From: Boris Burkov <boris@bur.io>
To: linux-btrfs@vger.kernel.org, kernel-team@fb.com
Subject: [PATCH v3] btrfs: fix dio continue after short write due to buffer page fault
Date: Tue, 7 Mar 2023 12:49:30 -0800 [thread overview]
Message-ID: <6733f2fac24b674d9f60dc1093de30513c099629.1678212067.git.boris@bur.io> (raw)
If an application is doing direct io to a btrfs file and experiences a
page fault reading from the write buffer, iomap will issue a partial
bio, and allow the fs to keep going. However, there was a subtle bug in
this code path in the btrfs dio iomap implementation that led to the
partial write ending up as a gap in the file's extents and to be read
back as zeros.
The sequence of events in a partial write, lightly summarized and
trimmed down for brevity is as follows:
====WRITING TASK====
btrfs_direct_write
__iomap_dio_write
iomap_iter
btrfs_dio_iomap_begin # create full ordered extent
iomap_dio_bio_iter
bio_iov_iter_get_pages # page fault; partial read
submit_bio # partial bio
iomap_iter
btrfs_dio_iomap_end
btrfs_mark_ordered_io_finished # sets BTRFS_ORDERED_IOERR;
# submit to finish_ordered_fn wq
fault_in_iov_iter_readable # btrfs_direct_write detects partial write
__iomap_dio_write
iomap_iter
btrfs_dio_iomap_begin # create second partial ordered extent
iomap_dio_bio_iter
bio_iov_iter_get_pages # read all of remainder
submit_bio # partial bio with all of remainder
iomap_iter
btrfs_dio_iomap_end # nothing exciting to do with ordered io
====DIO ENDIO====
==FIRST PARTIAL BIO==
btrfs_dio_end_io
btrfs_mark_ordered_io_finished # bytes_left > 0
# don't submit to finish_ordered_fn wq
==SECOND PARTIAL BIO==
btrfs_dio_end_io
btrfs_mark_ordered_io_finished # bytes_left == 0
# submit to finish_ordered_fn wq
====BTRFS FINISH ORDERED WQ====
==FIRST PARTIAL BIO==
btrfs_finish_ordered_io # called by dio_iomap_end_io, sees
# BTRFS_ORDERED_IOERR, just drops the
# ordered_extent
==SECOND PARTIAL BIO==
btrfs_finish_ordered_io # called by btrfs_dio_end_io, writes out file
# extents, csums, etc...
The essence of the problem is that while btrfs_direct_write and iomap
properly interact to submit all the correct bios, there is insufficient
logic in the btrfs dio functions (btrfs_dio_iomap_begin,
btrfs_dio_submit_io, btrfs_dio_end_io, and btrfs_dio_iomap_end) to
ensure that every bio is at least a part of a completed ordered_extent.
And it is completing an ordered_extent that results in crucial
functionality like inserting a file extent item for the range in the
subvolume/fs tree.
More specifically, btrfs_dio_end_io treats the ordered extent as
unfinished but btrfs_dio_iomap_end sets BTRFS_ORDERED_IOERR on it.
Thus, the finish io work doesn't result in file extents, csums, etc...
In the aftermath, such a file behaves as though it has a hole in it,
instead of the purportedly written data.
We considered a few options for fixing the bug:
1. treat the partial bio as if we had truncated the file, which would
result in properly finishing it.
2. split the ordered extent when submitting a partial bio.
3. cache the ordered extent across calls to __iomap_dio_rw in
iter->private, so that we could reuse it and correctly apply several
bios to it.
I had trouble with 1, and it felt the most like a hack, so I tried 2
and 3. Since 3 has the benefit of also not creating an extra file
extent, and avoids an ordered extent lookup during bio submission, it
felt like the best option.
A quick summary of the changes necessary to implement this cached
ordered_extent behavior:
- btrfs_direct_write keeps track of an ordered_extent for the duration
of a call, possible across several __iomap_dio_rws.
- zero the btrfs_dio_data before using it, since its fields constitute
state now.
- btrfs_dio_write uses dio_data to pass this ordered extent into and out
of __iomap_dio_rw.
- when the write is done, put the ordered_extent.
- if the short write happens to be length 0, then we _don't_ get an
extra bio, so we do need to cancel the ordered_extent like we used
to (and ditch the cached ordered extent)
- if the short write ordered_extent has an error on it, drop the cached
ordered extent, as before.
- in btrfs_dio_iomap_begin, if the cached ordered extent is present,
skip all the work of creating it, just look up the extent mapping and
jump to setting up the iomap. (This part could likely be more
elegant..)
Thanks to Josef, Christoph, and Filipe with their help figuring out the
bug and the fix.
