[PATCH] btrfs: Prevent nowait or async read from doing sync IO

From: Martin Raiber <martin@urbackup.org>
To: linux-btrfs@vger.kernel.org
Cc: io-uring@vger.kernel.org, Martin Raiber <martin@urbackup.org>
Subject: [PATCH] btrfs: Prevent nowait or async read from doing sync IO
Date: Fri, 8 Jan 2021 00:02:48 +0000	[thread overview]
Message-ID: <01020176df4d86ba-658b4ef1-1b4a-464f-afe4-fb69ca60e04e-000000@eu-west-1.amazonses.com> (raw)

When reading from btrfs file via io_uring I get following
call traces:

[<0>] wait_on_page_bit+0x12b/0x270
[<0>] read_extent_buffer_pages+0x2ad/0x360
[<0>] btree_read_extent_buffer_pages+0x97/0x110
[<0>] read_tree_block+0x36/0x60
[<0>] read_block_for_search.isra.0+0x1a9/0x360
[<0>] btrfs_search_slot+0x23d/0x9f0
[<0>] btrfs_lookup_csum+0x75/0x170
[<0>] btrfs_lookup_bio_sums+0x23d/0x630
[<0>] btrfs_submit_data_bio+0x109/0x180
[<0>] submit_one_bio+0x44/0x70
[<0>] extent_readahead+0x37a/0x3a0
[<0>] read_pages+0x8e/0x1f0
[<0>] page_cache_ra_unbounded+0x1aa/0x1f0
[<0>] generic_file_buffered_read+0x3eb/0x830
[<0>] io_iter_do_read+0x1a/0x40
[<0>] io_read+0xde/0x350
[<0>] io_issue_sqe+0x5cd/0xed0
[<0>] __io_queue_sqe+0xf9/0x370
[<0>] io_submit_sqes+0x637/0x910
[<0>] __x64_sys_io_uring_enter+0x22e/0x390
[<0>] do_syscall_64+0x33/0x80
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9

Prevent those by setting IOCB_NOIO before calling
generic_file_buffered_read.

Async read has the same problem. So disable that by removing
FMODE_BUF_RASYNC. This was added with commit
8730f12b7962b21ea9ad2756abce1e205d22db84 ("btrfs: flag files as
supporting buffered async reads") with 5.9. Io_uring will read
the data via worker threads if it can't be read without sync IO
this way.

Signed-off-by: Martin Raiber <martin@urbackup.org>
---
 fs/btrfs/file.c | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 0e41459b8..8bb561f6d 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -3589,7 +3589,7 @@ static loff_t btrfs_file_llseek(struct file *file, loff_t offset, int whence)
 
 static int btrfs_file_open(struct inode *inode, struct file *filp)
 {
-	filp->f_mode |= FMODE_NOWAIT | FMODE_BUF_RASYNC;
+	filp->f_mode |= FMODE_NOWAIT;
 	return generic_file_open(inode, filp);
 }
 
@@ -3639,7 +3639,18 @@ static ssize_t btrfs_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
 			return ret;
 	}
 
-	return generic_file_buffered_read(iocb, to, ret);
+	if (iocb->ki_flags & IOCB_NOWAIT)
+		iocb->ki_flags |= IOCB_NOIO;
+
+	ret = generic_file_buffered_read(iocb, to, ret);
+
+	if (iocb->ki_flags & IOCB_NOWAIT) {
+		iocb->ki_flags &= ~IOCB_NOIO;
+		if (ret == 0)
+			ret = -EAGAIN;
+	}
+
+	return ret;
 }
 
 const struct file_operations btrfs_file_operations = {
-- 
2.30.0

