Re: [PATCH] btrfs: qgroup: Always free PREALLOC META reserve in btrfs_delalloc_release_extents()

[David Sterba <dsterba@suse.cz>]

On Mon, Oct 14, 2019 at 02:34:51PM +0800, Qu Wenruo wrote:
> [Background]
> Btrfs qgroup uses two types of reserved space for METADATA space,
> PERTRANS and PREALLOC.
> 
> PERTRANS is metadata space reserved for each transaction started by
> btrfs_start_transaction().
> While PREALLOC is for delalloc, where we reserve space before joining a
> transaction, and finally it will be converted to PERTRANS after the
> writeback is done.
> 
> [Inconsistency]
> However there is inconsistency in how we handle PREALLOC metadata space.
> 
> The most obvious one is:
> In btrfs_buffered_write():
> 	btrfs_delalloc_release_extents(BTRFS_I(inode), reserve_bytes, true);
> 
> We always free qgroup PREALLOC meta space.
> 
> While in btrfs_truncate_block():
> 	btrfs_delalloc_release_extents(BTRFS_I(inode), blocksize, (ret != 0));
> 
> We only free qgroup PREALLOC meta space when something went wrong.
> 
> [The Correct Behavior]
> The correct behavior should be the one in btrfs_buffered_write(), we
> should always free PREALLOC metadata space.
> 
> The reason is, the btrfs_delalloc_* mechanism works by:
> - Reserve metadata first, even it's not necessary
>   In btrfs_delalloc_reserve_metadata()
> 
> - Free the unused metadata space
>   Normally in:
>   btrfs_delalloc_release_extents()
>   |- btrfs_inode_rsv_release()
>      Here we do calculation on whether we should release or not.
> 
> E.g. for 64K buffered write, the metadata rsv works like:
> 
> /* The first page */
> reserve_meta:	num_bytes=calc_inode_reservations()
> free_meta:	num_bytes=0
> total:		num_bytes=calc_inode_reservations()
> /* The first page caused one outstanding extent, thus needs metadata
>    rsv */
> 
> /* The 2nd page */
> reserve_meta:	num_bytes=calc_inode_reservations()
> free_meta:	num_bytes=calc_inode_reservations()
> total:		not changed
> /* The 2nd page doesn't cause new outstanding extent, needs no new meta
>    rsv, so we free what we have reserved */
> 
> /* The 3rd~16th pages */
> reserve_meta:	num_bytes=calc_inode_reservations()
> free_meta:	num_bytes=calc_inode_reservations()
> total:		not changed (still space for one outstanding extent)
> 
> This means, if btrfs_delalloc_release_extents() determines to free some
> space, then those space should be freed NOW.
> So for qgroup, we should call btrfs_qgroup_free_meta_prealloc() other
> than btrfs_qgroup_convert_reserved_meta().
> 
> The good news is:
> - The callers are not that hot
>   The hottest caller is in btrfs_buffered_write(), which is already
>   fixed by commit 336a8bb8e36a ("btrfs: Fix wrong
>   btrfs_delalloc_release_extents parameter"). Thus it's not that
>   easy to cause false EDQUOT.
> 
> - The trans commit in advance for qgroup would hide the bug
>   Since commit f5fef4593653 ("btrfs: qgroup: Make qgroup async transaction
>   commit more aggressive"), when btrfs qgroup metadata free space is slow,
>   it will try to commit transaction and free the wrongly converted
>   PERTRANS space, so it's not that easy to hit such bug.
> 
> [FIX]
> So to fix the problem, remove the @qgroup_free parameter for
> btrfs_delalloc_release_extents(), and always pass true to
> btrfs_inode_rsv_release().
> 
> Reported-by: Filipe Manana <fdmanana@suse.com>
> Fixes: 43b18595d660 ("btrfs: qgroup: Use separate meta reservation type for delalloc")
> Signed-off-by: Qu Wenruo <wqu@suse.com>

Added to 5.4 queue, thanks. I'll add tag for stable 4.19, though there
are conflicts, not even it's applicable on top of 5.3.