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Subject[PATCH 6.1/7] mm: thp: replace DEBUG_VM BUG with VM_WARN when unmap fails for split
From: Yang Shi <shy828301@gmail.com>

When debugging the bug reported by Wang Yugui [1], try_to_unmap() may
fail, but the first VM_BUG_ON_PAGE() just checks page_mapcount() however
it may miss the failure when head page is unmapped but other subpage is
mapped. Then the second DEBUG_VM BUG() that check total mapcount would
catch it. This may incur some confusion. And this is not a fatal issue,
so consolidate the two DEBUG_VM checks into one VM_WARN_ON_ONCE_PAGE().

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

Signed-off-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
---
v5: Rediffed by Hugh to fit after 6/7 in his mm/thp series; Cc stable.
v4: Updated the subject and commit log per Hugh.
Reordered the patches per Hugh.
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 | 26 ++++++++------------------
1 file changed, 8 insertions(+), 18 deletions(-)

--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2350,19 +2350,19 @@ 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;

- /* Make sure that the BUGs will not bite */
+ /* If warning below, prevent a race in the mapped accounting */
if (IS_ENABLED(CONFIG_DEBUG_VM))
ttu_flags |= TTU_SYNC;

- unmap_success = try_to_unmap(page, ttu_flags);
- VM_BUG_ON_PAGE(!unmap_success, page);
+ 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)
@@ -2673,7 +2673,7 @@ int split_huge_page_to_list(struct page
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 extra_pins, ret;
pgoff_t end;

VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
@@ -2732,7 +2732,6 @@ int split_huge_page_to_list(struct page
}

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

/* block interrupt reentry in xa_lock and spinlock */
local_irq_disable();
@@ -2750,9 +2749,7 @@ int split_huge_page_to_list(struct page

/* 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 (page_ref_freeze(head, 1 + extra_pins)) {
if (!list_empty(page_deferred_list(head))) {
ds_queue->split_queue_len--;
list_del(page_deferred_list(head));
@@ -2772,16 +2769,9 @@ int split_huge_page_to_list(struct page
__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));
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