Re: [PATCH v4 2/3] xfs: introduce in-core global counter of allocbt blocks

[Chandan Babu R <chandanrlinux@gmail.com>]

On 27 Apr 2021 at 17:03, Brian Foster wrote:
> On Tue, Apr 27, 2021 at 03:58:16PM +0530, Chandan Babu R wrote:
>> On 23 Apr 2021 at 18:40, Brian Foster wrote:
>> > Introduce an in-core counter to track the sum of all allocbt blocks
>> > used by the filesystem. This value is currently tracked per-ag via
>> > the ->agf_btreeblks field in the AGF, which also happens to include
>> > rmapbt blocks. A global, in-core count of allocbt blocks is required
>> > to identify the subset of global ->m_fdblocks that consists of
>> > unavailable blocks currently used for allocation btrees. To support
>> > this calculation at block reservation time, construct a similar
>> > global counter for allocbt blocks, populate it on first read of each
>> > AGF and update it as allocbt blocks are used and released.
>> >
>> > Signed-off-by: Brian Foster <bfoster@redhat.com>
>> > ---
>> >  fs/xfs/libxfs/xfs_alloc.c       | 12 ++++++++++++
>> >  fs/xfs/libxfs/xfs_alloc_btree.c |  2 ++
>> >  fs/xfs/xfs_mount.h              |  6 ++++++
>> >  3 files changed, 20 insertions(+)
>> >
>> > diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
>> > index aaa19101bb2a..144e2d68245c 100644
>> > --- a/fs/xfs/libxfs/xfs_alloc.c
>> > +++ b/fs/xfs/libxfs/xfs_alloc.c
>> > @@ -3036,6 +3036,7 @@ xfs_alloc_read_agf(
>> >  	struct xfs_agf		*agf;		/* ag freelist header */
>> >  	struct xfs_perag	*pag;		/* per allocation group data */
>> >  	int			error;
>> > +	uint32_t		allocbt_blks;
>> >
>> >  	trace_xfs_alloc_read_agf(mp, agno);
>> >
>> > @@ -3066,6 +3067,17 @@ xfs_alloc_read_agf(
>> >  		pag->pagf_refcount_level = be32_to_cpu(agf->agf_refcount_level);
>> >  		pag->pagf_init = 1;
>> >  		pag->pagf_agflreset = xfs_agfl_needs_reset(mp, agf);
>> > +
>> > +		/*
>> > +		 * Update the global in-core allocbt block counter. Filter
>> > +		 * rmapbt blocks from the on-disk counter because those are
>> > +		 * managed by perag reservation.
>> > +		 */
>> > +		if (pag->pagf_btreeblks > be32_to_cpu(agf->agf_rmap_blocks)) {
>> 
>> pag->pagf_btreeblks gets incremented everytime a block is allocated to refill
>> AGFL (via xfs_alloc_get_freelist()). Apart from the allobt trees, blocks for
>> Rmap btree also get allocated from AGFL. Hence pag->pagf_btreeblks must be
>> larger than agf->agf_rmap_blocks.
>> 
>
> This function is actually to consume a block from the AGFL (as opposed
> to refill it).

Sorry, I meant to say "allocate a block from AGFL".

>
>> Can you please describe the scenario in which pag->pagf_btreeblks has a value
>> that is <= agf->agf_rmap_blocks?
>> 
>
> Ah, this was just an initialization quirk. I originally had an assert
> here and based the logic on the assumption that pagf_btreeblks >=
> agf_rmap_blocks, but alas:
>
> # mkfs.xfs -f -mrmapbt <dev>
> ...
> # xfs_db -c "agf 0" -c "p rmapblocks" -c "p btreeblks" <dev>
> rmapblocks = 1
> btreeblks = 0
> #

Thanks for clarifying my doubt.

The patch looks good to me.

Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>

-- 
chandan