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for-5.16-deadlock-fix-tag

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Merge tag 'for-5.16-deadlock-fix-tag' of git://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux

Pull btrfs fix from David Sterba:
 "Fix for a deadlock when direct/buffered IO is done on a mmaped file
  and a fault happens (details in the patch). There's a fstest
  generic/647 that triggers the problem and makes testing hard"

* tag 'for-5.16-deadlock-fix-tag' of git://git.kernel.org/pub/scm/linux/kernel/git/kdave/linux:
  btrfs: fix deadlock due to page faults during direct IO reads and writes
This commit is contained in:
Linus Torvalds 2021-11-10 16:50:57 -08:00
commit 6070dcc8e5
1 changed files with 123 additions and 16 deletions
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139
fs/btrfs/file.c
View File

@ -1912,16 +1912,17 @@ static ssize_t check_direct_IO(struct btrfs_fs_info *fs_info,
static ssize_t btrfs_direct_write(struct kiocb *iocb, struct iov_iter *from)
{
const bool is_sync_write = (iocb->ki_flags & IOCB_DSYNC);
struct file *file = iocb->ki_filp;
struct inode *inode = file_inode(file);
struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb);
loff_t pos;
ssize_t written = 0;
ssize_t written_buffered;
size_t prev_left = 0;
loff_t endbyte;
ssize_t err;
unsigned int ilock_flags = 0;
struct iomap_dio *dio = NULL;
if (iocb->ki_flags & IOCB_NOWAIT)
ilock_flags |= BTRFS_ILOCK_TRY;
@ -1964,23 +1965,80 @@ relock:
goto buffered;
}
dio = __iomap_dio_rw(iocb, from, &btrfs_dio_iomap_ops, &btrfs_dio_ops,
0, 0);
/*
* We remove IOCB_DSYNC so that we don't deadlock when iomap_dio_rw()
* calls generic_write_sync() (through iomap_dio_complete()), because
* that results in calling fsync (btrfs_sync_file()) which will try to
* lock the inode in exclusive/write mode.
*/
if (is_sync_write)
iocb->ki_flags &= ~IOCB_DSYNC;
/*
* The iov_iter can be mapped to the same file range we are writing to.
* If that's the case, then we will deadlock in the iomap code, because
* it first calls our callback btrfs_dio_iomap_begin(), which will create
* an ordered extent, and after that it will fault in the pages that the
* iov_iter refers to. During the fault in we end up in the readahead
* pages code (starting at btrfs_readahead()), which will lock the range,
* find that ordered extent and then wait for it to complete (at
* btrfs_lock_and_flush_ordered_range()), resulting in a deadlock since
* obviously the ordered extent can never complete as we didn't submit
* yet the respective bio(s). This always happens when the buffer is
* memory mapped to the same file range, since the iomap DIO code always
* invalidates pages in the target file range (after starting and waiting
* for any writeback).
*
* So here we disable page faults in the iov_iter and then retry if we
* got -EFAULT, faulting in the pages before the retry.
*/
again:
from->nofault = true;
err = iomap_dio_rw(iocb, from, &btrfs_dio_iomap_ops, &btrfs_dio_ops,
IOMAP_DIO_PARTIAL, written);
from->nofault = false;
/* No increment (+=) because iomap returns a cumulative value. */
if (err > 0)
written = err;
if (iov_iter_count(from) > 0 && (err == -EFAULT || err > 0)) {
const size_t left = iov_iter_count(from);
/*
* We have more data left to write. Try to fault in as many as
* possible of the remainder pages and retry. We do this without
* releasing and locking again the inode, to prevent races with
* truncate.
*
* Also, in case the iov refers to pages in the file range of the
* file we want to write to (due to a mmap), we could enter an
* infinite loop if we retry after faulting the pages in, since
* iomap will invalidate any pages in the range early on, before
* it tries to fault in the pages of the iov. So we keep track of
* how much was left of iov in the previous EFAULT and fallback
* to buffered IO in case we haven't made any progress.
*/
if (left == prev_left) {
err = -ENOTBLK;
} else {
fault_in_iov_iter_readable(from, left);
prev_left = left;
goto again;
}
}
btrfs_inode_unlock(inode, ilock_flags);
if (IS_ERR_OR_NULL(dio)) {
err = PTR_ERR_OR_ZERO(dio);
if (err < 0 && err != -ENOTBLK)
goto out;
} else {
written = iomap_dio_complete(dio);
}
/*
* Add back IOCB_DSYNC. Our caller, btrfs_file_write_iter(), will do
* the fsync (call generic_write_sync()).
*/
if (is_sync_write)
iocb->ki_flags |= IOCB_DSYNC;
if (written < 0 || !iov_iter_count(from)) {
err = written;
/* If 'err' is -ENOTBLK then it means we must fallback to buffered IO. */
if ((err < 0 && err != -ENOTBLK) || !iov_iter_count(from))
goto out;
}
buffered:
pos = iocb->ki_pos;
@ -2005,7 +2063,7 @@ buffered:
invalidate_mapping_pages(file->f_mapping, pos >> PAGE_SHIFT,
endbyte >> PAGE_SHIFT);
out:
return written ? written : err;
return err < 0 ? err : written;
}
static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
@ -3659,6 +3717,8 @@ static int check_direct_read(struct btrfs_fs_info *fs_info,
static ssize_t btrfs_direct_read(struct kiocb *iocb, struct iov_iter *to)
{
struct inode *inode = file_inode(iocb->ki_filp);
size_t prev_left = 0;
ssize_t read = 0;
ssize_t ret;
if (fsverity_active(inode))
@ -3668,10 +3728,57 @@ static ssize_t btrfs_direct_read(struct kiocb *iocb, struct iov_iter *to)
return 0;
btrfs_inode_lock(inode, BTRFS_ILOCK_SHARED);
again:
/*
* This is similar to what we do for direct IO writes, see the comment
* at btrfs_direct_write(), but we also disable page faults in addition
* to disabling them only at the iov_iter level. This is because when
* reading from a hole or prealloc extent, iomap calls iov_iter_zero(),
* which can still trigger page fault ins despite having set ->nofault
* to true of our 'to' iov_iter.
*
* The difference to direct IO writes is that we deadlock when trying
* to lock the extent range in the inode's tree during he page reads
* triggered by the fault in (while for writes it is due to waiting for
* our own ordered extent). This is because for direct IO reads,
* btrfs_dio_iomap_begin() returns with the extent range locked, which
* is only unlocked in the endio callback (end_bio_extent_readpage()).
*/
pagefault_disable();
to->nofault = true;
ret = iomap_dio_rw(iocb, to, &btrfs_dio_iomap_ops, &btrfs_dio_ops,
0, 0);
IOMAP_DIO_PARTIAL, read);
to->nofault = false;
pagefault_enable();
/* No increment (+=) because iomap returns a cumulative value. */
if (ret > 0)
read = ret;
if (iov_iter_count(to) > 0 && (ret == -EFAULT || ret > 0)) {
const size_t left = iov_iter_count(to);
if (left == prev_left) {
/*
* We didn't make any progress since the last attempt,
* fallback to a buffered read for the remainder of the
* range. This is just to avoid any possibility of looping
* for too long.
*/
ret = read;
} else {
/*
* We made some progress since the last retry or this is
* the first time we are retrying. Fault in as many pages
* as possible and retry.
*/
fault_in_iov_iter_writeable(to, left);
prev_left = left;
goto again;
}
}
btrfs_inode_unlock(inode, BTRFS_ILOCK_SHARED);
return ret;
return ret < 0 ? ret : read;
}
static ssize_t btrfs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)