[PATCH v2 0/2] Refactor of vma_merge and new merge call

[PATCH v2 0/2] Refactor of vma_merge and new merge call

[Jakub Matěna]

I am currently working on my master's thesis trying to increase
number of merges of VMAs currently failing because of page offset
incompatibility and difference in their anon_vmas. The following
refactor and added merge call included in this series is just two
smaller upgrades I created along the way.

The rest of the work is still being worked on but was send to mailing
list as a RFC:
https://lore.kernel.org/all/20220516125405.1675-1-matenajakub@gmail.com/

This patch series is based on tag v5.18-rc2. This is second version,
I forgot to include one header file in the first version.

Jakub Matěna (2):
  mm: refactor of vma_merge()
  mm: add merging after mremap resize

 mm/mmap.c   | 87 +++++++++++++++++++++++------------------------------
 mm/mremap.c |  8 +++--
 2 files changed, 43 insertions(+), 52 deletions(-)

-- 
2.35.1

[PATCH v2 1/2] [PATCH 1/2] mm: refactor of vma_merge()

[Jakub Matěna]

Refactor vma_merge() to make it shorter and more understandable.
Main change is the elimination of code duplicity in the case of
merge next check. This is done by first doing checks and caching
the results before executing the merge itself. The variable 'area' is
divided into 'mid' and 'res' as previously it was used for two purposes,
as the middle VMA between prev and next and also as the result of the
merge itself. Exit paths are also unified.

Signed-off-by: Jakub Matěna <matenajakub@gmail.com>
---
 mm/mmap.c | 87 +++++++++++++++++++++++--------------------------------
 1 file changed, 37 insertions(+), 50 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index 3aa839f81e63..bd0dde2ad3e2 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1170,8 +1170,10 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
 			struct anon_vma_name *anon_name)
 {
 	pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
-	struct vm_area_struct *area, *next;
-	int err;
+	struct vm_area_struct *mid, *next, *res;
+	int err = -1;
+	bool merge_prev = false;
+	bool merge_next = false;
 
 	/*
 	 * We later require that vma->vm_flags == vm_flags,
@@ -1181,75 +1183,60 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
 		return NULL;
 
 	next = vma_next(mm, prev);
-	area = next;
-	if (area && area->vm_end == end)		/* cases 6, 7, 8 */
+	mid = next;
+	if (next && next->vm_end == end)		/* cases 6, 7, 8 */
 		next = next->vm_next;
 
 	/* verify some invariant that must be enforced by the caller */
 	VM_WARN_ON(prev && addr <= prev->vm_start);
-	VM_WARN_ON(area && end > area->vm_end);
+	VM_WARN_ON(mid && end > mid->vm_end);
 	VM_WARN_ON(addr >= end);
 
-	/*
-	 * Can it merge with the predecessor?
-	 */
+	/* Can we merge the predecessor? */
 	if (prev && prev->vm_end == addr &&
 			mpol_equal(vma_policy(prev), policy) &&
 			can_vma_merge_after(prev, vm_flags,
 					    anon_vma, file, pgoff,
 					    vm_userfaultfd_ctx, anon_name)) {
-		/*
-		 * OK, it can.  Can we now merge in the successor as well?
-		 */
-		if (next && end == next->vm_start &&
-				mpol_equal(policy, vma_policy(next)) &&
-				can_vma_merge_before(next, vm_flags,
-						     anon_vma, file,
-						     pgoff+pglen,
-						     vm_userfaultfd_ctx, anon_name) &&
-				is_mergeable_anon_vma(prev->anon_vma,
-						      next->anon_vma, NULL)) {
-							/* cases 1, 6 */
-			err = __vma_adjust(prev, prev->vm_start,
-					 next->vm_end, prev->vm_pgoff, NULL,
-					 prev);
-		} else					/* cases 2, 5, 7 */
-			err = __vma_adjust(prev, prev->vm_start,
-					 end, prev->vm_pgoff, NULL, prev);
-		if (err)
-			return NULL;
-		khugepaged_enter_vma_merge(prev, vm_flags);
-		return prev;
+		merge_prev = true;
 	}
-
-	/*
-	 * Can this new request be merged in front of next?
-	 */
+	/* Can we merge the successor? */
 	if (next && end == next->vm_start &&
 			mpol_equal(policy, vma_policy(next)) &&
 			can_vma_merge_before(next, vm_flags,
 					     anon_vma, file, pgoff+pglen,
 					     vm_userfaultfd_ctx, anon_name)) {
+		merge_next = true;
+	}
+	/* Can we merge both the predecessor and the successor? */
+	if (merge_prev && merge_next &&
+			is_mergeable_anon_vma(prev->anon_vma,
+				next->anon_vma, NULL)) {	 /* cases 1, 6 */
+		err = __vma_adjust(prev, prev->vm_start,
+					next->vm_end, prev->vm_pgoff, NULL,
+					prev);
+		res = prev;
+	} else if (merge_prev) {			/* cases 2, 5, 7 */
+		err = __vma_adjust(prev, prev->vm_start,
+					end, prev->vm_pgoff, NULL, prev);
+		res = prev;
+	} else if (merge_next) {
 		if (prev && addr < prev->vm_end)	/* case 4 */
 			err = __vma_adjust(prev, prev->vm_start,
-					 addr, prev->vm_pgoff, NULL, next);
-		else {					/* cases 3, 8 */
-			err = __vma_adjust(area, addr, next->vm_end,
-					 next->vm_pgoff - pglen, NULL, next);
-			/*
-			 * In case 3 area is already equal to next and
-			 * this is a noop, but in case 8 "area" has
-			 * been removed and next was expanded over it.
-			 */
-			area = next;
-		}
-		if (err)
-			return NULL;
-		khugepaged_enter_vma_merge(area, vm_flags);
-		return area;
+					addr, prev->vm_pgoff, NULL, next);
+		else					/* cases 3, 8 */
+			err = __vma_adjust(mid, addr, next->vm_end,
+					next->vm_pgoff - pglen, NULL, next);
+		res = next;
 	}
 
