[v3 PATCH 2/2] mm: thp: check page_mapped instead of page_mapcount for split

Yang Shi
2021-05-25T16:21:54Z

When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
return false positive for PTE-mapped THP since page_mapcount() is used
to check if the THP is unmapped, but it just checks compound mapount and
head page's mapcount. If the THP is PTE-mapped and head page is not
mapped, it may return false positive.

The try_to_unmap() has been changed to void function, so check
page_mapped() after it. And changed BUG_ON to WARN_ON since it is not a
fatal issue.

[1] https://lore.kernel.org/linux-mm/20210412180659.B9E3.409509F4@e16-tech.com/

Reviewed-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Yang Shi <shy828301@gmail.com>
---
v3: Incorporated the comments from Hugh. Keep Zi Yan's reviewed-by tag
since there is no fundamental change against v2.
v2: Removed dead code and updated the comment of try_to_unmap() per Zi
Yan.
mm/huge_memory.c | 17 +++++------------
1 file changed, 5 insertions(+), 12 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 80fe642d742d..72d81d8e01b1 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2343,6 +2343,8 @@ static void unmap_page(struct page *page)
ttu_flags |= TTU_SPLIT_FREEZE;

try_to_unmap(page, ttu_flags);
+
+ VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);
}

static void remap_page(struct page *page, unsigned int nr)
@@ -2653,7 +2655,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
struct deferred_split *ds_queue = get_deferred_split_queue(head);
struct anon_vma *anon_vma = NULL;
struct address_space *mapping = NULL;
- int count, mapcount, extra_pins, ret;
+ int mapcount, extra_pins, ret;
pgoff_t end;

VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
@@ -2712,7 +2714,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
}

unmap_page(head);
- VM_BUG_ON_PAGE(compound_mapcount(head), head);

/* block interrupt reentry in xa_lock and spinlock */
local_irq_disable();
@@ -2730,7 +2731,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)

/* Prevent deferred_split_scan() touching ->_refcount */
spin_lock(&ds_queue->split_queue_lock);
- count = page_count(head);
mapcount = total_mapcount(head);
if (!mapcount && page_ref_freeze(head, 1 + extra_pins)) {
if (!list_empty(page_deferred_list(head))) {
@@ -2752,16 +2752,9 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
__split_huge_page(page, list, end);
ret = 0;
} else {
- if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
- pr_alert("total_mapcount: %u, page_count(): %u\n",
- mapcount, count);
- if (PageTail(page))
- dump_page(head, NULL);
- dump_page(page, "total_mapcount(head) > 0");
- BUG();
- }
spin_unlock(&ds_queue->split_queue_lock);
-fail: if (mapping)
+fail:
+ if (mapping)
xa_unlock(&mapping->i_pages);
local_irq_enable();
remap_page(head, thp_nr_pages(head));
--
2.26.2