[PATCH 37/46] mm: fix deadlock of mremap() against UFFDIO_MOVE
David Hildenbrand (Arm)
david at kernel.org
Mon Oct 26 16:17:57 PDT 2026
From: Chris Mason <mason at kernel.org>
With per-VMA locks, UFFDIO_MOVE can run concurrently with mremap()
on different VMAs in the same mm. Those VMAs can share page tables, so
both operations can acquire the same pair of page-table locks.
UFFDIO_MOVE orders the locks by address, while mremap() takes the source
lock first. These conflicting orders can deadlock; mremap() holding
mmap_lock for writing no longer helps.
Move double_pt_lock() and double_pt_unlock() to mm/internal.h and use
them for all page-table moves in mremap(). Preserve the source entry
checks after acquiring both locks. Remove the comments claiming that
exclusive mmap_lock makes lock ordering irrelevant.
In move_ptes(), make sure to just recheck source and destination PMD
after taking the locks and drop the overly-long comment.
Fix generated by an LLM and sent by Chris Mason in private.
Fixes: 867a43a34ff8 ("userfaultfd: use per-vma locks in userfaultfd operations")
Cc: stable at vger.kernel.org
Signed-off-by: Chris Mason <mason at kernel.org>
[ david: use new helpers consistently; simplify description and comments;
move_ptes() fixup ]
Signed-off-by: David Hildenbrand (Arm) <david at kernel.org>
---
include/linux/userfaultfd_k.h | 2 -
mm/huge_memory.c | 19 ++++------
mm/hugetlb.c | 14 ++-----
mm/internal.h | 28 ++++++++++++++
mm/mremap.c | 87 +++++++++++++++++--------------------------
mm/userfaultfd.c | 27 --------------
6 files changed, 72 insertions(+), 105 deletions(-)
diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h
index a14b8a9ffb7b1..565e98f342dc8 100644
--- a/include/linux/userfaultfd_k.h
+++ b/include/linux/userfaultfd_k.h
@@ -153,8 +153,6 @@ extern int mrwprotect_range(struct userfaultfd_ctx *ctx, unsigned long start,
unsigned long len, bool enable_rwp);
/* move_pages */
-void double_pt_lock(spinlock_t *ptl1, spinlock_t *ptl2);
-void double_pt_unlock(spinlock_t *ptl1, spinlock_t *ptl2);
int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pmd_t dst_pmdval,
struct vm_area_struct *dst_vma,
struct vm_area_struct *src_vma,
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 1b50cc2b35351..08e58cda7d37d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2645,15 +2645,11 @@ bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr,
return false;
}
- /*
- * We don't have to worry about the ordering of src and dst
- * ptlocks because exclusive mmap_lock prevents deadlock.
- */
- old_ptl = __pmd_trans_huge_lock(old_pmd, vma);
- if (old_ptl) {
- new_ptl = pmd_lockptr(mm, new_pmd);
- if (new_ptl != old_ptl)
- spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
+ old_ptl = pmd_lockptr(mm, old_pmd);
+ new_ptl = pmd_lockptr(mm, new_pmd);
+
+ double_pt_lock(old_ptl, new_ptl);
+ if (likely(pmd_is_huge(*old_pmd))) {
pmd = pmdp_huge_get_and_clear(mm, old_addr, old_pmd);
if (pmd_present(pmd))
force_flush = true;
@@ -2682,11 +2678,10 @@ bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr,
set_pmd_at(mm, new_addr, new_pmd, pmd);
if (force_flush)
flush_pmd_tlb_range(vma, old_addr, old_addr + PMD_SIZE);
- if (new_ptl != old_ptl)
- spin_unlock(new_ptl);
- spin_unlock(old_ptl);
+ double_pt_unlock(old_ptl, new_ptl);
return true;
}
+ double_pt_unlock(old_ptl, new_ptl);
return false;
}
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index da5d04edf7c10..84141527fe73d 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5153,15 +5153,9 @@ static void move_huge_pte(struct vm_area_struct *vma, unsigned long old_addr,
spinlock_t *src_ptl, *dst_ptl;
pte_t pte;
- dst_ptl = huge_pte_lock(h, mm, dst_pte);
+ dst_ptl = huge_pte_lockptr(h, mm, dst_pte);
src_ptl = huge_pte_lockptr(h, mm, src_pte);
-
- /*
- * We don't have to worry about the ordering of src and dst ptlocks
- * because exclusive mmap_lock (or the i_mmap_lock) prevents deadlock.
