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mm/huge_memory: properly handle refcounted PUD mappings on fork

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copy_huge_pud() copies device-DAX entries without taking a folio reference
or updating mapcounts and the child's RSS.

For example, MADV_GUARD_INSTALL sets VM_MAYBE_GUARD on a device-DAX VMA,
forcing copy_page_range() to traverse the whole VMA on fork. Any remaining
huge PUDs are then copied without accounting for the new mappings.
Unmapping them in the child decrements accounting that was never added.

Use vm_normal_page_pud() to identify refcounted mappings. Take a folio
reference and add the child's PUD rmap and RSS accounting before installing
the copied entry. Pass src and dst VMAs consistently instead of src and
dst MM. Move the TODO.

Fixes: aed877c2b4 ("device/dax: properly refcount device dax pages when mapping")
Cc: stable@vger.kernel.org
Reviewed-by: Zi Yan <ziy@nvidia.com>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Lance Yang <lance.yang@linux.dev>
Tested-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Link: https://patch.msgid.link/20261031-fixes_core-v3-11-d6437171ecc3@kernel.org
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
This commit is contained in:
David Hildenbrand (Arm) 2026-10-31 10:39:40 +01:00
parent f09171b67e
commit d90bdc02e3
3 changed files with 22 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
include/linux/huge_mm.h
View File

@ -12,9 +12,8 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long addr,
struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma);
bool huge_pmd_set_accessed(struct vm_fault *vmf);
int copy_huge_pud(struct mm_struct *dst_mm, struct mm_struct *src_mm,
pud_t *dst_pud, pud_t *src_pud, unsigned long addr,
struct vm_area_struct *vma);
int copy_huge_pud(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
pud_t *dst_pud, pud_t *src_pud, unsigned long addr);
#ifdef CONFIG_HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
void huge_pud_set_accessed(struct vm_fault *vmf, pud_t orig_pud);

27
mm/huge_memory.c
View File

@ -2072,11 +2072,13 @@ void touch_pud(struct vm_area_struct *vma, unsigned long addr,
update_mmu_cache_pud(vma, addr, pud);
}
int copy_huge_pud(struct mm_struct *dst_mm, struct mm_struct *src_mm,
pud_t *dst_pud, pud_t *src_pud, unsigned long addr,
struct vm_area_struct *vma)
int copy_huge_pud(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
pud_t *dst_pud, pud_t *src_pud, unsigned long addr)
{
struct mm_struct *dst_mm = dst_vma->vm_mm;
struct mm_struct *src_mm = src_vma->vm_mm;
spinlock_t *dst_ptl, *src_ptl;
struct page *page;
pud_t pud;
int ret;
@ -2089,11 +2091,20 @@ int copy_huge_pud(struct mm_struct *dst_mm, struct mm_struct *src_mm,
if (unlikely(!pud_trans_huge(pud)))
goto out_unlock;
/*
* TODO: once we support anonymous pages, use
* folio_try_dup_anon_rmap_*() and split if duplicating fails.
*/
if (vma_is_cow_mapping(vma) && pud_write(pud)) {
page = vm_normal_page_pud(src_vma, addr, pud);
if (page) {
struct folio *folio = page_folio(page);
folio_get(folio);
/*
* TODO: once we support anonymous pages, use
* folio_try_dup_anon_rmap_*() and split if duplicating fails.
*/
folio_add_file_rmap_pud(folio, page, dst_vma);
add_mm_counter(dst_mm, mm_counter_file(folio), HPAGE_PUD_NR);
}
if (vma_is_cow_mapping(src_vma) && pud_write(pud)) {
pudp_set_wrprotect(src_mm, addr, src_pud);
pud = pud_wrprotect(pud);
}

4
mm/memory.c
View File

@ -1463,7 +1463,6 @@ copy_pud_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
unsigned long end)
{
struct mm_struct *dst_mm = dst_vma->vm_mm;
struct mm_struct *src_mm = src_vma->vm_mm;
pud_t *src_pud, *dst_pud;
unsigned long next;
@ -1477,8 +1476,7 @@ copy_pud_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma,
int err;
VM_BUG_ON_VMA(next-addr != HPAGE_PUD_SIZE, src_vma);
err = copy_huge_pud(dst_mm, src_mm,
dst_pud, src_pud, addr, src_vma);
err = copy_huge_pud(dst_vma, src_vma, dst_pud, src_pud, addr);
if (err == -ENOMEM)
return -ENOMEM;
if (!err)