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

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Mar 16, 2022 at 03:17:27PM -0400, Brian Foster wrote:
> On Wed, Mar 16, 2022 at 02:48:43PM -0400, Brian Foster wrote:
> > 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:
> > > > > > > From: Darrick J. Wong <djwong@kernel.org>
> > > > > > > 
> > > > > > > xfs_reserve_blocks controls the size of the user-visible free space
> > > > > > > reserve pool.  Given the difference between the current and requested
> > > > > > > pool sizes, it will try to reserve free space from fdblocks.  However,
> > > > > > > the amount requested from fdblocks is also constrained by the amount of
> > > > > > > space that we think xfs_mod_fdblocks will give us.  We'll keep trying to
> > > > > > > reserve space so long as xfs_mod_fdblocks returns ENOSPC.
> > > > > > > 
> > > > > > > In commit fd43cf600cf6, we decided that xfs_mod_fdblocks should not hand
> > > > > > > out the "free space" used by the free space btrees, because some portion
> > > > > > > of the free space btrees hold in reserve space for future btree
> > > > > > > expansion.  Unfortunately, xfs_reserve_blocks' estimation of the number
> > > > > > > of blocks that it could request from xfs_mod_fdblocks was not updated to
> > > > > > > include m_allocbt_blks, so if space is extremely low, the caller hangs.
> > > > > > > 
> > > > > > > Fix this by including m_allocbt_blks in the estimation, and modify the
> > > > > > > loop so that it will not retry infinitely.
> > > > > > > 
> > > > > > > Found by running xfs/306 (which formats a single-AG 20MB filesystem)
> > > > > > > with an fstests configuration that specifies a 1k blocksize and a
> > > > > > > specially crafted log size that will consume 7/8 of the space (17920
> > > > > > > blocks, specifically) in that AG.
> > > > > > > 
> > > > > > > Cc: Brian Foster <bfoster@redhat.com>
> > > > > > > Fixes: fd43cf600cf6 ("xfs: set aside allocation btree blocks from block reservation")
> > > > > > > Signed-off-by: Darrick J. Wong <djwong@kernel.org>
> > > > > > > ---
> > > > > > >  fs/xfs/xfs_fsops.c |   16 ++++++++++++----
> > > > > > >  1 file changed, 12 insertions(+), 4 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/fs/xfs/xfs_fsops.c b/fs/xfs/xfs_fsops.c
> > > > > > > index 33e26690a8c4..78b6982ea5b0 100644
> > > > > > > --- a/fs/xfs/xfs_fsops.c
> > > > > > > +++ b/fs/xfs/xfs_fsops.c
> > > > > > > @@ -379,6 +379,7 @@ xfs_reserve_blocks(
> > > > > > >  	int64_t			fdblks_delta = 0;
> > > > > > >  	uint64_t		request;
> > > > > > >  	int64_t			free;
> > > > > > > +	unsigned int		tries;
> > > > > > >  	int			error = 0;
> > > > > > >  
> > > > > > >  	/* If inval is null, report current values and return */
> > > > > > > @@ -432,9 +433,16 @@ xfs_reserve_blocks(
> > > > > > >  	 * perform a partial reservation if the request exceeds free space.
> > > > > > >  	 */
> > > > > > >  	error = -ENOSPC;
> > > > > > > -	do {
> > > > > > > -		free = percpu_counter_sum(&mp->m_fdblocks) -
> > > > > > > -						mp->m_alloc_set_aside;
> > > > > > > +	for (tries = 0; tries < 30 && error == -ENOSPC; tries++) {
> > > > > > 
> > > > > > Any reason for the magic number of retries as opposed to perhaps just
> > > > > > not retrying at all?
> > > > > 
> > > > > I /think/ the origins of the loop was commit dbcabad19aa9 ("[XFS] Fix
> > > > > block reservation mechanism."), where I guess Dave decided that we
> > > > > should loop forever trying to satisfy a request from userspace to
> > > > > increase the reserve pool.  OFC you and I have been patching this
> > > > > function to fix all its horrible warts over the years, so maybe you're
> > > > > right that this should only try once...
> > > > > 
> > > > > (For the mount time default reservation, we should only iterate the loop
> > > > > once (provided the accounting is correct ;) since nobody else is
> > > > > touching the free space counters.)
> > > > > 
> > > > > > This seems a little odd when you think about it
> > > > > > given that the request is already intended to take available space into
> > > > > > account and modify the request from userspace. OTOH, another
> > > > > > consideration could be to retry some (really large?) number of times and
> > > > > > then bail out if we happen to iterate without an observable change in
> > > > > > free space (i.e., something is wrong), however I suppose that could be
> > > > > > racy as well. *shrug*
> > > > > 
> > > > > ...but if you're the sysadmin desperately trying to increase the size of
> > > > > the reserve pool when the fs is running near ENOSPC, you're going to be
> > > > > racing with fdblocks bouncing up and down.  The @free samples that we
> > > > > take here in the loop body are indeed racy since we can't tell the
> > > > > difference between @free being unchanged from the last iteration because
