+ mm-hugetlb-fix-copy_hugetlb_page_range-to-use-pt_share_count.patch added to mm-hotfixes-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-hugetlb-fix-copy_hugetlb_page_range-to-use-pt_share_count.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-hugetlb-fix-copy_hugetlb_page_range-to-use-pt_share_count.patch

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

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------------------------------------------------------
From: Jane Chu <jane.chu@oracle.com>
Subject: mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count
Date: Tue, 9 Sep 2025 12:43:57 -0600

commit 59d9094df3d79 introduced ->pt_share_count dedicated to hugetlb PMD
share count tracking, but omitted fixing copy_hugetlb_page_range(),
leaving the function relying on page_count() for tracking that no longer
works.

When lazy page table copy for hugetlb is disabled (commit bcd51a3c679d),
fork()'ing with hugetlb PMD sharing quickly locks up -

[  239.446559] watchdog: BUG: soft lockup - CPU#75 stuck for 27s!
[  239.446611] RIP: 0010:native_queued_spin_lock_slowpath+0x7e/0x2e0
[  239.446631] Call Trace:
[  239.446633]  <TASK>
[  239.446636]  _raw_spin_lock+0x3f/0x60
[  239.446639]  copy_hugetlb_page_range+0x258/0xb50
[  239.446645]  copy_page_range+0x22b/0x2c0
[  239.446651]  dup_mmap+0x3e2/0x770
[  239.446654]  dup_mm.constprop.0+0x5e/0x230
[  239.446657]  copy_process+0xd17/0x1760
[  239.446660]  kernel_clone+0xc0/0x3e0
[  239.446661]  __do_sys_clone+0x65/0xa0
[  239.446664]  do_syscall_64+0x82/0x930
[  239.446668]  ? count_memcg_events+0xd2/0x190
[  239.446671]  ? syscall_trace_enter+0x14e/0x1f0
[  239.446676]  ? syscall_exit_work+0x118/0x150
[  239.446677]  ? arch_exit_to_user_mode_prepare.constprop.0+0x9/0xb0
[  239.446681]  ? clear_bhb_loop+0x30/0x80
[  239.446684]  ? clear_bhb_loop+0x30/0x80
[  239.446686]  entry_SYSCALL_64_after_hwframe+0x76/0x7e

There are two options to resolve the potential latent issue:
  1. remove the PMD sharing awareness from copy_hugetlb_page_range(),
  2. fix it.
This patch opts for the second option.

Link: https://lkml.kernel.org/r/20250909184357.569259-1-jane.chu@oracle.com
Fixes: 59d9094df3d79 ("mm: hugetlb: independent PMD page table shared
count")
Signed-off-by: Jane Chu <jane.chu@oracle.com>
Cc:
Cc: David Hildenbrand <david@redhat.com>
Cc: Jane Chu <jane.chu@oracle.com>
Cc: Liu Shixin <liushixin2@huawei.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/hugetlb.c |   13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

--- a/mm/hugetlb.c~mm-hugetlb-fix-copy_hugetlb_page_range-to-use-pt_share_count
+++ a/mm/hugetlb.c
@@ -5594,18 +5594,13 @@ int copy_hugetlb_page_range(struct mm_st
 			break;
 		}
 
-		/*
-		 * If the pagetables are shared don't copy or take references.
-		 *
-		 * dst_pte == src_pte is the common case of src/dest sharing.
-		 * However, src could have 'unshared' and dst shares with
-		 * another vma. So page_count of ptep page is checked instead
-		 * to reliably determine whether pte is shared.
-		 */
-		if (page_count(virt_to_page(dst_pte)) > 1) {
+#ifdef CONFIG_HUGETLB_PMD_PAGE_TABLE_SHARING
+		/* If the pagetables are shared don't copy or take references. */
+		if (ptdesc_pmd_pts_count(virt_to_ptdesc(dst_pte)) > 0) {
 			addr |= last_addr_mask;
 			continue;
 		}
+#endif
 
 		dst_ptl = huge_pte_lock(h, dst, dst_pte);
 		src_ptl = huge_pte_lockptr(h, src, src_pte);
_

Patches currently in -mm which might be from jane.chu@oracle.com are

mm-hugetlb-fix-copy_hugetlb_page_range-to-use-pt_share_count.patch