Fixes: 51bd9563b678 ("btrfs: fix deadlock due to page faults during direct IO reads and writes")
Link: https://bugzilla.redhat.com/show_bug.cgi?id=2169947
Link: https://lore.kernel.org/linux-btrfs/aa1fb69e-b613-47aa-a99e-a0a2c9ed273f@app.fastmail.com/
Link: https://pastebin.com/3SDaH8C6
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>
---
Changelog:
v3:
- handle BTRFS_IOERR set on the ordered_extent in btrfs_dio_iomap_end.
If the bio fails before we loop in the submission loop and exit from
the loop early, we never submit a second bio covering the rest of the
extent range, resulting in leaking the ordered_extent, which hangs umount.
We can distinguish this from a short write in btrfs_dio_iomap_end by
checking the ordered_extent.
v2:
- rename new ordered extent function
- pull the new function into a prep patch
- reorganize how the ordered_extent is stored/passed around to avoid so
many annoying memsets and exposing it to fs/btrfs/file.c
- lots of small code style improvements
- remove unintentional whitespace changes
- commit message improvements
- various ASSERTs for clarity/debugging
fs/btrfs/btrfs_inode.h | 1 +
fs/btrfs/file.c | 11 +++++-
fs/btrfs/inode.c | 76 +++++++++++++++++++++++++++++++-----------
3 files changed, 68 insertions(+), 20 deletions(-)
diff --git a/fs/btrfs/btrfs_inode.h b/fs/btrfs/btrfs_inode.h
index 49a92aa65de1..87020aa58121 100644
--- a/fs/btrfs/btrfs_inode.h
+++ b/fs/btrfs/btrfs_inode.h
@@ -516,6 +516,7 @@ ssize_t btrfs_do_encoded_write(struct kiocb *iocb, struct iov_iter *from,
ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter,
size_t done_before);
struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter,
+ struct btrfs_ordered_extent **ordered_extent,
size_t done_before);
extern const struct dentry_operations btrfs_dentry_operations;
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 5cc5a1faaef5..ec5c5355906b 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1465,6 +1465,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
ssize_t err;
unsigned int ilock_flags = 0;
struct iomap_dio *dio;
+ struct btrfs_ordered_extent *ordered_extent = NULL;
if (iocb->ki_flags & IOCB_NOWAIT)
ilock_flags |= BTRFS_ILOCK_TRY;
@@ -1526,7 +1527,7 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
* got -EFAULT, faulting in the pages before the retry.
*/
from->nofault = true;
- dio = btrfs_dio_write(iocb, from, written);
+ dio = btrfs_dio_write(iocb, from, &ordered_extent, written);
from->nofault = false;
/*
@@ -1569,6 +1570,14 @@ static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
goto relock;
}
}
+ /*
+ * We can't loop back to btrfs_dio_write, so we can drop the cached
+ * ordered extent. Typically btrfs_dio_iomap_end will run and put the
+ * ordered_extent, but this is needed to clean up in case of an error
+ * path breaking out of iomap_iter before the final iomap_end call.
+ */
+ if (ordered_extent)
+ btrfs_put_ordered_extent(ordered_extent);
/*
* If 'err' is -ENOTBLK or we have not written all data, then it means
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 8f07d59e8193..94f31697a278 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -81,6 +81,7 @@ struct btrfs_dio_data {
struct extent_changeset *data_reserved;
bool data_space_reserved;
bool nocow_done;
+ struct btrfs_ordered_extent *ordered;
};
struct btrfs_dio_private {
@@ -6963,6 +6964,7 @@ struct extent_map *btrfs_get_extent(struct btrfs_inode *inode,
}
static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode,
+ struct btrfs_dio_data *dio_data,
const u64 start,
const u64 len,
const u64 orig_start,
@@ -6973,7 +6975,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode,
const int type)
{
struct extent_map *em = NULL;
- int ret;
+ struct btrfs_ordered_extent *ordered;
if (type != BTRFS_ORDERED_NOCOW) {
em = create_io_em(inode, start, len, orig_start, block_start,
@@ -6983,18 +6985,21 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode,
if (IS_ERR(em))
goto out;
}
- ret = btrfs_add_ordered_extent(inode, start, len, len, block_start,
- block_len, 0,
- (1 << type) |
- (1 << BTRFS_ORDERED_DIRECT),
- BTRFS_COMPRESS_NONE);
- if (ret) {
+ ordered = btrfs_alloc_ordered_extent(inode, start, len, len,
+ block_start, block_len, 0,
+ (1 << type) |
+ (1 << BTRFS_ORDERED_DIRECT),
+ BTRFS_COMPRESS_NONE);
+ if (IS_ERR(ordered)) {
if (em) {
free_extent_map(em);
