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Subject[PATCH 4.9 3/9] 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, split_queue_lock, tree_lock
in split_huge_page_to_list(); adapted to early version of unmap_page().

Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
mm/huge_memory.c | 29 ++++++++++-------------------
1 file changed, 10 insertions(+), 19 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 14cd0ef33b62..177ca028b986 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1891,7 +1891,7 @@ static void unmap_page(struct page *page)
{
enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS |
TTU_RMAP_LOCKED;
- int i, ret;
+ int i;

VM_BUG_ON_PAGE(!PageHead(page), page);

@@ -1899,15 +1899,16 @@ static void unmap_page(struct page *page)
ttu_flags |= TTU_MIGRATION;

/* We only need TTU_SPLIT_HUGE_PMD once */
- ret = try_to_unmap(page, ttu_flags | TTU_SPLIT_HUGE_PMD);
- for (i = 1; !ret && i < HPAGE_PMD_NR; i++) {
+ try_to_unmap(page, ttu_flags | TTU_SPLIT_HUGE_PMD);
+ for (i = 1; i < HPAGE_PMD_NR; i++) {
/* Cut short if the page is unmapped */
if (page_count(page) == 1)
return;

- ret = try_to_unmap(page + i, ttu_flags);
+ try_to_unmap(page + i, ttu_flags);
}
- VM_BUG_ON_PAGE(ret, page + i - 1);
+
+ VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);
}

static void remap_page(struct page *page)
@@ -2137,7 +2138,7 @@ 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;
@@ -2200,7 +2201,6 @@ 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)
@@ -2226,9 +2226,7 @@ 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));
@@ -2239,16 +2237,9 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
__split_huge_page(page, list, end, flags);
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)
spin_unlock(&mapping->tree_lock);
spin_unlock_irqrestore(zone_lru_lock(page_zone(head)), flags);
remap_page(head);
--
2.30.2


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