The patch titled Subject: mm/hugetlb: fix avoid_reserve to allow taking folio from subpool has been added to the -mm mm-unstable branch. Its filename is mm-hugetlb-fix-avoid_reserve-to-allow-taking-folio-from-subpool.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Peter Xu peterx@redhat.com Subject: mm/hugetlb: fix avoid_reserve to allow taking folio from subpool Date: Tue, 7 Jan 2025 15:39:56 -0500

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 04f2cbe35699 ("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 d85f69b0b533 ("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: d85f69b0b533 ("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 Cc: Breno Leitao leitao@debian.org Cc: Muchun Song muchun.song@linux.dev Cc: Naoya Horiguchi nao.horiguchi@gmail.com Cc: Oscar Salvador osalvador@suse.de 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 ---

mm/hugetlb.c | 22 +++------------------- 1 file changed, 3 insertions(+), 19 deletions(-)

--- a/mm/hugetlb.c~mm-hugetlb-fix-avoid_reserve-to-allow-taking-folio-from-subpool +++ a/mm/hugetlb.c @@ -1394,8 +1394,7 @@ static unsigned long available_huge_page

static struct folio *dequeue_hugetlb_folio_vma(struct hstate *h, struct vm_area_struct *vma, - unsigned long address, int avoid_reserve, - long chg) + unsigned long address, long chg) { struct folio *folio = NULL; struct mempolicy *mpol; @@ -1411,10 +1410,6 @@ static struct folio *dequeue_hugetlb_fol if (!vma_has_reserves(vma, chg) && !available_huge_pages(h)) goto err;

- /* If reserves cannot be used, ensure enough pages are in the pool */ - if (avoid_reserve && !available_huge_pages(h)) - goto err; - gfp_mask = htlb_alloc_mask(h); nid = huge_node(vma, address, gfp_mask, &mpol, &nodemask);

@@ -1430,7 +1425,7 @@ static struct folio *dequeue_hugetlb_fol folio = dequeue_hugetlb_folio_nodemask(h, gfp_mask, nid, nodemask);

- if (folio && !avoid_reserve && vma_has_reserves(vma, chg)) { + if (folio && vma_has_reserves(vma, chg)) { folio_set_hugetlb_restore_reserve(folio); h->resv_huge_pages--; } @@ -3047,17 +3042,6 @@ struct folio *alloc_hugetlb_folio(struct gbl_chg = hugepage_subpool_get_pages(spool, 1); if (gbl_chg < 0) goto out_end_reservation; - - /* - * Even though there was no reservation in the region/reserve - * map, there could be reservations associated with the - * subpool that can be used. This would be indicated if the - * return value of hugepage_subpool_get_pages() is zero. - * However, if avoid_reserve is specified we still avoid even - * the subpool reservations. - */ - if (avoid_reserve) - gbl_chg = 1; }

/* If this allocation is not consuming a reservation, charge it now. @@ -3080,7 +3064,7 @@ struct folio *alloc_hugetlb_folio(struct * from the global free pool (global change). gbl_chg == 0 indicates * a reservation exists for the allocation. */ - folio = dequeue_hugetlb_folio_vma(h, vma, addr, avoid_reserve, gbl_chg); + folio = dequeue_hugetlb_folio_vma(h, vma, addr, gbl_chg); if (!folio) { spin_unlock_irq(&hugetlb_lock); folio = alloc_buddy_hugetlb_folio_with_mpol(h, vma, addr); _

Patches currently in -mm which might be from peterx@redhat.com are

mm-hugetlb-fix-avoid_reserve-to-allow-taking-folio-from-subpool.patch mm-hugetlb-stop-using-avoid_reserve-flag-in-fork.patch mm-hugetlb-rename-avoid_reserve-to-cow_from_owner.patch mm-hugetlb-clean-up-map-global-resv-accounting-when-allocate.patch mm-hugetlb-simplify-vma_has_reserves.patch mm-hugetlb-drop-vma_has_reserves.patch mm-hugetlb-unify-restore-reserve-accounting-for-new-allocations.patch