Re: [PATCH v3 0/4] Fix HugeTLB subpool used_hpages tracking

[Karl Mehltretter <kmehltretter@gmail.com>]

On Wed, 16 Sep 2026 20:13:07 -0700 Andrew Morton <akpm@linux-foundation.org> wrote:
> So if downstream people (-stable maintainers, others) follow our
> recommendations, some kernels will get two of these patches, other
> kernel versions will get three and some lucky kernels might get all
> four.  Are you confident that the patches can be split apart in this
> fashion and still produce a good result?

Two things I noticed while testing this series on v7.3-rc3 (x86_64
QEMU, one CPU):

1. It overlaps with two fixes already queued in mm.git, so I've added
   their authors to Cc:

   - 3/4 rewrites the same out_subpool_put: block as Zhao Li's
     "mm/hugetlb: fix max-only subpool accounting on
     alloc_hugetlb_folio failure" (mm-hotfixes-unstable), and has the
     same Fixes: tag.
   - 2/4 rewrites the same out_put_pages: block as Jinmeng Zhou's
     "mm/hugetlb: fix subpool minimum reservation rollback"
     (mm-unstable).

   3/4 doesn't apply to mm-hotfixes-unstable, and 2/4-4/4 don't apply
   to mm-unstable. If I read them right, the queued fixes only handle
   mounts with size=, while 2/4 and 3/4 also cover min_size mounts, so
   they would probably replace them.

2. On splitting: 1/4 applies cleanly on top of both queued fixes, but
   in my tests it made min_size-only mounts worse on its own. As far
   as I can tell, that's because it starts tracking used_hpages on
   those mounts, while the two error paths only release it after 2/4
   and 3/4.

   HugePages_Rsvd, expected value in parentheses. Q = the two queued
   fixes, wrap = 18446744073709551615:

                                   rc3   +Q  +1/4  +Q+1/4  +1/4..4/4
   min_size=4M only:
     SIGBUS faults [1], no files (2)  2    2     0      0      2
   min_size=8M only:
     failed mmap [2], umounted (0)    0    0     3      3      0
   size=8M,min_size=4M:
     SIGBUS faults [1], no files (2)  0    0     0      0      2
   size=10M,min_size=8M:
     failed mmap [2], umounted (0)  wrap   0  wrap      0      0

   With 1/4 alone, the min_size-only mount loses its reservation, and
   in the failed-mmap case three huge pages stay reserved after umount,
   presumably because the subpool is never freed.

So it looks to me like 1/4 shouldn't go anywhere without 2/4 and 3/4.
I haven't tested older stable trees. With all four patches applied,
all the cases above give the expected values, and so does the
partial-truncate case from the 1/4 changelog (Rsvd drops to 0 after
the truncate instead of staying at 1).

I'm happy to rerun these tests on a rebased v4.

[1] https://lore.kernel.org/r/20260923065714.20781-1-kmehltretter@gmail.com/
[2] the scenario from Jinmeng's changelog:
    https://lore.kernel.org/20260907132055.26696-1-zhoujinmeng@bytedance.com

Thanks,
Karl