- */
- if (src_ptl != dst_ptl)
- spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
+ double_pt_lock(dst_ptl, src_ptl);
pte = huge_ptep_get_and_clear(mm, old_addr, src_pte, sz);
@@ -5190,9 +5184,7 @@ static void move_huge_pte(struct vm_area_struct *vma, unsigned long old_addr,
set_huge_pte_at(mm, new_addr, dst_pte, pte, sz);
}
- if (src_ptl != dst_ptl)
- spin_unlock(src_ptl);
- spin_unlock(dst_ptl);
+ double_pt_unlock(dst_ptl, src_ptl);
}
int move_hugetlb_page_tables(struct vm_area_struct *vma,
diff --git a/mm/internal.h b/mm/internal.h
index b1b0f29d2216e..7e4d88b2fb02c 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -619,6 +619,34 @@ static inline void force_page_cache_readahead(struct address_space *mapping,
force_page_cache_ra(&ractl, nr_to_read);
}
+/*
+ * Take two page table locks at the same level in address order such that
+ * we cannot deadlock.
+ */
+static inline void double_pt_lock(spinlock_t *ptl1, spinlock_t *ptl2)
+ __acquires(ptl1)
+ __acquires(ptl2)
+{
+ if (ptl1 > ptl2)
+ swap(ptl1, ptl2);
+ spin_lock(ptl1);
+ if (ptl1 != ptl2)
+ spin_lock_nested(ptl2, SINGLE_DEPTH_NESTING);
+ else
+ __acquire(ptl2);
+}
+
+static inline void double_pt_unlock(spinlock_t *ptl1, spinlock_t *ptl2)
+ __releases(ptl1)
+ __releases(ptl2)
+{
+ spin_unlock(ptl1);
+ if (ptl1 != ptl2)
+ spin_unlock(ptl2);
+ else
+ __release(ptl2);
+}
+
unsigned find_lock_entries(struct address_space *mapping, pgoff_t *start,
pgoff_t end, struct folio_batch *fbatch, pgoff_t *indices);
unsigned find_get_entries(struct address_space *mapping, pgoff_t *start,
diff --git a/mm/mremap.c b/mm/mremap.c
index 1237718848f5c..70b7264aa6bf5 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -202,7 +202,7 @@ static int move_ptes(struct pagetable_move_control *pmc,
struct mm_struct *mm = vma->vm_mm;
pte_t *old_ptep, *new_ptep;
pte_t old_pte, pte;
- pmd_t dummy_pmdval;
+ pmd_t old_pmdval, new_pmdval;
spinlock_t *old_ptl, *new_ptl;
bool force_flush = false;
unsigned long old_addr = pmc->old_addr;
@@ -234,31 +234,30 @@ static int move_ptes(struct pagetable_move_control *pmc,
if (pmc->need_rmap_locks)
take_rmap_locks(vma);
- /*
- * We don't have to worry about the ordering of src and dst
- * pte locks because exclusive mmap_lock prevents deadlock.
- */
- old_ptep = pte_offset_map_lock(mm, old_pmd, old_addr, &old_ptl);
+ old_ptep = pte_offset_map_rw_nolock(mm, old_pmd, old_addr, &old_pmdval,
+ &old_ptl);
if (!old_ptep) {
err = -EAGAIN;
goto out;
}
- /*
- * Now new_pte is none, so collapse_scan_file() path can not find
- * this by traversing file->f_mapping, so there is no concurrency with
- * retract_page_tables(). In addition, we already hold the exclusive
- * mmap_lock, so this new_pte page is stable, so there is no need to get
- * pmdval and do pmd_same() check.
- */
- new_ptep = pte_offset_map_rw_nolock(mm, new_pmd, new_addr, &dummy_pmdval,
- &new_ptl);
+ new_ptep = pte_offset_map_rw_nolock(mm, new_pmd, new_addr, &new_pmdval,
+ &new_ptl);
if (!new_ptep) {
- pte_unmap_unlock(old_ptep, old_ptl);
+ pte_unmap(old_ptep);
+ err = -EAGAIN;
+ goto out;
+ }
+
+ double_pt_lock(old_ptl, new_ptl);
+ if (unlikely(!pmd_same(old_pmdval, pmdp_get_lockless(old_pmd)) ||
+ !pmd_same(new_pmdval, pmdp_get_lockless(new_pmd)))) {
+ double_pt_unlock(old_ptl, new_ptl);
+ pte_unmap(new_ptep);
+ pte_unmap(old_ptep);
err = -EAGAIN;
goto out;
}
- if (new_ptl != old_ptl)
- spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
+
flush_tlb_batched_pending(vma->vm_mm);
lazy_mmu_mode_enable();
@@ -317,10 +316,9 @@ static int move_ptes(struct pagetable_move_control *pmc,
lazy_mmu_mode_disable();
if (force_flush)
flush_tlb_range(vma, old_end - len, old_end);
- if (new_ptl != old_ptl)
- spin_unlock(new_ptl);
+ double_pt_unlock(old_ptl, new_ptl);
pte_unmap(new_ptep - 1);
- pte_unmap_unlock(old_ptep - 1, old_ptl);
+ pte_unmap(old_ptep - 1);
out:
if (pmc->need_rmap_locks)
drop_rmap_locks(vma);
@@ -395,14 +393,9 @@ static bool move_normal_pmd(struct pagetable_move_control *pmc,
if (WARN_ON_ONCE(!pmd_none(*new_pmd)))
return false;
- /*
- * We don't have to worry about the ordering of src and dst
- * ptlocks because exclusive mmap_lock prevents deadlock.
