[PATCHSET,v2,0/6] xfs: fix incorrect reserve pool calculations and reporting

Message

Darrick J. Wong March 17, 2022, 9:20 p.m. UTC

Hi all,

Last week, I was 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.  This resulted mount hanging on
the infinite loop in xfs_reserve_blocks because we overestimate the
number of blocks that xfs_mod_fdblocks will give us, and the code
retries forever.

Initially, this was a single patch that fixed the calculation and
transformed the loop into a finite loop.  However, further discussion
revealed several more issues:

 * People had a hard time grokking that the "alloc_set_aside" is
   actually the amount of space we hide so that a bmbt split will always
   succeed;
 * The author didn't understand what XFS_ALLOC_AGFL_RESERVE actually
   mean or whether it was correctly calculated;
 * The "alloc_set_aside" underestimated the blocks needed to handle any
   bmap btree split;
 * Both statfs and XFS_IOC_FSCOUNTS forgot to exclude the amount of
   space used by the free space btrees on behalf of per-AG reservations,
   leading to overreporting of available space;
 * The loop control change really should have been separate.

Hence, this patchset has now grown to six patches to address each of
these issues separately.

If you're going to start using this mess, you probably ought to just
pull from my git trees, which are linked below.

This is an extraordinary way to destroy everything.  Enjoy!
Comments and questions are, as always, welcome.

--D

kernel git tree:
https://git.kernel.org/cgit/linux/kernel/git/djwong/xfs-linux.git/log/?h=reserve-block-fixes-5.18
---
 fs/xfs/libxfs/xfs_alloc.c |   30 +++++++++++++++++++++++-------
 fs/xfs/libxfs/xfs_alloc.h |    3 +--
 fs/xfs/libxfs/xfs_sb.c    |    2 --
 fs/xfs/xfs_fsops.c        |   24 +++++++++++++++++-------
 fs/xfs/xfs_log_recover.c  |    2 +-
 fs/xfs/xfs_mount.c        |    9 ++++++++-
 fs/xfs/xfs_mount.h        |   45 ++++++++++++++++++++++++++++++++++++++++++++-
 fs/xfs/xfs_super.c        |    4 +---
 8 files changed, 95 insertions(+), 24 deletions(-)