[Devel] [PATCH RHEL9 COMMIT] fs/ext4: relax inappropriate memory alignment check

[Konstantin Khorenko]

The commit is pushed to "branch-rh9-5.14.0-362.8.1.vz9.35.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh9-5.14.0-362.8.1.vz9.35.14
------>
commit acfd366ca1f7b4ac266f5a8e5c67d9c251f99ef3
Author: Alexey Kuznetsov <kuznet at virtuozzo.com>
Date:   Wed Feb 28 00:58:19 2024 +0800

    fs/ext4: relax inappropriate memory alignment check
    
    The bug is ancient, ext4 has been broken in mainstream since 2014,
    I guess this means nobody cares. Though right place of this
    patch is mainstream of course.
    
    It destroys performance of direct io in vz9 compared to vz7, where
    this place was still correct. Note, our memory buffers are aligned
    to 512 and we physically cannot get it even more coarse
    and there are exactly no reasons it would be not enough. Well,
    except for stupid bugs like this.
    
    Comment from the patch:
    
    /* Memory alignment has nothing to do with io alignment. All the goal
     * of ext4_unaligned_io() is to check when direct io requires some
     * fuss about unaligned modifications to ext4 file layout. Memorywise,
     * iomap handles all the required dma alignment troubles. So,
     * this check is entirely wrong and I can guess it was a copy-n-paste
     * error in original patch 9b884164.. "convert ext4 to ->write_iter()."
     * Yet, just to be on safe side let's leave minimal test for alignment
     * to 512 as bdev level used not to handle this well and 512
     * is old good traditional limit.
     */
    
    https://pmc.acronis.work/browse/VSTOR-79527
    
    Signed-off-by: Alexey Kuznetsov <kuznet at virtuozzo.com>
    
    Feature: fix ms/fs
---
 fs/ext4/file.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/fs/ext4/file.c b/fs/ext4/file.c
index 52cc0261e5dc..3a90225521e8 100644
--- a/fs/ext4/file.c
+++ b/fs/ext4/file.c
@@ -63,6 +63,9 @@ static bool ext4_should_use_dio(struct kiocb *iocb, struct iov_iter *iter)
 	if (dio_align == 1)
 		return true;
 
+	/* Not reached. So, line below is meaningless */
+	WARN_ON_ONCE(1);
+
 	return IS_ALIGNED(iocb->ki_pos | iov_iter_alignment(iter), dio_align);
 }
 
@@ -187,7 +190,20 @@ ext4_unaligned_io(struct inode *inode, struct iov_iter *from, loff_t pos)
 	struct super_block *sb = inode->i_sb;
 	unsigned long blockmask = sb->s_blocksize - 1;
 
-	if ((pos | iov_iter_alignment(from)) & blockmask)
+	if (pos & blockmask)
+		return true;
+
+	/* Memory alignment has nothing to do with io alignment. All the goal
+	 * of ext4_unaligned_io() is to check when direct io requires some
+	 * fuss about unaligned modifications to ext4 file layout. Memorywise,
+	 * iomap handles all the required dma alignment troubles. So,
+	 * this check is entirely wrong and I can guess it was a copy-n-paste
+	 * error in original patch 9b884164.. "convert ext4 to ->write_iter()."
+	 * Yet, just to be on safe side let's leave minimal test for alignment
+	 * to 512 as bdev level used not to handle this well and 512
+	 * is old good traditional limit.
+	 */
+	if (iov_iter_alignment(from) & 511)
 		return true;
 
 	return false;