在 2025/1/9 13:15, Qu Wenruo 写道:
在 2025/1/9 08:22, Boris Burkov 写道:
On Thu, Dec 12, 2024 at 04:43:55PM +1030, Qu Wenruo wrote:
[BUG] When running btrfs with block size (4K) smaller than page size (64K, aarch64), there is a very high chance to crash the kernel at generic/750, with the following messages: (before the call traces, there are 3 extra debug messages added)
BTRFS warning (device dm-3): read-write for sector size 4096 with page size 65536 is experimental BTRFS info (device dm-3): checking UUID tree hrtimer: interrupt took 5451385 ns BTRFS error (device dm-3): cow_file_range failed, root=4957 inode=257 start=1605632 len=69632: -28 BTRFS error (device dm-3): run_delalloc_nocow failed, root=4957 inode=257 start=1605632 len=69632: -28 BTRFS error (device dm-3): failed to run delalloc range, root=4957 ino=257 folio=1572864 submit_bitmap=8-15 start=1605632 len=69632: -28 ------------[ cut here ]------------ WARNING: CPU: 2 PID: 3020984 at ordered-data.c:360 can_finish_ordered_extent+0x370/0x3b8 [btrfs] CPU: 2 UID: 0 PID: 3020984 Comm: kworker/u24:1 Tainted: G OE 6.13.0-rc1-custom+ #89 Tainted: [O]=OOT_MODULE, [E]=UNSIGNED_MODULE Hardware name: QEMU KVM Virtual Machine, BIOS unknown 2/2/2022 Workqueue: events_unbound btrfs_async_reclaim_data_space [btrfs] pc : can_finish_ordered_extent+0x370/0x3b8 [btrfs] lr : can_finish_ordered_extent+0x1ec/0x3b8 [btrfs] Call trace: can_finish_ordered_extent+0x370/0x3b8 [btrfs] (P) can_finish_ordered_extent+0x1ec/0x3b8 [btrfs] (L) btrfs_mark_ordered_io_finished+0x130/0x2b8 [btrfs] extent_writepage+0x10c/0x3b8 [btrfs] extent_write_cache_pages+0x21c/0x4e8 [btrfs] btrfs_writepages+0x94/0x160 [btrfs] do_writepages+0x74/0x190 filemap_fdatawrite_wbc+0x74/0xa0 start_delalloc_inodes+0x17c/0x3b0 [btrfs] btrfs_start_delalloc_roots+0x17c/0x288 [btrfs] shrink_delalloc+0x11c/0x280 [btrfs] flush_space+0x288/0x328 [btrfs] btrfs_async_reclaim_data_space+0x180/0x228 [btrfs] process_one_work+0x228/0x680 worker_thread+0x1bc/0x360 kthread+0x100/0x118 ret_from_fork+0x10/0x20 ---[ end trace 0000000000000000 ]--- BTRFS critical (device dm-3): bad ordered extent accounting, root=4957 ino=257 OE offset=1605632 OE len=16384 to_dec=16384 left=0 BTRFS critical (device dm-3): bad ordered extent accounting, root=4957 ino=257 OE offset=1622016 OE len=12288 to_dec=12288 left=0 Unable to handle kernel NULL pointer dereference at virtual address 0000000000000008 BTRFS critical (device dm-3): bad ordered extent accounting, root=4957 ino=257 OE offset=1634304 OE len=8192 to_dec=4096 left=0 CPU: 1 UID: 0 PID: 3286940 Comm: kworker/u24:3 Tainted: G W OE 6.13.0-rc1-custom+ #89 Hardware name: QEMU KVM Virtual Machine, BIOS unknown 2/2/2022 Workqueue: btrfs_work_helper [btrfs] (btrfs-endio-write) pstate: 404000c5 (nZcv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--) pc : process_one_work+0x110/0x680 lr : worker_thread+0x1bc/0x360 Call trace: process_one_work+0x110/0x680 (P) worker_thread+0x1bc/0x360 (L) worker_thread+0x1bc/0x360 kthread+0x100/0x118 ret_from_fork+0x10/0x20 Code: f84086a1 f9000fe1 53041c21 b9003361 (f9400661) ---[ end trace 0000000000000000 ]--- Kernel panic - not syncing: Oops: Fatal exception SMP: stopping secondary CPUs SMP: failed to stop secondary CPUs 2-3 Dumping ftrace buffer: (ftrace buffer empty) Kernel Offset: 0x275bb9540000 from 0xffff800080000000 PHYS_OFFSET: 0xffff8fbba0000000 CPU features: 0x100,00000070,00801250,8201720b
[CAUSE] The above warning is triggered immediately after the delalloc range failure, this happens in the following sequence:
- Range [1568K, 1636K) is dirty
1536K 1568K 1600K 1636K 1664K | |/////////|////////| |
Where 1536K, 1600K and 1664K are page boundaries (64K page size)
Enter extent_writepage() for page 1536K
Enter run_delalloc_nocow() with locked page 1536K and range
[1568K, 1636K) This is due to the inode has preallocated extents.