> > > > > someone freed a block and someone else immediately consumed it, or a
> > > > > totally idle system.
> > > > > 
> > > > > Either way, it's better than hanging the whole system. :)
> > > > > 
> > > > 
> > > > Yeah.. I'm not bothered much by whether we retry once, 42 times or
> > > > forever. I think what this boils down to for me is whether it's worth
> > > > the risk of a behavior change of an -ENOSPC return causing something
> > > > unexpected for some random user or use case. Could we just do this in
> > > > two separate patches? Patch 1 fixes the calculation and targets stable,
> > > > patch 2 does whatever to the retry loop that potentially changes retry
> > > > semantics (and doesn't really need backporting)..?
> > > 
> > > Splitting the two patches sounds good to me.
> > > 
> > > > > What if I augment the loop control with a comment capturing some of this:
> > > > > 
> > > > > 	/*
> > > > > 	 * The loop body estimates how many blocks it can request from
> > > > > 	 * fdblocks to stash in the reserve pool.  This is a classic
> > > > > 	 * TOCTOU race since fdblocks updates are not always coordinated
> > > > > 	 * via m_sb_lock.  We also cannot tell if @free remaining
> > > > > 	 * unchanged between iterations is due to an idle system or
> > > > > 	 * freed blocks being consumed immediately, so we'll try a
> > > > > 	 * finite number of times to satisfy the request.
> > > > > 	 */
> > > > > 	for (tries = 0; tries < 30...) {
> > > > > 
> > > > > > 
> > > > > > > +		/*
> > > > > > > +		 * The reservation pool cannot take space that xfs_mod_fdblocks
> > > > > > > +		 * will not give us.  This includes the per-AG set-aside space
> > > > > > > +		 * and free space btree blocks that are not available for
> > > > > > > +		 * allocation due to per-AG metadata reservations.
> > > > > > > +		 */
> > > > > > > +		free = percpu_counter_sum(&mp->m_fdblocks);
> > > > > > > +		free -= mp->m_alloc_set_aside;
> > > > > > > +		free -= atomic64_read(&mp->m_allocbt_blks);
> > > > > > 
> > > > > > Seems reasonable. Do we want to consider ->m_allocbt_blks in other
> > > > > > places where ->m_alloc_set_aside is used (i.e. xfs_fs_statfs(), etc.)?
> > > > > > Not sure how much it matters for space reporting purposes, but if so, it
> > > > > > might also be worth reconsidering the usefulness of a static field and
> > > > > > initialization helper (i.e. xfs_alloc_set_aside()) if the majority of
> > > > > > uses involve a dynamic calculation (due to ->m_allocbt_blks).
> > > > > 
> > > > > When I was writing this patch, I very nearly decided to make those three
> > > > > lines above their own helper.  I didn't see any other spots that looked
> > > > > like obvious candidates for such a calculation outside of statfs.
> > > > > 
> > > > 
> > > > Indeed..
> > > > 
> > > > > Subtracting m_allocbt_blks from statfs' avail field is a behavior
> > > > > change, since we always used to consider bnobt blocks as available.  We
> > > > > don't have an exact count of how many blocks are needed to hide the
> > > > > per-ag reserved extents, so in the end we have to decide whether we want
> > > > > to hear complaints about over- or under-estimation of available blocks.
> > > > > 
> > > > > So I think the statfs stuff is a separate patch. :)
> > > > > 
> > > > 
> > > > 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.
> > > 
> > > 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:
> > > 
> > > xfs_db> btheight bmapbt -n 4294967296 -b 512
> > > bmapbt: worst case per 512-byte block: 13 records (leaf) / 13 keyptrs (node)
> > > level 0: 4294967296 records, 330382100 blocks
> > > level 1: 330382100 records, 25414008 blocks
> > > level 2: 25414008 records, 1954924 blocks
> > > level 3: 1954924 records, 150379 blocks
> > > level 4: 150379 records, 11568 blocks
> > > level 5: 11568 records, 890 blocks
> > > level 6: 890 records, 69 blocks
> > > level 7: 69 records, 6 blocks
> > > level 8: 6 records, 1 block
> > > 9 levels, 357913945 blocks total
> > > 
> > > The root level is in the inode, so we need 8 blocks to handle a full
> > > split on a 512b block filesystem.
> > > 
> > > Granted, it's at least correct for 4k+ block filesystems:
> > > 
> > > xfs_db> btheight bmapbt -n 4294967296
> > > bmapbt: worst case per 4096-byte block: 125 records (leaf) / 125 keyptrs (node)
> > > level 0: 4294967296 records, 34359739 blocks
> > > level 1: 34359739 records, 274878 blocks
> > > level 2: 274878 records, 2200 blocks
> > > level 3: 2200 records, 18 blocks
> > > level 4: 18 records, 1 block
> > > 5 levels, 34636836 blocks total
> > > 
> > > 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);
> > > }
> > > 
> > 
> > I'm not familiar enough with the original context, but that seems
> > reasonable to me at a glance. If we do want to change this, I'd again
> > just suggest a separate patch since presumably there isn't any known bug
> > report or anything associated with the current code.

