Re: [RFC PATCH v4 33/34] ext4: don't mark IOMAP_F_DIRTY for buffer write

Dave Chinner
2024-05-01T09:27:40Z

On Wed, Apr 10, 2024 at 11:03:12PM +0800, Zhang Yi wrote:
> From: Zhang Yi <yi.zhang@huawei.com>
>
> The data sync dirty check in ext4_inode_datasync_dirty() is expansive
> since jbd2_transaction_committed() holds journal->j_state lock when
> journal is enabled, it costs a lot in high-concurrency iomap buffered
> read/write paths, but we never check IOMAP_F_DIRTY in these cases, so
> let's check it only in swap file, dax and direct IO cases. Tested by
> Unixbench on 100GB ramdisk:
>
> ./Run -c 128 -i 10 fstime fsbuffer fsdisk
>
> == without this patch ==
> 128 CPUs in system; running 128 parallel copies of tests
>
> File Copy 1024 bufsize 2000 maxblocks 6332521.0 KBps
> File Copy 256 bufsize 500 maxblocks 1639726.0 KBps
> File Copy 4096 bufsize 8000 maxblocks 24018572.0 KBps
>
> == with this patch ==
> 128 CPUs in system; running 128 parallel copies of tests
>
> File Copy 1024 bufsize 2000 maxblocks 49229257.0 KBps
> File Copy 256 bufsize 500 maxblocks 24057510.0 KBps
> File Copy 4096 bufsize 8000 maxblocks 75704437.0 KBps
>
> Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
> ---
> fs/ext4/inode.c | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 1cb219d347af..269503749ef5 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -3281,9 +3281,13 @@ static void ext4_set_iomap(struct inode *inode, struct iomap *iomap,
> * there is no other metadata changes being made or are pending.
> */
> iomap->flags = 0;
> - if (ext4_inode_datasync_dirty(inode) ||
> - offset + length > i_size_read(inode))
> - iomap->flags |= IOMAP_F_DIRTY;
> + if ((flags & (IOMAP_DAX | IOMAP_REPORT)) ||
> + ((flags & (IOMAP_WRITE | IOMAP_DIRECT)) ==
> + (IOMAP_WRITE | IOMAP_DIRECT))) {
> + if (offset + length > i_size_read(inode) ||
> + ext4_inode_datasync_dirty(inode))
> + iomap->flags |= IOMAP_F_DIRTY;
> + }

NACK. This just adds a nasty landmine that anyone working on the
iomap infrastructure can step on. i.e. any time we add a new check
for IOMAP_F_DIRTY in the generic infrastructure, ext4 is going to
break because it won't set the IOMAP_F_DIRTY flag correctly.

If checking an inode is dirty is expensive on ext4, then make it
less expensive and everyone will benefit.

/me goes and looks at jbd2_transaction_committed()

Oh, it it's just a sequence number comparison, and it needs a lock
because it has to dereference the running/committed transactions
structures to get the current sequence numbers. Why not just store
the commiting/running transaction tids in the journal_t, and then
you can sample them without needing any locking and the whole
ext4_inode_datasync_dirty() scalability problem goes away...

-Dave.
--
Dave Chinner
david@fromorbit.com