Re: [PATCH] xfs: don't include bnobt blocks when reserving free block pool

[Dave Chinner <david@fromorbit.com>]

On Wed, Mar 16, 2022 at 11:17:26AM -0700, Darrick J. Wong wrote:
> On Wed, Mar 16, 2022 at 01:29:01PM -0400, Brian Foster wrote:
> > On Wed, Mar 16, 2022 at 09:32:16AM -0700, Darrick J. Wong wrote:
> > > On Wed, Mar 16, 2022 at 07:28:18AM -0400, Brian Foster wrote:
> > > > On Mon, Mar 14, 2022 at 11:08:47AM -0700, Darrick J. Wong wrote:
> > Similar deal as above.. I'm more interested in a potential cleanup of
> > the code that helps prevent this sort of buglet for the next user of
> > ->m_alloc_set_aside that will (expectedly) have no idea about this
> > subtle quirk than I am about what's presented in the free space
> > counters. ISTM that we ought to just ditch ->m_alloc_set_aside, replace
> > the existing xfs_alloc_set_aside() with an XFS_ALLOC_FS_RESERVED() macro
> > or something that just does the (agcount << 3) thing, and then define a
> 
> I'm not sure that the current xfs_alloc_set_aside code is correct.
> Right now it comes with this comment:
> 
> "We need to reserve 4 fsbs _per AG_ for the freelist and 4 more to
> handle a potential split of the file's bmap btree."
>
> I think the first part ("4 fsbs _per AG_ for the freelist") is wrong.
> AFAICT, that part refers to the number of blocks we need to keep free in
> case we have to replenish a completely empty AGFL.  The hardcoded value
> of 4 seems wrong, since xfs_alloc_min_freelist() is what _fix_freelist
> uses to decide how big the AGFL needs to be, and it returns 6 on a
> filesystem that has rmapbt enabled.  So I think XFS_ALLOC_AGFL_RESERVE
> is wrong here and should be replaced with the function call.

Back when I wrote that code (circa 2007, IIRC), that was actually
correct according to the reservations that were made when freeing
an extent at ENOSPC.

We needed 4 blocks for the AGFL fixup to always succeed  - 2 blocks
for each BNO and CNT btrees, and, IIRC, the extent free reservation
was just 4 blocks at that time. Hence the 4+4 value.

However, you are right that rmap also adds another per-ag btree that
is allocated from the agfl and that set_aside() should be taking
that into accout. That said, I think that xfs_alloc_min_freelist()
might even be wrong by just adding 2 blocks to the AGFL for the
rmapbt.

That is, at ENOSPC the rmapbt can be a *big* btree. It's not like
the BNO and CNT btrees which are completely empty at that point in
time; the RMAP tree could be one level below max height, and freeing
a single block could split a rmap rec and trigger a full height RMAP
split.

So the minimum free list length in that case is 2 + 2 + MAX_RMAP_HEIGHT.

> I also think the second part ("and 4 more to handle a split of the
> file's bmap btree") is wrong.  If we're really supposed to save enough
> blocks to handle a bmbt split, then I think this ought to be
> (mp->m_bm_maxlevels[0] - 1), not 4, right?  According to xfs_db, bmap
> btrees can be 9 levels tall:

Yes, we've changed the BMBT reservations in the years since that
code was written to handle max height reservations correctly, too.
So, like the RMAP btree reservation, we probably should be reserving
MAX_BMAP_HEIGHT in the set-aside calculation.

refcount btree space is handled by the ag_resv code and blocks
aren't allocated from the AGFL, so I don't think we need to take
taht into account for xfs_alloc_set_aside().

> So in the end, I think that calculation should become:
> 
> unsigned int
> xfs_alloc_set_aside(
> 	struct xfs_mount	*mp)
> {
> 	unsigned int		min-agfl = xfs_alloc_min_freelist(mp, NULL);
> 
> 	return mp->m_sb.sb_agcount * (min_agfl + mp->m_bm_maxlevels[0] - 1);
> }

*nod*, but with the proviso that xfs_alloc_min_freelist() doesn't
appear to be correct, either....

Also, that's a fixed value for the physical geometry of the
filesystem, so it should be calculated once at mount time and stored
in the xfs_mount (and only updated if needed at growfs time)...

> > new xfs_alloc_set_aside() that combines the macro calculation with
> > ->m_allocbt_blks. Then the whole "set aside" concept is calculated and
> > documented in one place. Hm?
> 
> I think I'd rather call the new function xfs_fdblocks_avail() over
> reusing an existing name, because I fear that zapping an old function
> and replacing it with a new function with the same name will cause
> confusion for anyone backporting patches or reading code after an
> absence.
> 
> Also the only reason we have a mount variable and a function (instead of
> a macro) is that Dave asked me to change the codebase away from the
> XFS_ALLOC_AG_MAX_USABLE/XFS_ALLOC_SET_ASIDE macros as part of merging
> reflink.

Yeah, macros wrapping a variable or repeated constant calculation
are bad, and it's something we've been cleaning up for a long
time...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com