+ mm-do-not-attempt-second-merge-for-file-backed-vmas.patch added to mm-hotfixes-unstable branch

+ mm-do-not-attempt-second-merge-for-file-backed-vmas.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

The patch titled
     Subject: mm: do not attempt second merge for file-backed VMAs
has been added to the -mm mm-hotfixes-unstable branch.  Its filename is
     mm-do-not-attempt-second-merge-for-file-backed-vmas.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-do-not-attempt-second-merge-for-file-backed-vmas.patch

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

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------------------------------------------------------
From: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Subject: mm: do not attempt second merge for file-backed VMAs
Date: Tue, 22 Oct 2024 21:40:59 +0100

Previously, we'd always try to merge a file-backed VMA if its flags were
changed by the driver.

This however is rarely meaningful as typically the flags would be changed
to VM_PFNMAP or other VM_SPECIAL flags which are inherently unmergable.

In cases where it is meaningful (for instance DAX) it is doubtful that this
optimisation is worth the effort and maintenance risk of having to unwind
state and perform a merge.

Since we've observed bugs and resource leaks due to complexity in this
area, it is simply not acceptable to have a 'nice to have' optimisation
like this complicating an already very complicated code path, so let's
simply eliminate it.

Link: https://lkml.kernel.org/r/7b6bf6165080505feb5e00b313d2c84c25015e45.1729628198.git.lorenzo.stoakes@oracle.com
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Jann Horn <jannh@google.com>
Cc: Liam R. Howlett <Liam.Howlett@Oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: <stable@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/vma.c |   39 ++-------------------------------------
 1 file changed, 2 insertions(+), 37 deletions(-)

--- a/mm/vma.c~mm-do-not-attempt-second-merge-for-file-backed-vmas
+++ a/mm/vma.c
@@ -2260,8 +2260,7 @@ static int __mmap_prepare(struct mmap_st
 	return 0;
 }
 
-static int __mmap_new_file_vma(struct mmap_state *map, struct vm_area_struct *vma,
-			       struct vm_area_struct **mergep)
+static int __mmap_new_file_vma(struct mmap_state *map, struct vm_area_struct *vma)
 {
 	struct vma_iterator *vmi = map->vmi;
 	struct vma_merge_struct *vmg = map->vmg;
@@ -2291,34 +2290,6 @@ static int __mmap_new_file_vma(struct mm
 			(vma->vm_flags & VM_MAYWRITE));
 
 	vma_iter_config(vmi, vmg->start, vmg->end);
-	/*
-	 * If flags changed after mmap_file(), we should try merge
-	 * vma again as we may succeed this time.
-	 */
-	if (unlikely(map->flags != vma->vm_flags && vmg->prev)) {
-		struct vm_area_struct *merge;
-
-		vmg->flags = vma->vm_flags;
-		/* If this fails, state is reset ready for a reattempt. */
-		merge = vma_merge_new_range(vmg);
-
-		if (merge) {
-			/*
-			 * ->mmap() can change vma->vm_file and fput
-			 * the original file. So fput the vma->vm_file
-			 * here or we would add an extra fput for file
-			 * and cause general protection fault
-			 * ultimately.
-			 */
-			fput(vma->vm_file);
-			vm_area_free(vma);
-			vma_iter_free(vmi);
-			*mergep = merge;
-		} else {
-			vma_iter_config(vmi, vmg->start, vmg->end);
-		}
-	}
-
 	map->flags = vma->vm_flags;
 	return 0;
 }
@@ -2341,7 +2312,6 @@ static int __mmap_new_vma(struct mmap_st
 {
 	struct vma_iterator *vmi = map->vmi;
 	struct vma_merge_struct *vmg = map->vmg;
-	struct vm_area_struct *merge = NULL;
 	int error = 0;
 	struct vm_area_struct *vma;
 
@@ -2365,7 +2335,7 @@ static int __mmap_new_vma(struct mmap_st
 	}
 
 	if (vmg->file)
-		error = __mmap_new_file_vma(map, vma, &merge);
+		error = __mmap_new_file_vma(map, vma);
 	else if (map->flags & VM_SHARED)
 		error = shmem_zero_setup(vma);
 	else
@@ -2374,9 +2344,6 @@ static int __mmap_new_vma(struct mmap_st
 	if (error)
 		goto free_iter_vma;
 
-	if (merge)
-		goto file_expanded;
-
 #ifdef CONFIG_SPARC64
 	/* TODO: Fix SPARC ADI! */
 	WARN_ON_ONCE(!arch_validate_flags(map->flags));
@@ -2393,8 +2360,6 @@ static int __mmap_new_vma(struct mmap_st
 	 * call covers the non-merge case.
 	 */
 	khugepaged_enter_vma(vma, map->flags);
-
-file_expanded:
 	ksm_add_vma(vma);
 
 	*vmap = vma;
_

Patches currently in -mm which might be from lorenzo.stoakes@oracle.com are

fork-do-not-invoke-uffd-on-fork-if-error-occurs.patch
fork-only-invoke-khugepaged-ksm-hooks-if-no-error.patch
mm-vma-add-expand-only-vma-merge-mode-and-optimise-do_brk_flags.patch
tools-testing-add-expand-only-mode-vma-test.patch
mm-avoid-unsafe-vma-hook-invocation-when-error-arises-on-mmap-hook.patch
mm-unconditionally-close-vmas-on-error.patch
mm-refactor-map_deny_write_exec.patch
mm-resolve-faulty-mmap_region-error-path-behaviour.patch
tools-testing-add-additional-vma_internalh-stubs.patch
mm-insolate-mmap-internal-logic-to-mm-vmac.patch
mm-refactor-__mmap_region.patch
mm-do-not-attempt-second-merge-for-file-backed-vmas.patch
selftests-mm-add-pkey_sighandler_xx-hugetlb_dio-to-gitignore.patch
mm-refactor-mm_access-to-not-return-null.patch
mm-refactor-mm_access-to-not-return-null-fix.patch
mm-madvise-unrestrict-process_madvise-for-current-process.patch
maple_tree-do-not-hash-pointers-on-dump-in-debug-mode.patch
tools-testing-fix-phys_addr_t-size-on-64-bit-systems.patch