-	return NULL;
+	/*
+	 * Cannot merge with predecessor or successor or error in __vma_adjust?
+	 */
+	if (err)
+		return NULL;
+	khugepaged_enter_vma_merge(res, vm_flags);
+	return res;
 }
 
 /*
-- 
2.35.1

[PATCH v2 2/2] [PATCH 2/2] mm: add merging after mremap resize

[Jakub Matěna]

When mremap call results in expansion, it might be possible to merge the
VMA with the next VMA which might become adjacent. This patch adds
vma_merge call after the expansion is done to try and merge.

Signed-off-by: Jakub Matěna <matenajakub@gmail.com>
---
 mm/mremap.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/mremap.c b/mm/mremap.c
index 303d3290b938..21dfa15e8200 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -9,6 +9,7 @@
  */
 
 #include <linux/mm.h>
+#include <linux/mm_inline.h>
 #include <linux/hugetlb.h>
 #include <linux/shm.h>
 #include <linux/ksm.h>
@@ -23,6 +24,7 @@
 #include <linux/mmu_notifier.h>
 #include <linux/uaccess.h>
 #include <linux/userfaultfd_k.h>
+#include <linux/mempolicy.h>
 
 #include <asm/cacheflush.h>
 #include <asm/tlb.h>
@@ -1022,8 +1024,11 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 				}
 			}
 
-			if (vma_adjust(vma, vma->vm_start, addr + new_len,
-				       vma->vm_pgoff, NULL)) {
+			vma = vma_merge(mm, vma, addr + old_len, addr + new_len,
+					vma->vm_flags, vma->anon_vma, vma->vm_file,
+					vma->vm_pgoff + (old_len >> PAGE_SHIFT), vma_policy(vma),
+					vma->vm_userfaultfd_ctx, anon_vma_name(vma));
+			if (!vma) {
 				vm_unacct_memory(pages);
 				ret = -ENOMEM;
 				goto out;
-- 
2.35.1

Re: [PATCH v2 2/2] [PATCH 2/2] mm: add merging after mremap resize

[Andrew Morton]

On Fri, 27 May 2022 23:17:08 +0200 Jakub Matěna <matenajakub@gmail.com> wrote:

> When mremap call results in expansion, it might be possible to merge the
> VMA with the next VMA which might become adjacent. This patch adds
> vma_merge call after the expansion is done to try and merge.

Thanks.

Would it make sense to add a test case for this in (I assume)
tools/testing/selftests/vm/mremap_test.c?  Maybe parse /proc/self/maps
and check that the merging occurred as expected?

Re: [PATCH v2 2/2] [PATCH 2/2] mm: add merging after mremap resize

[Jakub Matěna]

On Mon, May 30, 2022 at 1:47 AM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Fri, 27 May 2022 23:17:08 +0200 Jakub Matěna <matenajakub@gmail.com> wrote:
>
> > When mremap call results in expansion, it might be possible to merge the
> > VMA with the next VMA which might become adjacent. This patch adds
> > vma_merge call after the expansion is done to try and merge.
>
> Thanks.
>
> Would it make sense to add a test case for this in (I assume)
> tools/testing/selftests/vm/mremap_test.c?  Maybe parse /proc/self/maps
> and check that the merging occurred as expected?
>

It shouldn't be a problem and it makes sense. I will do it.

Jakub Matěna