[BUG]
When running the following fsx command (extracted from generic/127) on
subpage btrfs, it can create inline extent with regular extents:
fsx -q -l 262144 -o 65536 -S 191110531 -N 9057 -R -W $mnt/file >
/tmp/fsx
The offending extent would look like:
item 9 key (257 INODE_REF 256) itemoff 15703 itemsize 14
index 2 namelen 4 name: file
item 10 key (257 EXTENT_DATA 0) itemoff 14975 itemsize 728
generation 7 type 0 (inline)
inline extent data size 707 ram_bytes 707 compression 0 (none)
item 11 key (257 EXTENT_DATA 4096) itemoff 14922 itemsize 53
generation 7 type 2 (prealloc)
prealloc data disk byte 102346752 nr 4096
prealloc data offset 0 nr 4096
[CAUSE]
For subpage btrfs, the writeback is triggered in page unit, which means,
even if we just want to writeback range [16K, 20K) for 64K page system,
we will still try to writeback any dirty sector of range [0, 64K).
This is never a problem if sectorsize == PAGE_SIZE, but for subpage,
this can cause unexpected problems.
For above test case, the last several operations from fsx are:
9055 trunc from 0x40000 to 0x2c3
9057 falloc from 0x164c to 0x19d2 (0x386 bytes)
In operation 9055, we dirtied sector [0, 4096), then in falloc, we call
btrfs_wait_ordered_range(inode, start=4096, len=4096), only expecting to
writeback any dirty data in [4096, 8192), but nothing else.
Unfortunately, in subpage case, above btrfs_wait_ordered_range() will
trigger writeback of the range [0, 64K), which includes the data at [0,
4096).
And since at the call site, we haven't yet increased i_size, which is
still 707, this means cow_file_range() can insert an inline extent.
Resulting above inline + regular extent.
[WORKAROUND]
I don't really have any good short-term solution yet, as this means all
operations that would trigger writeback need to be reviewed for any
isize change.
So here I choose to disable inline extent creation for subpage case as a
workaround.
We have done tons of work just to avoid such extent, so I don't to
create an exception just for subpage.
This only affects inline extent creation, btrfs subpage support has no
problem reading existing inline extents at all.
Signed-off-by: Qu Wenruo <w...@suse.com>
---
fs/btrfs/inode.c | 18 ++++++++++++++++--
1 file changed, 16 insertions(+), 2 deletions(-)
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index e31a0521564e..5030bbf3a667 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -663,7 +663,11 @@ static noinline int compress_file_range(struct async_chunk
*async_chunk)
}
}
cont:
- if (start == 0) {
+ /*
+ * Check cow_file_range() for why we don't even try to create
+ * inline extent for subpage case.
+ */
+ if (start == 0 && fs_info->sectorsize == PAGE_SIZE) {
/* lets try to make an inline extent */
if (ret || total_in < actual_end) {
/* we didn't compress the entire range, try
@@ -1061,7 +1065,17 @@ static noinline int cow_file_range(struct btrfs_inode
*inode,
inode_should_defrag(inode, start, end, num_bytes, SZ_64K);
- if (start == 0) {
+ /*
+ * Due to the page size limit, for subpage we can only trigger the
+ * writeback for the dirty sectors of page, that means data writeback
+ * is doing more writeback than what we want.
+ *
+ * This is especially unexpected for some call sites like fallocate,
+ * where we only increase isize after everything is done.
+ * This means we can trigger inline extent even we didn't want.
+ * So here we skip inline extent creation completely.
+ */
+ if (start == 0 && fs_info->sectorsize == PAGE_SIZE) {
/* lets try to make an inline extent */
ret = cow_file_range_inline(inode, start, end, 0,
BTRFS_COMPRESS_NONE, NULL);
--
2.31.1
