FAILED: patch "[PATCH] mm/memory: fix PMD/PUD checks in follow_pfnmap_start()" failed to apply to 7.0-stable tree

The patch below does not apply to the 7.0-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-7.0.y
git checkout FETCH_HEAD
git cherry-pick -x 26e7888a0c89e36332c1e897e4887f69e1e9c751
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026050345-curtsy-basis-8bb5@gregkh' --subject-prefix 'PATCH 7.0.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 26e7888a0c89e36332c1e897e4887f69e1e9c751 Mon Sep 17 00:00:00 2001
From: "David Hildenbrand (Arm)" <[email protected]>
Date: Mon, 23 Mar 2026 21:20:18 +0100
Subject: [PATCH] mm/memory: fix PMD/PUD checks in follow_pfnmap_start()

follow_pfnmap_start() suffers from two problems:

(1) We are not re-fetching the pmd/pud after taking the PTL

Therefore, we are not properly stabilizing what the lock actually protects. If there is concurrent zapping, we would indicate to the caller that we found an entry, however, that entry might already have been invalidated, or contain a different PFN after taking the lock.

Properly use pmdp_get() / pudp_get() after taking the lock.

(2) pmd_leaf() / pud_leaf() are not well defined on non-present entries

pmd_leaf()/pud_leaf() could wrongly trigger on non-present entries.

There is no real guarantee that pmd_leaf()/pud_leaf() returns something reasonable on non-present entries. Most architectures indeed either perform a present check or make it work by smart use of flags.

However, for example loongarch checks the _PAGE_HUGE flag in pmd_leaf(), and always sets the _PAGE_HUGE flag in __swp_entry_to_pmd(). Whereby pmd_trans_huge() explicitly checks pmd_present(), pmd_leaf() does not do that.

Let's check pmd_present()/pud_present() before assuming "the is a present PMD leaf" when spotting pmd_leaf()/pud_leaf(), like other page table handling code that traverses user page tables does.

Given that non-present PMD entries are likely rare in VM_IO|VM_PFNMAP, (1) is likely more relevant than (2). It is questionable how often (1) would actually trigger, but let's CC stable to be sure.

This was found by code inspection.

index 6d54e5ec82f2..425e852a2eb7 100644
@@ -6824,11 +6824,16 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args)
 
 	pudp = pud_offset(p4dp, address);
 	pud = pudp_get(pudp);
-	if (pud_none(pud))
+	if (!pud_present(pud))
 		goto out;
 	if (pud_leaf(pud)) {
 		lock = pud_lock(mm, pudp);
-		if (!unlikely(pud_leaf(pud))) {
+		pud = pudp_get(pudp);
+
+		if (unlikely(!pud_present(pud))) {
+			spin_unlock(lock);
+			goto out;
+		} else if (unlikely(!pud_leaf(pud))) {
 			spin_unlock(lock);
 			goto retry;
 		}

@@ -6840,9 +6845,16 @@ int follow_pfnmap_start(struct follow_pfnmap_args *args)
 
 	pmdp = pmd_offset(pudp, address);
 	pmd = pmdp_get_lockless(pmdp);
+	if (!pmd_present(pmd))
+		goto out;
 	if (pmd_leaf(pmd)) {
 		lock = pmd_lock(mm, pmdp);
-		if (!unlikely(pmd_leaf(pmd))) {
+		pmd = pmdp_get(pmdp);
+
+		if (unlikely(!pmd_present(pmd))) {
+			spin_unlock(lock);
+			goto out;
+		} else if (unlikely(!pmd_leaf(pmd))) {
 			spin_unlock(lock);
 			goto retry;
 		}