btrfs_drop_extent_map_range(inode, start,
start + len - 1, false);
}
- em = ERR_PTR(ret);
+ em = ERR_PTR(PTR_ERR(ordered));
+ } else {
+ ASSERT(!dio_data->ordered);
+ dio_data->ordered = ordered;
}
out:
@@ -7002,6 +7007,7 @@ static struct extent_map *btrfs_create_dio_extent(struct btrfs_inode *inode,
}
static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode,
+ struct btrfs_dio_data *dio_data,
u64 start, u64 len)
{
struct btrfs_root *root = inode->root;
@@ -7017,7 +7023,8 @@ static struct extent_map *btrfs_new_extent_direct(struct btrfs_inode *inode,
if (ret)
return ERR_PTR(ret);
- em = btrfs_create_dio_extent(inode, start, ins.offset, start,
+ em = btrfs_create_dio_extent(inode, dio_data,
+ start, ins.offset, start,
ins.objectid, ins.offset, ins.offset,
ins.offset, BTRFS_ORDERED_REGULAR);
btrfs_dec_block_group_reservations(fs_info, ins.objectid);
@@ -7362,7 +7369,7 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map,
}
space_reserved = true;
- em2 = btrfs_create_dio_extent(BTRFS_I(inode), start, len,
+ em2 = btrfs_create_dio_extent(BTRFS_I(inode), dio_data, start, len,
orig_start, block_start,
len, orig_block_len,
ram_bytes, type);
@@ -7404,7 +7411,7 @@ static int btrfs_get_blocks_direct_write(struct extent_map **map,
goto out;
space_reserved = true;
- em = btrfs_new_extent_direct(BTRFS_I(inode), start, len);
+ em = btrfs_new_extent_direct(BTRFS_I(inode), dio_data, start, len);
if (IS_ERR(em)) {
ret = PTR_ERR(em);
goto out;
@@ -7508,6 +7515,17 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start,
}
}
+ if (dio_data->ordered) {
+ ASSERT(write);
+ em = btrfs_get_extent(BTRFS_I(inode), NULL, 0,
+ dio_data->ordered->file_offset,
+ dio_data->ordered->bytes_left);
+ if (IS_ERR(em)) {
+ ret = PTR_ERR(em);
+ goto err;
+ }
+ goto map_iomap;
+ }
memset(dio_data, 0, sizeof(*dio_data));
/*
@@ -7649,6 +7667,7 @@ static int btrfs_dio_iomap_begin(struct inode *inode, loff_t start,
else
free_extent_state(cached_state);
+map_iomap:
/*
* Translate extent map information to iomap.
* We trim the extents (and move the addr) even though iomap code does
@@ -7702,13 +7721,27 @@ static int btrfs_dio_iomap_end(struct inode *inode, loff_t pos, loff_t length,
if (submitted < length) {
pos += submitted;
length -= submitted;
- if (write)
- btrfs_mark_ordered_io_finished(BTRFS_I(inode), NULL,
- pos, length, false);
- else
+ if (write) {
+ struct btrfs_ordered_extent *ordered = dio_data->ordered;
+ if (submitted == 0 ||
+ test_bit(BTRFS_ORDERED_IOERR, &ordered->flags)) {
+ btrfs_mark_ordered_io_finished(BTRFS_I(inode),
+ NULL, pos,
+ length, false);
+ btrfs_put_ordered_extent(ordered);
+ dio_data->ordered = NULL;
+ }
+ } else {
unlock_extent(&BTRFS_I(inode)->io_tree, pos,
pos + length - 1, NULL);
+ }
ret = -ENOTBLK;
+ } else {
+ /* On the last bio, release our cached ordered_extent. */
+ if (write) {
+ btrfs_put_ordered_extent(dio_data->ordered);
+ dio_data->ordered = NULL;
+ }
}
if (write)
@@ -7771,19 +7804,24 @@ static const struct iomap_dio_ops btrfs_dio_ops = {
ssize_t btrfs_dio_read(struct kiocb *iocb, struct iov_iter *iter, size_t done_before)
{
- struct btrfs_dio_data data;
+ struct btrfs_dio_data data = { 0 };
return iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops,
IOMAP_DIO_PARTIAL, &data, done_before);
}
struct iomap_dio *btrfs_dio_write(struct kiocb *iocb, struct iov_iter *iter,
+ struct btrfs_ordered_extent **ordered_extent,
size_t done_before)
{
- struct btrfs_dio_data data;
+ struct btrfs_dio_data dio_data = { .ordered = *ordered_extent };
+ struct iomap_dio *dio;
- return __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops,
- IOMAP_DIO_PARTIAL, &data, done_before);
+ dio = __iomap_dio_rw(iocb, iter, &btrfs_dio_iomap_ops, &btrfs_dio_ops,
+ IOMAP_DIO_PARTIAL, &dio_data, done_before);
+ if (!IS_ERR_OR_NULL(dio))
+ *ordered_extent = dio_data.ordered;
+ return dio;
}
static int btrfs_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,
--
2.38.1
next reply other threads:[~2023-03-07 20:49 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-07 20:49 Boris Burkov [this message]
2023-03-08 16:00 ` [PATCH v3] btrfs: fix dio continue after short write due to buffer page fault Sweet Tea Dorminy
2023-03-09 17:40 ` David Sterba
2023-03-15 19:53 ` Boris Burkov
2023-03-15 19:52 ` David Sterba
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