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Peter Xu 58db7c5fbe mm/hugetlb: fix avoid_reserve to allow taking folio from subpool 
Patch series "mm/hugetlb: Refactor hugetlb allocation resv accounting",
v2.

This is a follow up on Ackerley's series here as replacement:

https://lore.kernel.org/r/cover.1728684491.git.ackerleytng@google.com

The goal of this series is to cleanup hugetlb resv accounting, especially
during folio allocation, to decouple a few things:

  - Hugetlb folios v.s. Hugetlbfs: IOW, the hope is in the future hugetlb
    folios can be allocated completely without hugetlbfs.

  - Decouple VMA v.s. hugetlb folio allocations: allocating a hugetlb folio
    should not always require a hugetlbfs VMA.  For example, either it got
    allocated from the inode level (see hugetlbfs_fallocate() where it used
    a pesudo VMA for allocation), or it can be allocated by other kernel
    subsystems.

It paves way for other users to allocate hugetlb folios out of either
system reservations, or subpools (instead of hugetlbfs, as a file system).
For longer term, this prepares hugetlb as a separate concept versus
hugetlbfs, so that hugetlb folios can be allocated by not only hugetlbfs
and other things.

Tests I've done:

- I had a reproducer in patch 1 for the bug I found, this will start to
  work after patch 1 or the whole set applied.

- Hugetlb regression tests (on x86_64 2MBs), includes:

  - All vmtests on hugetlbfs

  - libhugetlbfs test suite (which may fail some tests, but no new failures
    will be introduced by this series, so all such failures happen before
    this series so shouldn't be relevant).


This patch (of 7):

Since commit 04f2cbe356 ("hugetlb: guarantee that COW faults for a
process that called mmap(MAP_PRIVATE) on hugetlbfs will succeed"),
avoid_reserve was introduced for a special case of CoW on hugetlb private
mappings, and only if the owner VMA is trying to allocate yet another
hugetlb folio that is not reserved within the private vma reserved map.

Later on, in commit d85f69b0b5 ("mm/hugetlb: alloc_huge_page handle
areas hole punched by fallocate"), alloc_huge_page() enforced to not
consume any global reservation as long as avoid_reserve=true.  This
operation doesn't look correct, because even if it will enforce the
allocation to not use global reservation at all, it will still try to take
one reservation from the spool (if the subpool existed).  Then since the
spool reserved pages take from global reservation, it'll also take one
reservation globally.

Logically it can cause global reservation to go wrong.

I wrote a reproducer below, trigger this special path, and every run of
such program will cause global reservation count to increment by one, until
it hits the number of free pages:

  #define _GNU_SOURCE             /* See feature_test_macros(7) */
  #include <stdio.h>
  #include <fcntl.h>
  #include <errno.h>
  #include <unistd.h>
  #include <stdlib.h>
  #include <sys/mman.h>

  #define  MSIZE  (2UL << 20)

  int main(int argc, char *argv[])
  {
      const char *path;
      int *buf;
      int fd, ret;
      pid_t child;

      if (argc < 2) {
          printf("usage: %s <hugetlb_file>\n", argv[0]);
          return -1;
      }

      path = argv[1];

      fd = open(path, O_RDWR | O_CREAT, 0666);
      if (fd < 0) {
          perror("open failed");
          return -1;
      }

      ret = fallocate(fd, 0, 0, MSIZE);
      if (ret != 0) {
          perror("fallocate");
          return -1;
      }

      buf = mmap(NULL, MSIZE, PROT_READ|PROT_WRITE,
                 MAP_PRIVATE, fd, 0);

      if (buf == MAP_FAILED) {
          perror("mmap() failed");
          return -1;
      }

      /* Allocate a page */
      *buf = 1;

      child = fork();
      if (child == 0) {
          /* child doesn't need to do anything */
          exit(0);
      }

      /* Trigger CoW from owner */
      *buf = 2;

      munmap(buf, MSIZE);
      close(fd);
      unlink(path);

      return 0;
  }

It can only reproduce with a sub-mount when there're reserved pages on the
spool, like:

  # sysctl vm.nr_hugepages=128
  # mkdir ./hugetlb-pool
  # mount -t hugetlbfs -o min_size=8M,pagesize=2M none ./hugetlb-pool

Then run the reproducer on the mountpoint:

  # ./reproducer ./hugetlb-pool/test

Fix it by taking the reservation from spool if available.  In general,
avoid_reserve is IMHO more about "avoid vma resv map", not spool's.

I copied stable, however I have no intention for backporting if it's not a
clean cherry-pick, because private hugetlb mapping, and then fork() on top
is too rare to hit.