- */
- old_ptl = pmd_lock(mm, old_pmd);
+ old_ptl = pmd_lockptr(mm, old_pmd);
new_ptl = pmd_lockptr(mm, new_pmd);
- if (new_ptl != old_ptl)
- spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
+ double_pt_lock(old_ptl, new_ptl);
pmd = *old_pmd;
@@ -418,9 +411,7 @@ static bool move_normal_pmd(struct pagetable_move_control *pmc,
pmd_populate(mm, new_pmd, pmd_pgtable(pmd));
flush_tlb_range(vma, pmc->old_addr, pmc->old_addr + PMD_SIZE);
out_unlock:
- if (new_ptl != old_ptl)
- spin_unlock(new_ptl);
- spin_unlock(old_ptl);
+ double_pt_unlock(old_ptl, new_ptl);
return res;
}
@@ -452,14 +443,10 @@ static bool move_normal_pud(struct pagetable_move_control *pmc,
if (WARN_ON_ONCE(!pud_none(*new_pud)))
return false;
- /*
- * We don't have to worry about the ordering of src and dst
- * ptlocks because exclusive mmap_lock prevents deadlock.
- */
- old_ptl = pud_lock(mm, old_pud);
+ old_ptl = pud_lockptr(mm, old_pud);
new_ptl = pud_lockptr(mm, new_pud);
- if (new_ptl != old_ptl)
- spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
+
+ double_pt_lock(old_ptl, new_ptl);
pud = *old_pud;
if (unlikely(pud_none(pud)))
@@ -473,9 +460,7 @@ static bool move_normal_pud(struct pagetable_move_control *pmc,
pud_populate(mm, new_pud, pud_pgtable(pud));
flush_tlb_range(vma, pmc->old_addr, pmc->old_addr + PUD_SIZE);
out_unlock:
- if (new_ptl != old_ptl)
- spin_unlock(new_ptl);
- spin_unlock(old_ptl);
+ double_pt_unlock(old_ptl, new_ptl);
return true;
}
@@ -503,16 +488,14 @@ static bool move_huge_pud(struct pagetable_move_control *pmc,
if (WARN_ON_ONCE(!pud_none(*new_pud)))
return false;
- /*
- * We don't have to worry about the ordering of src and dst
- * ptlocks because exclusive mmap_lock prevents deadlock.
- */
- old_ptl = __pud_trans_huge_lock(old_pud, vma);
- if (unlikely(!old_ptl))
- return false;
+ old_ptl = pud_lockptr(mm, old_pud);
new_ptl = pud_lockptr(mm, new_pud);
- if (new_ptl != old_ptl)
- spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
+
+ double_pt_lock(old_ptl, new_ptl);
+ if (unlikely(!pud_trans_huge(*old_pud))) {
+ double_pt_unlock(old_ptl, new_ptl);
+ return false;
+ }
/* Clear the pud */
pud = pudp_huge_get_and_clear(mm, pmc->old_addr, old_pud);
@@ -523,9 +506,7 @@ static bool move_huge_pud(struct pagetable_move_control *pmc,
/* mark soft_ditry when we add pud level soft dirty support */
set_pud_at(mm, pmc->new_addr, new_pud, pud);
flush_pud_tlb_range(vma, pmc->old_addr, pmc->old_addr + HPAGE_PUD_SIZE);
- if (new_ptl != old_ptl)
- spin_unlock(new_ptl);
- spin_unlock(old_ptl);
+ double_pt_unlock(old_ptl, new_ptl);
return true;
}
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 710e913c73bfa..76de84510340c 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1206,33 +1206,6 @@ int mrwprotect_range(struct userfaultfd_ctx *ctx, unsigned long start,
return 0;
}
-void double_pt_lock(spinlock_t *ptl1,
- spinlock_t *ptl2)
- __acquires(ptl1)
- __acquires(ptl2)
-{
- if (ptl1 > ptl2)
- swap(ptl1, ptl2);
- /* lock in virtual address order to avoid lock inversion */
- spin_lock(ptl1);
- if (ptl1 != ptl2)
- spin_lock_nested(ptl2, SINGLE_DEPTH_NESTING);
- else
- __acquire(ptl2);
-}
-
-void double_pt_unlock(spinlock_t *ptl1,
- spinlock_t *ptl2)
- __releases(ptl1)
- __releases(ptl2)
-{
- spin_unlock(ptl1);
- if (ptl1 != ptl2)
- spin_unlock(ptl2);
- else
- __release(ptl2);
-}
-
static inline bool is_pte_pages_stable(pte_t *dst_pte, pte_t *src_pte,
pte_t orig_dst_pte, pte_t orig_src_pte,
pmd_t *dst_pmd, pmd_t dst_pmdval)
--
2.43.0
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