The patch titled
Subject: mm/hugetlb: stop using avoid_reserve flag in fork()
has been added to the -mm mm-unstable branch. Its filename is
mm-hugetlb-stop-using-avoid_reserve-flag-in-fork.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-hugetlb-stop-using-avoid_reserve-flag-in-fork.patch
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@xxxxxxxxxx>
Subject: mm/hugetlb: stop using avoid_reserve flag in fork()
Date: Tue, 7 Jan 2025 15:39:57 -0500
When fork() and stumble on top of a dma-pinned hugetlb private page, CoW
must happen during fork() to guarantee dma coherency.
In this specific path, hugetlb pages need to be allocated for the child
process. Stop using avoid_reserve=1 flag here: it's not required to be
used here, as dest_vma (which is destined to be a MAP_PRIVATE hugetlb vma)
will have no private vma resv map, and that will make sure it won't be
able to use a vma reservation later.
No functional change intended with this change. Said that, it's still
wanted to do this, so as to reduce the usage of avoid_reserve to the only
one user, which is also why this flag was introduced initially in commit
04f2cbe35699 ("hugetlb: guarantee that COW faults for a process that
called mmap(MAP_PRIVATE) on hugetlbfs will succeed"). I don't see whoever
else should set it at all.
Further patch will clean up resv accounting based on this.
Link: https://lkml.kernel.org/r/20250107204002.2683356-3-peterx@xxxxxxxxxx
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
Cc: Ackerley Tng <ackerleytng@xxxxxxxxxx>
Cc: Breno Leitao <leitao@xxxxxxxxxx>
Cc: Muchun Song <muchun.song@xxxxxxxxx>
Cc: Naoya Horiguchi <nao.horiguchi@xxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Roman Gushchin <roman.gushchin@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/hugetlb.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/mm/hugetlb.c~mm-hugetlb-stop-using-avoid_reserve-flag-in-fork
+++ a/mm/hugetlb.c
@@ -5369,7 +5369,7 @@ again:
spin_unlock(src_ptl);
spin_unlock(dst_ptl);
/* Do not use reserve as it's private owned */
- new_folio = alloc_hugetlb_folio(dst_vma, addr, 1);
+ new_folio = alloc_hugetlb_folio(dst_vma, addr, 0);
if (IS_ERR(new_folio)) {
folio_put(pte_folio);
ret = PTR_ERR(new_folio);
_
Patches currently in -mm which might be from peterx@xxxxxxxxxx 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
