+ fs-proc-task_mmu-reduce-scope-of-lazy-mmu-region.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: fs/proc/task_mmu: reduce scope of lazy mmu region
has been added to the -mm mm-unstable branch.  Its filename is
     fs-proc-task_mmu-reduce-scope-of-lazy-mmu-region.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/fs-proc-task_mmu-reduce-scope-of-lazy-mmu-region.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Ryan Roberts <ryan.roberts@arm.com>
Subject: fs/proc/task_mmu: reduce scope of lazy mmu region
Date: Mon, 3 Mar 2025 14:15:36 +0000

Update the way arch_[enter|leave]_lazy_mmu_mode() is called in
pagemap_scan_pmd_entry() to follow the normal pattern of holding the ptl
for user space mappings.  As a result the scope is reduced to only the pte
table, but that's where most of the performance win is.

While I believe there wasn't technically a bug here, the original scope
made it easier to accidentally nest or, worse, accidentally call something
like kmap() which would expect an immediate mode pte modification but it
would end up deferred.

Link: https://lkml.kernel.org/r/20250303141542.3371656-3-ryan.roberts@arm.com
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juegren Gross <jgross@suse.com>
Cc: Matthew Wilcow (Oracle) <willy@infradead.org>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 fs/proc/task_mmu.c |   11 ++++-------
 1 file changed, 4 insertions(+), 7 deletions(-)

--- a/fs/proc/task_mmu.c~fs-proc-task_mmu-reduce-scope-of-lazy-mmu-region
+++ a/fs/proc/task_mmu.c
@@ -2479,22 +2479,19 @@ static int pagemap_scan_pmd_entry(pmd_t
 	spinlock_t *ptl;
 	int ret;
 
-	arch_enter_lazy_mmu_mode();
-
 	ret = pagemap_scan_thp_entry(pmd, start, end, walk);
-	if (ret != -ENOENT) {
-		arch_leave_lazy_mmu_mode();
+	if (ret != -ENOENT)
 		return ret;
-	}
 
 	ret = 0;
 	start_pte = pte = pte_offset_map_lock(vma->vm_mm, pmd, start, &ptl);
 	if (!pte) {
-		arch_leave_lazy_mmu_mode();
 		walk->action = ACTION_AGAIN;
 		return 0;
 	}
 
+	arch_enter_lazy_mmu_mode();
+
 	if ((p->arg.flags & PM_SCAN_WP_MATCHING) && !p->vec_out) {
 		/* Fast path for performing exclusive WP */
 		for (addr = start; addr != end; pte++, addr += PAGE_SIZE) {
@@ -2563,8 +2560,8 @@ flush_and_return:
 	if (flush_end)
 		flush_tlb_range(vma, start, addr);
 
-	pte_unmap_unlock(start_pte, ptl);
 	arch_leave_lazy_mmu_mode();
+	pte_unmap_unlock(start_pte, ptl);
 
 	cond_resched();
 	return ret;
_

Patches currently in -mm which might be from ryan.roberts@arm.com are

mm-dont-skip-arch_sync_kernel_mappings-in-error-paths.patch
mm-ioremap-pass-pgprot_t-to-ioremap_prot-instead-of-unsigned-long.patch
mm-fix-lazy-mmu-docs-and-usage.patch
fs-proc-task_mmu-reduce-scope-of-lazy-mmu-region.patch
sparc-mm-disable-preemption-in-lazy-mmu-mode.patch
sparc-mm-avoid-calling-arch_enter-leave_lazy_mmu-in-set_ptes.patch
revert-x86-xen-allow-nesting-of-same-lazy-mode.patch