- Enter cow_file_range() with locked page 1536K and range
[1568K, 1636K)
- btrfs_reserve_extent() only reserved two extents
The main loop of cow_file_range() only reserved two data extents,
Now we have:
1536K 1568K 1600K 1636K 1664K | |<-->|<--->|/|///////| | 1584K 1596K Range [1568K, 1596K) has ordered extent reserved.
- btrfs_reserve_extent() failed inside cow_file_range() for file offset
1596K This is already a bug in our space reservation code, but for now let's focus on the error handling path.
Now cow_file_range() returned -ENOSPC.
- btrfs_run_delalloc_range() do error cleanup <<< ROOT CAUSE
Call btrfs_cleanup_ordered_extents() with locked folio 1536K and range [1568K, 1636K)
Function btrfs_cleanup_ordered_extents() normally needs to skip the ranges inside the folio, as it will normally be cleaned up by extent_writepage().
Such split error handling is already problematic in the first place.
What's worse is the folio range skipping itself, which is not taking subpage cases into consideration at all, it will only skip the range if the page start >= the range start. In our case, the page start < the range start, since for subpage cases we can have delalloc ranges inside the folio but not covering the folio.
So it doesn't skip the page range at all. This means all the ordered extents, both [1568K, 1584K) and [1584K, 1596K) will be marked as IOERR.
And those two ordered extents have no more pending ios, it is marked finished, and *QUEUED* to be deleted from the io tree.
- extent_writepage() do error cleanup
Call btrfs_mark_ordered_io_finished() for the range [1536K, 1600K).
Although ranges [1568K, 1584K) and [1584K, 1596K) are finished, the deletion from io tree is async, it may or may not happen at this timing.
If the ranges are not yet removed, we will do double cleaning on those ranges, triggers the above ordered extent warnings.
In theory there are other bugs, like the cleanup in extent_writepage() can cause double accounting on ranges that are submitted async (compression for example).
But that's much harder to trigger because normally we do not mix regular and compression delalloc ranges.
[FIX] The folio range split is already buggy and not subpage compatible, it's introduced a long time ago where subpage support is not even considered.
So instead of splitting the ordered extents cleanup into the folio range and out of folio range, do all the cleanup inside writepage_delalloc().
- Pass @NULL as locked_folio for btrfs_cleanup_ordered_extents() in
btrfs_run_delalloc_range()
- Skip the btrfs_cleanup_ordered_extents() if writepage_delalloc()
failed
So all ordered extents are only cleaned up by btrfs_run_delalloc_range().
- Handle the ranges that already have ordered extents allocated
If part of the folio already has ordered extent allocated, and btrfs_run_delalloc_range() failed, we also need to cleanup that range.
Now we have a concentrated error handling for ordered extents during btrfs_run_delalloc_range().
Great investigation and writeup, thanks!
Thanks a lot of the review!
The explanation and fix both make sense to me. I traced the change in error handling and I see how we are avoiding double ending the ordered_extent. So with that said, feel free to add: Reviewed-by: Boris Burkov boris@bur.io
However, I would like to request one thing, if I may. While this is all still relatively fresh in your mind, could you please document the intended behavior of the various functions (at least the ones you modify/reason about) with regards to:
- cleanup state of the various objects involved like ordered_extents
and subpages (e.g., writepage_delalloc cleans up ordered extents, so callers should not, etc.)
The subpage one should not be considered as something special, it's really just some kind of enhanced page flags for subpage cases.
Thus I'll not explicitly mention the subpage bitmap, but directly mention the involved flags (in this particular case, folio Ordered and Locked flags).
- return values (e.g., when precisely does btrfs_run_delalloc_range
return >= 0 ?)
My bad, I should update the comment in commit d034cdb4cc8a ("btrfs: lock subpage ranges in one go for writepage_delalloc()").
