[RFC PATCH 6/6] mm: Change dup_mmap() recovery

[Lorenzo Stoakes]

On Mon, Aug 18, 2025 at 04:12:59PM +0100, Lorenzo Stoakes wrote:
> It seems your series clashes with my mm flags change, and this patch is a
> victim, it won't apply against mm-new at all :(

I rebased it/fixed mm flags stuff locally (manually) so I could test, so I
thought - why not actually share that here to make your life easier :P

It's attached, if I didn't mess up with neomutt (big IF)

Cheers, Lorenzo
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>From 16b7f9d7042de8cb7d6f86b815d3075cedcf1132 Mon Sep 17 00:00:00 2001
From: "Liam R. Howlett" <Liam.Howlett at oracle.com>
Date: Mon, 18 Aug 2025 16:19:46 +0100
Subject: [PATCH RFC 6/6] mm: change dup_mmap() recovery

When the dup_mmap() fails during the vma duplication or setup, don't
write the XA_ZERO entry in the vma tree.  Instead, destroy the tree and
free the new resources, leaving an empty vma tree.

Using XA_ZERO introduced races where the vma could be found between
dup_mmap() dropping all locks and exit_mmap() taking the locks.  The
race can occur because the mm can be reached through the other trees
via successfully copied vmas and other methods such as the swapoff code.

XA_ZERO was marking the location to stop vma removal and pagetable
freeing.  The newly created arguments to the unmap_vmas() and
free_pgtables() serve this function.

Replacing the XA_ZERO entry use with the new argument list also means
the checks for xa_is_zero() are no longer necessary so these are also
removed.

Signed-off-by: Liam R. Howlett <Liam.Howlett at oracle.com>
---
 mm/memory.c |  6 +-----
 mm/mmap.c   | 39 ++++++++++++++++++++++++++++-----------
 2 files changed, 29 insertions(+), 16 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 9f584c5e4f43..218b496dc9ad 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -387,8 +387,6 @@ void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
 		 * be 0.  This will underflow and is okay.
 		 */
 		next = mas_find(mas, tree_max - 1);
-		if (unlikely(xa_is_zero(next)))
-			next = NULL;

 		/*
 		 * Hide vma from rmap and truncate_pagecache before freeing
@@ -407,8 +405,6 @@ void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
 		while (next && next->vm_start <= vma->vm_end + PMD_SIZE) {
 			vma = next;
 			next = mas_find(mas, tree_max - 1);
-			if (unlikely(xa_is_zero(next)))
-				next = NULL;
 			if (mm_wr_locked)
 				vma_start_write(vma);
 			unlink_anon_vmas(vma);
@@ -2106,7 +2102,7 @@ void unmap_vmas(struct mmu_gather *tlb, struct ma_state *mas,
 				 mm_wr_locked);
 		hugetlb_zap_end(vma, &details);
 		vma = mas_find(mas, tree_end - 1);
-	} while (vma && likely(!xa_is_zero(vma)));
+	} while (vma);
 	mmu_notifier_invalidate_range_end(&range);
 }

diff --git a/mm/mmap.c b/mm/mmap.c
index b49df82817ba..ecfeb1987156 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1288,7 +1288,7 @@ void exit_mmap(struct mm_struct *mm)
 	arch_exit_mmap(mm);

 	vma = vma_next(&vmi);
-	if (!vma || unlikely(xa_is_zero(vma))) {
+	if (!vma) {
 		/* Can happen if dup_mmap() received an OOM */
 		mmap_read_unlock(mm);
 		mmap_write_lock(mm);
@@ -1858,20 +1858,37 @@ __latent_entropy int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
 		ksm_fork(mm, oldmm);
 		khugepaged_fork(mm, oldmm);
 	} else {
+		unsigned long max;

 		/*
-		 * The entire maple tree has already been duplicated. If the
-		 * mmap duplication fails, mark the failure point with
-		 * XA_ZERO_ENTRY. In exit_mmap(), if this marker is encountered,
-		 * stop releasing VMAs that have not been duplicated after this
-		 * point.
+		 * The entire maple tree has already been duplicated, but
+		 * replacing the vmas failed at mpnt (which could be NULL if
+		 * all were allocated but the last vma was not fully set up. Use
+		 * the start address of the failure point to clean up the half
+		 * initialized tree.
 		 */
-		if (mpnt) {
-			mas_set_range(&vmi.mas, mpnt->vm_start, mpnt->vm_end - 1);
-			mas_store(&vmi.mas, XA_ZERO_ENTRY);
-			/* Avoid OOM iterating a broken tree */
-			mm_flags_set(MMF_OOM_SKIP, mm);
+		if (!mm->map_count) {
+			/* zero vmas were written to the new tree. */
+			max = 0;
+		} else if (mpnt) {
+			/* mid-tree failure */
+			max = mpnt->vm_start;
+		} else {
+			/* All vmas were written to the new tree */
+			max = ULONG_MAX;
 		}
+
+		/* Hide mm from oom killer because the memory is being freed */
+		mm_flags_set(MMF_OOM_SKIP, mm);
+		if (max) {
+			vma_iter_set(&vmi, 0);
+			tmp = vma_next(&vmi);
+			flush_cache_mm(mm);
+			unmap_region(&vmi.mas, tmp, 0, max, max, NULL, NULL);
+			charge = tear_down_vmas(mm, &vmi, tmp, max);
+			vm_unacct_memory(charge);
+		}
+		__mt_destroy(&mm->mm_mt);
 		/*
 		 * The mm_struct is going to exit, but the locks will be dropped
 		 * first.  Set the mm_struct as unstable is advisable as it is
--
2.50.1