[RFC PATCH v2 4/4] [PATCH 4/4] mm: add tracing for VMA merges

Jakub Matěna
2022-03-11T17:46:05Z

Adds trace support for vma_merge to measure successful and unsuccessful
merges of two VMAs with distinct anon_vmas and also trace support for
merges made possible by update of page offset made possible by a previous
patch in this series.

Signed-off-by: Jakub Matěna <matenajakub@gmail.com>
---
include/trace/events/mmap.h | 83 +++++++++++++++++++++++++++++++++++++
mm/internal.h | 12 ++++++
mm/mmap.c | 69 ++++++++++++++++--------------
3 files changed, 133 insertions(+), 31 deletions(-)

diff --git a/include/trace/events/mmap.h b/include/trace/events/mmap.h
index 4661f7ba07c0..bad7abe4899c 100644
--- a/include/trace/events/mmap.h
+++ b/include/trace/events/mmap.h
@@ -6,6 +6,27 @@
#define _TRACE_MMAP_H

#include <linux/tracepoint.h>
+#include <../mm/internal.h>
+
+#define AV_MERGE_TYPES \
+ EM(MERGE_FAILED) \
+ EM(AV_MERGE_FAILED) \
+ EM(AV_MERGE_NULL) \
+ EM(AV_MERGE_SAME) \
+ EMe(AV_MERGE_DIFFERENT)
+
+#undef EM
+#undef EMe
+#define EM(a) TRACE_DEFINE_ENUM(a);
+#define EMe(a) TRACE_DEFINE_ENUM(a);
+
+AV_MERGE_TYPES
+
+#undef EM
+#undef EMe
+
+#define EM(a) { a, #a },
+#define EMe(a) { a, #a }

TRACE_EVENT(vm_unmapped_area,

@@ -42,6 +63,68 @@ TRACE_EVENT(vm_unmapped_area,
__entry->low_limit, __entry->high_limit, __entry->align_mask,
__entry->align_offset)
);
+
+TRACE_EVENT(vm_av_merge,
+
+ TP_PROTO(int merged, enum vma_merge_res merge_prev,
+ enum vma_merge_res merge_next, enum vma_merge_res merge_both),
+
+ TP_ARGS(merged, merge_prev, merge_next, merge_both),
+
+ TP_STRUCT__entry(
+ __field(int, merged)
+ __field(enum vma_merge_res, predecessor_different_av)
+ __field(enum vma_merge_res, successor_different_av)
+ __field(enum vma_merge_res, predecessor_with_successor_different_av)
+ __field(int, same_count)
+ __field(int, diff_count)
+ __field(int, failed_count)
+ ),
+
+ TP_fast_assign(
+ __entry->merged = merged == 0;
+ __entry->predecessor_different_av = merge_prev;
+ __entry->successor_different_av = merge_next;
+ __entry->predecessor_with_successor_different_av = merge_both;
+ __entry->same_count = (merge_prev == AV_MERGE_SAME) +
+ (merge_next == AV_MERGE_SAME) +
+ (merge_both == AV_MERGE_SAME);
+ __entry->diff_count = (merge_prev == AV_MERGE_DIFFERENT) +
+ (merge_next == AV_MERGE_DIFFERENT) +
+ (merge_both == AV_MERGE_DIFFERENT);
+ __entry->failed_count = (merge_prev == AV_MERGE_FAILED) +
+ (merge_next == AV_MERGE_FAILED) +
+ (merge_both == AV_MERGE_FAILED);
+ ),
+
+ TP_printk("merged=%d predecessor=%s successor=%s predecessor_with_successor=%s same_count=%d diff_count=%d failed_count=%d",
+ __entry->merged,
+ __print_symbolic(__entry->predecessor_different_av, AV_MERGE_TYPES),
+ __print_symbolic(__entry->successor_different_av, AV_MERGE_TYPES),
+ __print_symbolic(__entry->predecessor_with_successor_different_av, AV_MERGE_TYPES),
+ __entry->same_count, __entry->diff_count, __entry->failed_count)
+
+);
+
+TRACE_EVENT(vm_pgoff_merge,
+
+ TP_PROTO(struct vm_area_struct *vma, bool anon_pgoff_updated),
+
+ TP_ARGS(vma, anon_pgoff_updated),
+
+ TP_STRUCT__entry(
+ __field(bool, faulted)
+ __field(bool, updated)
+ ),
+
+ TP_fast_assign(
+ __entry->faulted = vma->anon_vma;
+ __entry->updated = anon_pgoff_updated;
+ ),
+
+ TP_printk("faulted=%d updated=%d\n",
+ __entry->faulted, __entry->updated)
+);
#endif

/* This part must be outside protection */
diff --git a/mm/internal.h b/mm/internal.h
index d80300392a19..860169612192 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -34,6 +34,18 @@ struct folio_batch;
/* Do not use these with a slab allocator */
#define GFP_SLAB_BUG_MASK (__GFP_DMA32|__GFP_HIGHMEM|~__GFP_BITS_MASK)