Still better fix it here before too late.
- anything else you think would be helpful for reasoning about these
functions in an abstract way while you are at it.
That request is obviously optional for landing these fixes, but I really think it would help if we went through the bother every time we deciphered one of these undocumented paths. A restatement of your best understanding of the behavior now will really pay off for the next person reading this code :)
And since we're here, just a quick note for the patch 4/5, that although those two patches are fixing error handling, they are still mostly backport oriented fixes (although I'm 100% sure it will cause conflicts).
They still are doing cross-layer error handling. E.g. the ordered extents cleanup are not inside cow_file_range(), but done by btrfs_run_delalloc_range().
I'm going to properly fix all those cross-layer error handling in the next series soon.
Thanks, Qu
Thanks, Boris
Cc: stable@vger.kernel.org # 5.15+ Fixes: d1051d6ebf8e ("btrfs: Fix error handling in btrfs_cleanup_ordered_extents") Signed-off-by: Qu Wenruo wqu@suse.com
fs/btrfs/extent_io.c | 37 ++++++++++++++++++++++++++++++++----- fs/btrfs/inode.c | 2 +- 2 files changed, 33 insertions(+), 6 deletions(-)
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c index 9725ff7f274d..417c710c55ca 100644 --- a/fs/btrfs/extent_io.c +++ b/fs/btrfs/extent_io.c @@ -1167,6 +1167,12 @@ static noinline_for_stack int writepage_delalloc(struct btrfs_inode *inode, * last delalloc end. */ u64 last_delalloc_end = 0; + /* + * Save the last successfully ran delalloc range end (exclusive). + * This is for error handling to avoid ranges with ordered extent created + * but no IO will be submitted due to error. + */
nit: last_finished what? I feel this name or comment could use some extra work.
I can enhance it to @last_finished_delalloc_end and update the comment.
Thanks, Qu
+ u64 last_finished = page_start; u64 delalloc_start = page_start; u64 delalloc_end = page_end; u64 delalloc_to_write = 0; @@ -1235,11 +1241,19 @@ static noinline_for_stack int writepage_delalloc(struct btrfs_inode *inode, found_len = last_delalloc_end + 1 - found_start;
if (ret >= 0) { + /* + * Some delalloc range may be created by previous folios. + * Thus we still need to clean those range up during error + * handling. + */ + last_finished = found_start; /* No errors hit so far, run the current delalloc range. */ ret = btrfs_run_delalloc_range(inode, folio, found_start, found_start + found_len - 1, wbc); + if (ret >= 0) + last_finished = found_start + found_len; } else { /* * We've hit an error during previous delalloc range, @@ -1274,8 +1288,21 @@ static noinline_for_stack int writepage_delalloc(struct btrfs_inode *inode,
delalloc_start = found_start + found_len; } - if (ret < 0) + /* + * It's possible we have some ordered extents created before we hit + * an error, cleanup non-async successfully created delalloc ranges. + */ + if (unlikely(ret < 0)) { + unsigned int bitmap_size = min( + (last_finished - page_start) >> fs_info->sectorsize_bits, + fs_info->sectors_per_page);
+ for_each_set_bit(bit, &bio_ctrl->submit_bitmap, bitmap_size) + btrfs_mark_ordered_io_finished(inode, folio, + page_start + (bit << fs_info->sectorsize_bits), + fs_info->sectorsize, false); return ret; + } out: if (last_delalloc_end) delalloc_end = last_delalloc_end; @@ -1509,13 +1536,13 @@ static int extent_writepage(struct folio *folio, struct btrfs_bio_ctrl *bio_ctrl
bio_ctrl->wbc->nr_to_write--;
-done: - if (ret) { + if (ret) btrfs_mark_ordered_io_finished(BTRFS_I(inode), folio, page_start, PAGE_SIZE, !ret); - mapping_set_error(folio->mapping, ret); - }
+done: + if (ret < 0) + mapping_set_error(folio->mapping, ret); /* * Only unlock ranges that are submitted. As there can be some async * submitted ranges inside the folio. diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index c4997200dbb2..d41bb47d59fb 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -2305,7 +2305,7 @@ int btrfs_run_delalloc_range(struct btrfs_inode *inode, struct folio *locked_fol
out: if (ret < 0) - btrfs_cleanup_ordered_extents(inode, locked_folio, start, + btrfs_cleanup_ordered_extents(inode, NULL, start, end - start + 1); return ret; } -- 2.47.1