mm: thp: replace DEBUG_VM BUG with VM_WARN when unmap fails for split

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>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 84ab735..6d2a011 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2352,15 +2352,15 @@ static void unmap_page(struct page *page)
 {
        enum ttu_flags ttu_flags = TTU_IGNORE_MLOCK | TTU_SYNC |
                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);
+
+       VM_WARN_ON_ONCE_PAGE(page_mapped(page), page);
 }
 
 static void remap_page(struct page *page, unsigned int nr)
@@ -2671,7 +2671,7 @@ 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 extra_pins, ret;
        pgoff_t end;
 
        VM_BUG_ON_PAGE(is_huge_zero_page(head), head);
@@ -2730,7 +2730,6 @@ 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();
@@ -2748,9 +2747,7 @@ 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 (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));
@@ -2770,16 +2767,9 @@ 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));