Link: https://lkml.kernel.org/r/20250107204002.2683356-1-peterx@redhat.com
Link: https://lkml.kernel.org/r/20250107204002.2683356-2-peterx@redhat.com
Fixes: d85f69b0b5 ("mm/hugetlb: alloc_huge_page handle areas hole punched by fallocate")
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Tested-by: Ackerley Tng <ackerleytng@google.com>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Breno Leitao <leitao@debian.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
2025-01-25 20:22:30 -08:00
arch mm: introduce ctor/dtor at PGD level 2025-01-25 20:22:24 -08:00
block block: avoid to reuse hctx not removed from cpuhp callback list 2024-12-18 07:25:37 -07:00
certs sign-file,extract-cert: use pkcs11 provider for OPENSSL MAJOR >= 3 2024-09-20 19:52:48 +03:00
crypto crypto: rsassa-pkcs1 - Copy source data for SG list 2024-12-10 13:34:05 +08:00
Documentation Docs/mm/damon/design: update for removal of DAMON debugfs interface 2025-01-25 20:22:29 -08:00
drivers mm/fake-numa: allow later numa node hotplug 2025-01-25 20:22:29 -08:00
fs lib/list_debug.c: add object information in case of invalid object 2025-01-25 20:22:23 -08:00
include mm/memmap: prevent double scanning of memmap by kmemleak 2025-01-25 20:22:30 -08:00
init - The series "resource: A couple of cleanups" from Andy Shevchenko 2024-11-25 16:09:48 -08:00
io_uring io_uring/kbuf: use pre-committed buffer address for non-pollable file 2025-01-03 09:38:37 -07:00
ipc - The series "resource: A couple of cleanups" from Andy Shevchenko 2024-11-25 16:09:48 -08:00
kernel mm: convert mm_lock_seq to a proper seqcount 2025-01-13 22:40:50 -08:00
lib lib/list_debug.c: add object information in case of invalid object 2025-01-25 20:22:23 -08:00
LICENSES LICENSES: add 0BSD license text 2024-09-01 20:43:24 -07:00
mm mm/hugetlb: fix avoid_reserve to allow taking folio from subpool 2025-01-25 20:22:30 -08:00
net Nothing major here. Over the last two weeks we gathered only around 2025-01-03 14:36:54 -08:00
rust workqueue: Fixes for v6.13-rc5 2025-01-03 15:03:56 -08:00
samples samples/damon/prcl: implement schemes setup 2025-01-13 22:40:57 -08:00
scripts scripts/decode_stacktrace.sh: fix decoding of lines with an additional info 2025-01-12 19:03:34 -08:00
security selinux/stable-6.13 PR 20241217 2024-12-18 12:10:15 -08:00
sound sound fixes for 6.13-rc6 2025-01-03 10:54:51 -08:00
tools kunit: configs: remove configs for DAMON debugfs interface tests 2025-01-25 20:22:29 -08:00
usr kbuild: Drop support for include/asm-<arch> in headers_check.pl 2024-12-21 11:43:17 +09:00
virt VFIO updates for v6.13 2024-11-27 12:57:03 -08:00
.clang-format clang-format: Update with v6.11-rc1's for_each macro list 2024-08-02 13:20:31 +02:00
.clippy.toml rust: enable Clippy's check-private-items 2024-10-07 21:39:57 +02:00
.cocciconfig
.editorconfig .editorconfig: remove trim_trailing_whitespace option 2024-06-13 16:47:52 +02:00
.get_maintainer.ignore MAINTAINERS: Retire Ralf Baechle 2024-11-12 15:48:59 +01:00
.gitattributes .gitattributes: set diff driver for Rust source code files 2023-05-31 17:48:25 +02:00
.gitignore Kbuild updates for v6.13 2024-11-30 13:41:50 -08:00
.mailmap mailmap: modify the entry for Mathieu Othacehe 2024-12-30 17:59:08 -08:00
.rustfmt.toml rust: add .rustfmt.toml 2022-09-28 09:02:20 +02:00
COPYING COPYING: state that all contributions really are covered by this file 2020-02-10 13:32:20 -08:00
CREDITS cgroup: Changes for v6.13 2024-11-20 09:54:49 -08:00
Kbuild Kbuild updates for v6.1 2022-10-10 12:00:45 -07:00
Kconfig kbuild: ensure full rebuild when the compiler is updated 2020-05-12 13:28:33 +09:00
MAINTAINERS MAINTAINERS: update MEMORY MAPPING section 2025-01-13 22:40:59 -08:00
Makefile Linux 6.13-rc6 2025-01-05 14:13:40 -08:00
README README: Fix spelling 2024-03-18 03:36:32 -06:00

README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.