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Subject[PATCH 4.19 10/34] mm: thp: replace DEBUG_VM BUG with VM_WARN when unmap fails for split
Date
From: Yang Shi <shy828301@gmail.com>

[ Upstream commit 504e070dc08f757bccaed6d05c0f53ecbfac8a23 ]

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.

As 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/

Link: https://lkml.kernel.org/r/d0f0db68-98b8-ebfb-16dc-f29df24cf012@google.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: Alistair Popple <apopple@nvidia.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jue Wang <juew@google.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Peter Xu <peterx@redhat.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Wang Yugui <wangyugui@e16-tech.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Note on stable backport: fixed up variables and split_queue_lock in
split_huge_page_to_list(), and conflict on ttu_flags in unmap_page().

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
mm/huge_memory.c | 24 +++++++-----------------
1 file changed, 7 insertions(+), 17 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 78c1ad5f8109..4400957d8e4e 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2431,15 +2431,15 @@ static void unmap_page(struct page *page)
{
enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS |
TTU_RMAP_LOCKED | TTU_SPLIT_HUGE_PMD | TTU_SYNC;
- 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);
+
+ VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);
}

static void remap_page(struct page *page)
@@ -2698,7 +2698,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
struct pglist_data *pgdata = NODE_DATA(page_to_nid(head));
struct anon_vma *anon_vma = NULL;
struct address_space *mapping = NULL;
- int count, mapcount, extra_pins, ret;
+ int extra_pins, ret;
bool mlocked;
unsigned long flags;
pgoff_t end;
@@ -2760,7 +2760,6 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)

mlocked = PageMlocked(page);
unmap_page(head);
- VM_BUG_ON_PAGE(compound_mapcount(head), head);

/* Make sure the page is not on per-CPU pagevec as it takes pin */
if (mlocked)
@@ -2786,9 +2785,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)

/* Prevent deferred_split_scan() touching ->_refcount */
spin_lock(&pgdata->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))) {
pgdata->split_queue_len--;
list_del(page_deferred_list(head));
@@ -2804,16 +2801,9 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
} else
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(&pgdata->split_queue_lock);
-fail: if (mapping)
+fail:
+ if (mapping)
xa_unlock(&mapping->i_pages);
spin_unlock_irqrestore(zone_lru_lock(page_zone(head)), flags);
remap_page(head);
--
2.30.2


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