Oh definitely, each of these little fixes nets their own patch.

> > > > 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.
> > > 
> > 
> > The logical change is basically just to include the allocbt block count
> > in the "set aside" calculation, not necessarily redefine what "set
> > aside" means, so I don't really see that as a problem. The function is
> > only used in a couple places, but we've managed to make it confusing by
> > conflating the naming of the function, xfs_mount field and local
> > variables in certain contexts.

Hmm.  The way I see things, the setaside is the *minimum* amount of free
space that we need to withhold from the user to guarantee that we can
always make progress in a file mapping update.  m_allocbt_blks is the
amount of space used by free space btrees (currently, the entire btree)
that we withhold from the user to guarantee that btree expansions from
the per-AG reservations can actually be filled.

The reserve pool looks to me like a user-controlled chunk of space that
we also withhold to avoid ENOSPC failure from within writeback; and the
per-AG reservations of course are even more space withheld to guarantee
btree expansion.

IOWs, there are four (I hope!) different free space pools that we use to
guarantee forward progress on this or that operation, and the only thing
they have in common is they don't get reported to userspace as available
blocks.  I don't know that I want to go mixing them too much.

Though, I agree that all of this is really confusing and needs a central
function that makes it obvious how we compute available blocks given all
the free space pools that we have.

> > BTW, do we ever really expect the "set aside" value to be of any
> > significance relative to the perag reservation values for any moderately
> > recent kernel?

Nope.  Any non-tiny filesystem with at least finobt=1 is going to have
way more per-AG reservation than either the setaside or the reserve
pool.

> > IIRC (most) perag res is not presented as free space, so
> > I'm curious if it would even be noticeable if we just fixed and used
> > xfs_alloc_set_aside() consistently across the board..

It isn't.  On a 500G test image with reflink, rmap, and finobt turned
on, the per-AG reservations consume ~630kblocks for each of four AGs of
about ~32.7mblocks apiece.  That's about 2%.  The reservation pool is
8192 blocks and the set-aside is 4*8=32 blocks, both of which are
rounding error by comparison.

At maximal free space fragmentation, the free space btrees would consume
(worst case) about 65kblocks per AG.  That might be noticeable.

> I suppose it's reasonable to want to include allocbt blocks as free
> space in general, though, since technically they are eventually usable
> blocks. So naming and whatnot aside, at the end the day I'm good with
> anything that refactors this code one way or another such that it's
> harder to repeat the same mistake.

Yeah, I at least like the idea of having a function that estimates how
much free space we can try to reserve without using reserve pool blocks.
I also am warming to the idea of telling users they can't have /any/ of
those reserved blocks... particularly since I haven't even gotten to the
"I fill my fs to 99% full and only then buy one more gigabyte of space,
wash, rinse, repeat" complaints.

--D

> 
> Brian
> 
> > Brian
> > 
> > > 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.
> > > 
> > > --D
> > > 
> > > > Brian
> > > > 
> > > > > --D
> > > > > 
> > > > > > 
> > > > > > Brian
> > > > > > 
> > > > > > >  		if (free <= 0)
> > > > > > >  			break;
> > > > > > >  
> > > > > > > @@ -459,7 +467,7 @@ xfs_reserve_blocks(
> > > > > > >  		spin_unlock(&mp->m_sb_lock);
> > > > > > >  		error = xfs_mod_fdblocks(mp, -fdblks_delta, 0);
> > > > > > >  		spin_lock(&mp->m_sb_lock);
> > > > > > > -	} while (error == -ENOSPC);
> > > > > > > +	}
> > > > > > >  
> > > > > > >  	/*
> > > > > > >  	 * Update the reserve counters if blocks have been successfully
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
>