Commit 14967a9c authored by Jane Chu's avatar Jane Chu Committed by Andrew Morton
Browse files

mm/hugetlb: fix copy_hugetlb_page_range() to use ->pt_share_count 

commit 59d9094d ("mm: hugetlb: independent PMD page table shared
count") 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, that is, revert commit
bcd51a3c ("hugetlb: lazy page table copies in fork()") fork()'ing with
hugetlb PMD sharing quickly lockup -

[  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. warn against PMD sharing in copy_hugetlb_page_range(),
  2. fix it.
This patch opts for the second option.
While at it, simplify the comment, the details are not actually relevant
anymore.

Link: https://lkml.kernel.org/r/20250916004520.1604530-1-jane.chu@oracle.com


Fixes: 59d9094d ("mm: hugetlb: independent PMD page table shared count")
Signed-off-by: default avatarJane Chu <jane.chu@oracle.com>
Reviewed-by: default avatarHarry Yoo <harry.yoo@oracle.com>
Acked-by: default avatarOscar Salvador <osalvador@suse.de>
Acked-by: default avatarDavid Hildenbrand <david@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liu Shixin <liushixin2@huawei.com>
Cc: Muchun Song <muchun.song@linux.dev>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 7b738765

include/linux/mm_types.h

+5 −0
Original line number Diff line number Diff line
@@ -631,6 +631,11 @@ static inline int ptdesc_pmd_pts_count(struct ptdesc *ptdesc) 
{

	return atomic_read(&ptdesc->pt_share_count);

}



static inline bool ptdesc_pmd_is_shared(struct ptdesc *ptdesc)

{

	return !!ptdesc_pmd_pts_count(ptdesc);

}

#else

static inline void ptdesc_pmd_pts_init(struct ptdesc *ptdesc)

{

mm/hugetlb.c

+5 −10
Original line number Diff line number Diff line
@@ -5594,18 +5594,13 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src, 
			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, there is nothing to do */

		if (ptdesc_pmd_is_shared(virt_to_ptdesc(dst_pte))) {

			addr |= last_addr_mask;

			continue;

		}

#endif



		dst_ptl = huge_pte_lock(h, dst, dst_pte);

		src_ptl = huge_pte_lockptr(h, src, src_pte);
@@ -7602,7 +7597,7 @@ int huge_pmd_unshare(struct mm_struct *mm, struct vm_area_struct *vma, 
	hugetlb_vma_assert_locked(vma);

	if (sz != PMD_SIZE)

		return 0;

	if (!ptdesc_pmd_pts_count(virt_to_ptdesc(ptep)))

	if (!ptdesc_pmd_is_shared(virt_to_ptdesc(ptep)))

		return 0;



	pud_clear(pud);