Re: [PATCH 3/3] ext4: fix race between writepages and remount

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On 2023/3/27 17:35, Jan Kara wrote:
On Thu 23-03-23 22:18:53, Baokun Li wrote:
On 2023/3/23 19:44, Jan Kara wrote:
---
   fs/ext4/ext4.h      |  3 ++-
   fs/ext4/ext4_jbd2.h |  9 +++++----
   fs/ext4/super.c     | 14 ++++++++++++++
   3 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index 08b29c289da4..f60967fa648f 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1703,7 +1703,8 @@ struct ext4_sb_info {
   	/*
   	 * Barrier between writepages ops and changing any inode's JOURNAL_DATA
-	 * or EXTENTS flag.
+	 * or EXTENTS flag or between changing SHOULD_DIOREAD_NOLOCK flag on
+	 * remount and writepages ops.
   	 */
   	struct percpu_rw_semaphore s_writepages_rwsem;
   	struct dax_device *s_daxdev;
diff --git a/fs/ext4/ext4_jbd2.h b/fs/ext4/ext4_jbd2.h
index 0c77697d5e90..d82bfcdd56e5 100644
--- a/fs/ext4/ext4_jbd2.h
+++ b/fs/ext4/ext4_jbd2.h
@@ -488,6 +488,9 @@ static inline int ext4_free_data_revoke_credits(struct inode *inode, int blocks)
   	return blocks + 2*(EXT4_SB(inode->i_sb)->s_cluster_ratio - 1);
   }
+/* delalloc is a temporary fix to prevent generic/422 test failures*/
+#define EXT4_MOUNT_SHOULD_DIOREAD_NOLOCK (EXT4_MOUNT_DIOREAD_NOLOCK | \
+					  EXT4_MOUNT_DELALLOC)
   /*
    * This function controls whether or not we should try to go down the
    * dioread_nolock code paths, which makes it safe to avoid taking
@@ -499,7 +502,8 @@ static inline int ext4_free_data_revoke_credits(struct inode *inode, int blocks)
    */
   static inline int ext4_should_dioread_nolock(struct inode *inode)
   {
-	if (!test_opt(inode->i_sb, DIOREAD_NOLOCK))
+	if (test_opt(inode->i_sb, SHOULD_DIOREAD_NOLOCK) !=
+	    EXT4_MOUNT_SHOULD_DIOREAD_NOLOCK)
   		return 0;
   	if (!S_ISREG(inode->i_mode))
   		return 0;
@@ -507,9 +511,6 @@ static inline int ext4_should_dioread_nolock(struct inode *inode)
   		return 0;
   	if (ext4_should_journal_data(inode))
   		return 0;
-	/* temporary fix to prevent generic/422 test failures */
-	if (!test_opt(inode->i_sb, DELALLOC))
-		return 0;
   	return 1;
   }
Is there a need for this SHOULD_DIOREAD_NOLOCK? When called from writeback
we will be protected by s_writepages_rwsem anyway. When called from other
places, we either decide to do dioread_nolock or don't but the situation
can change at any instant so I don't see how unifying this check would
help. And the new SHOULD_DIOREAD_NOLOCK somewhat obfuscates what's going
on.
We're thinking that the mount-related flags in
ext4_should_dioread_nolock() might be modified, such as DELALLOC being
removed because generic/422 test failures were fixed in some other way,
resulting in some unnecessary locking during remount, or for whatever
reason a mount-related flag was added to ext4_should_dioread_nolock(),
and we didn't make a synchronization change in __ext4_remount() that
would cause the problem to recur.  So we added this flag to this function
(instead of in ext4.h), so that when we change the mount option in
ext4_should_dioread_nolock(), we directly change this flag, and we don't
have to consider making synchronization changes in __ext4_remount().

We have checked where this function is called and there are two types of
calls to this function:
1. One category is ext4_do_writepages() and mpage_map_one_extent(), which
are protected by s_writepages_rwsem, the location of the problem;
2. The other type is in ext4_page_mkwrite(),
ext4_convert_inline_data_to_extent(), ext4_write_begin() to determine
whether to get the block using ext4_get_block_unwritten() or
ext4_get_block().

     1) If we just started fetching written blocks, it looks like there is no
problem;
     2) If we start getting unwritten blocks, when DIOREAD_NOLOCK is cleared
by remount,
         we will convert the blocks to written in ext4_map_blocks(). The
data=ordered mode ensures that we don't see stale data.
Yes. So do you agree that EXT4_MOUNT_SHOULD_DIOREAD_NOLOCK is not really
needed?

								Honza
Yes, I totally agree!
If we unconditionally grabbed s_writepages_rwsem when remounting,
there would be no mount option synchronization problem, and the flag
would be completely unnecessary.

I will send a patch V2 with the changes suggested by you.
--
With Best Regards,
Baokun Li
.



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