = cur_offset;
ret = can_nocow_file_extent(path, &found_key, inode, &nocow_args);
if (ret < 0) {
if (cow_start != (u64)-1)
cur_offset = cow_start;
if (ret < 0)
goto error;
} else if (ret == 0) {
if (ret == 0)
goto out_check;
}

ret = 0;
bg = btrfs_inc_nocow_writers(fs_info, nocow_args.disk_bytenr);
@@ -2133,9 +2127,9 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
if (cow_start != (u64)-1) {
ret = fallback_to_cow(inode, locked_page,
cow_start, found_key.offset - 1);
cow_start = (u64)-1;
if (ret)
goto error;
cow_start = (u64)-1;
}

nocow_end = cur_offset + nocow_args.num_bytes - 1;
@@ -2214,6 +2208,7 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
if (cow_start != (u64)-1) {
cur_offset = end;
ret = fallback_to_cow(inode, locked_page, cow_start, end);
cow_start = (u64)-1;
if (ret)
goto error;
}
@@ -2222,6 +2217,13 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
if (nocow)
btrfs_dec_nocow_writers(bg);

/*
* If an error happened while a COW region is outstanding, cur_offset
* needs to be reset to cow_start to ensure the COW region is unlocked
* as well.
*/
if (cow_start != (u64)-1)
cur_offset = cow_start;
if (ret && cur_offset < end)
extent_clear_unlock_delalloc(inode, cur_offset, end,
locked_page, EXTENT_LOCKED |

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