[v3 PATCH 1/2] mm: rmap: make try_to_unmap() void function

Yang Shi
2021-05-25T16:21:52Z

Currently try_to_unmap() return bool value by checking page_mapcount(),
however this may return false positive since page_mapcount() doesn't
check all subpages of compound page. The total_mapcount() could be used
instead, but its cost is higher since it traverses all subpages.

Actually the most callers of try_to_unmap() don't care about the
return value at all. So just need check if page is still mapped by
page_mapped() when necessary. And page_mapped() does bail out early
when it finds mapped subpage.

Suggested-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Yang Shi <shy828301@gmail.com>
---
include/linux/rmap.h | 2 +-
mm/huge_memory.c | 4 +---
mm/memory-failure.c | 13 ++++++-------
mm/rmap.c | 6 +-----
mm/vmscan.c | 3 ++-
5 files changed, 11 insertions(+), 17 deletions(-)

diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index def5c62c93b3..116cb193110a 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -194,7 +194,7 @@ static inline void page_dup_rmap(struct page *page, bool compound)
int page_referenced(struct page *, int is_locked,
struct mem_cgroup *memcg, unsigned long *vm_flags);

-bool try_to_unmap(struct page *, enum ttu_flags flags);
+void try_to_unmap(struct page *, enum ttu_flags flags);

/* Avoid racy checks */
#define PVMW_SYNC (1 << 0)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 19195fca1aee..80fe642d742d 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2336,15 +2336,13 @@ static void unmap_page(struct page *page)
{
enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK |
TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD;
- bool unmap_success;

VM_BUG_ON_PAGE(!PageHead(page), page);

if (PageAnon(page))
ttu_flags |= TTU_SPLIT_FREEZE;

- unmap_success = try_to_unmap(page, ttu_flags);
- VM_BUG_ON_PAGE(!unmap_success, page);
+ try_to_unmap(page, ttu_flags);
}

static void remap_page(struct page *page, unsigned int nr)
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 9dcc9bcea731..6dd53ff34825 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1126,7 +1126,7 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
collect_procs(hpage, &tokill, flags & MF_ACTION_REQUIRED);

if (!PageHuge(hpage)) {
- unmap_success = try_to_unmap(hpage, ttu);
+ try_to_unmap(hpage, ttu);
} else {
if (!PageAnon(hpage)) {
/*
@@ -1138,17 +1138,16 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
*/
mapping = hugetlb_page_mapping_lock_write(hpage);
if (mapping) {
- unmap_success = try_to_unmap(hpage,
- ttu|TTU_RMAP_LOCKED);
+ try_to_unmap(hpage, ttu|TTU_RMAP_LOCKED);
i_mmap_unlock_write(mapping);
- } else {
+ } else
pr_info("Memory failure: %#lx: could not lock mapping for mapped huge page\n", pfn);
- unmap_success = false;
- }
} else {
- unmap_success = try_to_unmap(hpage, ttu);
+ try_to_unmap(hpage, ttu);
}
}
+
+ unmap_success = !page_mapped(hpage);
if (!unmap_success)
pr_err("Memory failure: %#lx: failed to unmap page (mapcount=%d)\n",
pfn, page_mapcount(hpage));
diff --git a/mm/rmap.c b/mm/rmap.c
index a35cbbbded0d..728de421e43a 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1748,10 +1748,8 @@ static int page_not_mapped(struct page *page)
*
* Tries to remove all the page table entries which are mapping this
* page, used in the pageout path. Caller must hold the page lock.
- *
- * If unmap is successful, return true. Otherwise, false.
*/
-bool try_to_unmap(struct page *page, enum ttu_flags flags)
+void try_to_unmap(struct page *page, enum ttu_flags flags)
{
struct rmap_walk_control rwc = {
.rmap_one = try_to_unmap_one,
@@ -1776,8 +1774,6 @@ bool try_to_unmap(struct page *page, enum ttu_flags flags)
rmap_walk_locked(page, &rwc);
else
rmap_walk(page, &rwc);
-
- return !page_mapcount(page) ? true : false;
}

/**
diff --git a/mm/vmscan.c b/mm/vmscan.c
index f96d62159720..fa5052ace415 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1499,7 +1499,8 @@ static unsigned int shrink_page_list(struct list_head *page_list,
if (unlikely(PageTransHuge(page)))
flags |= TTU_SPLIT_HUGE_PMD;

- if (!try_to_unmap(page, flags)) {
+ try_to_unmap(page, flags);
+ if (page_mapped(page)) {
stat->nr_unmap_fail += nr_pages;
if (!was_swapbacked && PageSwapBacked(page))
stat->nr_lazyfree_fail += nr_pages;
--
2.26.2