+/*
+ * Following values indicate reason for merge success or failure.
+ */
+enum vma_merge_res {
+ MERGE_FAILED,
+ AV_MERGE_FAILED,
+ AV_MERGE_NULL,
+ MERGE_OK = AV_MERGE_NULL,
+ AV_MERGE_SAME,
+ AV_MERGE_DIFFERENT,
+};
+
void page_writeback_init(void);

static inline void *folio_raw_mapping(struct folio *folio)
diff --git a/mm/mmap.c b/mm/mmap.c
index ccb24862e670..663f8ec46f2c 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1064,21 +1064,21 @@ static inline int is_mergeable_anon_vma(struct anon_vma *anon_vma1,
*/
if ((!anon_vma1 || !anon_vma2) && (!vma ||
list_is_singular(&vma->anon_vma_chain)))
- return 1;
+ return AV_MERGE_NULL;
if (anon_vma1 == anon_vma2)
- return 1;
+ return AV_MERGE_SAME;
/*
* Different anon_vma but not shared by several processes
*/
else if ((anon_vma1 && anon_vma2) &&
(anon_vma1 == anon_vma1->root)
&& (rbt_no_children(anon_vma1)))
- return 1;
+ return AV_MERGE_DIFFERENT;
/*
* Different anon_vma and shared -> unmergeable
*/
else
- return 0;
+ return AV_MERGE_FAILED;
}

/*
@@ -1099,12 +1099,10 @@ can_vma_merge_before(struct vm_area_struct *vma, unsigned long vm_flags,
struct vm_userfaultfd_ctx vm_userfaultfd_ctx,
const char *anon_name)
{
- if (is_mergeable_vma(vma, file, vm_flags, vm_userfaultfd_ctx, anon_name) &&
- is_mergeable_anon_vma(anon_vma, vma->anon_vma, vma)) {
+ if (is_mergeable_vma(vma, file, vm_flags, vm_userfaultfd_ctx, anon_name))
if (vma->vm_pgoff == vm_pgoff)
- return 1;
- }
- return 0;
+ return is_mergeable_anon_vma(anon_vma, vma->anon_vma, vma);
+ return MERGE_FAILED;
}

/*
@@ -1121,14 +1119,13 @@ can_vma_merge_after(struct vm_area_struct *vma, unsigned long vm_flags,
struct vm_userfaultfd_ctx vm_userfaultfd_ctx,
const char *anon_name)
{
- if (is_mergeable_vma(vma, file, vm_flags, vm_userfaultfd_ctx, anon_name) &&
- is_mergeable_anon_vma(anon_vma, vma->anon_vma, vma)) {
+ if (is_mergeable_vma(vma, file, vm_flags, vm_userfaultfd_ctx, anon_name)) {
pgoff_t vm_pglen;
vm_pglen = vma_pages(vma);
if (vma->vm_pgoff + vm_pglen == vm_pgoff)
- return 1;
+ return is_mergeable_anon_vma(anon_vma, vma->anon_vma, vma);
}
- return 0;
+ return MERGE_FAILED;
}

/*
@@ -1185,8 +1182,14 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
pgoff_t pglen = (end - addr) >> PAGE_SHIFT;
struct vm_area_struct *area, *next;
int err = -1;
- bool merge_prev = false;
- bool merge_next = false;
+ /*
+ * Following three variables are used to store values
+ * indicating wheather this VMA and its anon_vma can
+ * be merged and also the type of failure or success.
+ */
+ enum vma_merge_res merge_prev = MERGE_FAILED;
+ enum vma_merge_res merge_both = MERGE_FAILED;
+ enum vma_merge_res merge_next = MERGE_FAILED;

/*
* We later require that vma->vm_flags == vm_flags,
@@ -1207,32 +1210,34 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,

/* Can we merge the predecessor? */
if (prev && prev->vm_end == addr &&
- mpol_equal(vma_policy(prev), policy) &&
- can_vma_merge_after(prev, vm_flags,
+ mpol_equal(vma_policy(prev), policy)) {
+ merge_prev = can_vma_merge_after(prev, vm_flags,
anon_vma, file, pgoff,
- vm_userfaultfd_ctx, anon_name)) {
- merge_prev = true;
- area = prev;
+ vm_userfaultfd_ctx, anon_name);
}
+
/* 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;
+ mpol_equal(policy, vma_policy(next))) {
+ merge_next = can_vma_merge_before(next, vm_flags,
+ anon_vma, file, pgoff+pglen,
+ vm_userfaultfd_ctx, anon_name);
}
+
/* 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 */
+ if (merge_prev >= MERGE_OK && merge_next >= MERGE_OK)
+ merge_both = is_mergeable_anon_vma(prev->anon_vma, next->anon_vma, NULL);
+
+ if (merge_both >= MERGE_OK) { /* cases 1, 6 */
err = __vma_adjust(prev, prev->vm_start,
next->vm_end, prev->vm_pgoff, NULL,
prev);
- } else if (merge_prev) { /* cases 2, 5, 7 */
+ area = prev;
+ } else if (merge_prev >= MERGE_OK) { /* cases 2, 5, 7 */
err = __vma_adjust(prev, prev->vm_start,
end, prev->vm_pgoff, NULL, prev);
- } else if (merge_next) {
+ area = prev;
+ } else if (merge_next >= MERGE_OK) {
if (prev && addr < prev->vm_end) /* case 4 */
err = __vma_adjust(prev, prev->vm_start,
addr, prev->vm_pgoff, NULL, next);
@@ -1246,7 +1251,7 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
*/
area = next;
}
-
+ trace_vm_av_merge(err, merge_prev, merge_next, merge_both);
/*
* Cannot merge with predecessor or successor or error in __vma_adjust?
*/
@@ -3321,6 +3326,8 @@ struct vm_area_struct *copy_vma(struct vm_area_struct **vmap,
/*
* Source vma may have been merged into new_vma
*/
+ trace_vm_pgoff_merge(vma, anon_pgoff_updated);
+
if (unlikely(vma_start >= new_vma->vm_start &&
vma_start < new_vma->vm_end)) {
/*
--
2.34.1