Re: [PATCH v2] writeback: keep cleanup_offline_cgwb()'s scan out of syncs

[Jan Kara]

On Sat 17-10-26 04:07:11, Liz Fong-Jones wrote:
> On a filesystem mounted with lazytime, sync() and syncfs() can return
> without persisting timestamp updates of inodes attached to a dying
> cgroup wb, so a crash after the sync loses them. File data and sizes are
> not affected.
> 
> Since commit f6988c90671e ("writeback: bound cleanup_offline_cgwb()
> rescans by rotating scanned inodes"), isw_prepare_wbs_switch() moves the
> inodes it walks to the newest end of the list. When a pass fills its isw
> before reaching an inode that got its lazytime update after a sync's
> cutoff, the walked inodes land past it on b_dirty_time, and the sync's
> queue_io(), which stops at the first inode dirtied after the cutoff,
> misses them. The late inode has to be one without a pending lazytime
> update; one already on b_dirty_time keeps its position.
> 
> The window is narrow. cleanup_offline_cgwbs_workfn() skips wbs with
> anything on b_dirty, b_io or b_more_io, so the scan has to land between
> wb_writeback() fixing dirtied_before and taking list_lock for its first
> queue_io(), unless a multi-pass drain started before the wb got dirty
> IO.
> 
> Take wb_switch_rwsem for read across the scan, as
> process_inode_switch_wbs() does for the switches.
> 
> Fixes: f6988c90671e ("writeback: bound cleanup_offline_cgwb() rescans by rotating scanned inodes")
> Cc: stable@xxxxxxxxxxxxxxx
> Suggested-by: Tejun Heo <tj@xxxxxxxxxx>
> Link: https://lore.kernel.org/all/0bc5d58533f10b4841807be3025daa11@xxxxxxxxxx/
> Assisted-by: Claude:claude-opus-5-5 checkpatch
> Assisted-by: Claude:claude-fable-5-1
> Acked-by: Tejun Heo <tj@xxxxxxxxxx>
> Signed-off-by: Liz Fong-Jones <lizf@xxxxxxxxxxxx>

Thanks! The fix looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
> Changes in v2, all from Tejun's review:
> - Correct the trigger and the reordering description, and note how
>   narrow the window is
> - Reword the code comment
> - Move the test details here and trim the description
> 
> Test, with a debug-only delay holding a syncfs's work on the wb between
> its cutoff and queue_io() to widen the window: 300 files on a cgroup's
> wb with lazytime atime updates, one more read after the cutoff, then
> the cgroup removed so the cleanup ran. Without this patch, syncfs()
> queued 48 of the 300 and after a crash 252 atime updates were lost, on
> ext4 (-I 4096, so neighbours' timestamps are not written along) in 3 of
> 3 runs and on XFS in 1 of 1; with it, none were lost. arm64 KVM guests,
> 7.3-rc7.
> 
> v1: https://lore.kernel.org/all/20261015114307.1041273-1-lizf@xxxxxxxxxxxx/
> 
>  fs/fs-writeback.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index ea3eb40bf828..372ac9785e03 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -789,6 +789,13 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
>  		new_wb = &wb->bdi->wb; /* wb_get() is noop for bdi's wb */
>  
>  	nr = 0;
> +	/*
> +	 * The scan moves walked inodes to the newest end of b_dirty_time,
> +	 * past ones it didn't reach. If one of those was dirtied after a
> +	 * sync's cutoff, the sync's queue_io() stops there and misses the
> +	 * moved ones. Exclude syncs as process_inode_switch_wbs() does.
> +	 */
> +	down_read(&wb->bdi->wb_switch_rwsem);
>  	spin_lock(&wb->list_lock);
>  	/*
>  	 * In addition to the inodes that have completed writeback, also switch
> @@ -803,6 +810,7 @@ bool cleanup_offline_cgwb(struct bdi_writeback *wb)
>  		restart = isw_prepare_wbs_switch(new_wb, isw, &wb->b_dirty_time,
>  						 &nr);
>  	spin_unlock(&wb->list_lock);
> +	up_read(&wb->bdi->wb_switch_rwsem);
>  
>  	/* no attached inodes? bail out */
>  	if (nr == 0) {
> 
> base-commit: 24a19b953e2a3d0ea5a8c4951a0ab8f32720c907
> -- 
> 2.53.0
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR