mm: remove VM_BUG_ON(PageSlab()) from page_mapcount()

Replace superfluous VM_BUG_ON() with comment about correct usage.

Technically reverts commit 1d148e2
("mm: add VM_BUG_ON_PAGE() to page_mapcount()"), but context have changed.

Function isolate_migratepages_block() runs some checks out of lru_lock
when choose pages for migration.  After checking PageLRU() it checks extra
page references by comparing page_count() and page_mapcount().  Between
these two checks page could be removed from lru, freed and taken by slab.

As a result this race triggers VM_BUG_ON(PageSlab()) in page_mapcount().
Race window is tiny. For certain workload this happens around once a year.

 page:ffffea0105ca9380 count:1 mapcount:0 mapping:ffff88ff7712c180 index:0x0 compound_mapcount: 0
 flags: 0x500000000008100(slab|head)
 raw: 0500000000008100 dead000000000100 dead000000000200 ffff88ff7712c180
 raw: 0000000000000000 0000000080200020 00000001ffffffff 0000000000000000
 page dumped because: VM_BUG_ON_PAGE(PageSlab(page))
 ------------[ cut here ]------------
 kernel BUG at ./include/linux/mm.h:628!
 invalid opcode: 0000 [#1] SMP NOPTI
 CPU: 77 PID: 504 Comm: kcompactd1 Tainted: G        W         4.19.109-27 #1
 Hardware name: Yandex T175-N41-Y3N/MY81-EX0-Y3N, BIOS R05 06/20/2019
 RIP: 0010:isolate_migratepages_block+0x986/0x9b0

Code in isolate_migratepages_block() was added in commit 119d6d5
("mm, compaction: avoid isolating pinned pages") before adding VM_BUG_ON
into page_mapcount().

This race has been predicted in 2015 by Vlastimil Babka (see link below).

Link: http://lkml.kernel.org/r/159032779896.957378.7852761411265662220.stgit@buzz
Link: https://lore.kernel.org/lkml/[email protected]/
Link: https://lore.kernel.org/linux-mm/158937872515.474360.5066096871639561424.stgit@buzz/T/ (v1)
Fixes: 1d148e2 ("mm: add VM_BUG_ON_PAGE() to page_mapcount()")
Signed-off-by: Konstantin Khlebnikov <[email protected]>
Acked-by: Hugh Dickins <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Stephen Rothwell <[email protected]>

include/linux/mm.h

@@ -782,6 +782,11 @@ static inline void *kvcalloc(size_t n, size_t size, gfp_t flags)
 
 extern void kvfree(const void *addr);
 
+/*
+ * Mapcount of compound page as a whole, not includes mapped sub-pages.
+ *
+ * Must be called only for compound pages or any their tail sub-pages.
+ */
 static inline int compound_mapcount(struct page *page)
 {
 	VM_BUG_ON_PAGE(!PageCompound(page), page);
@@ -801,10 +806,15 @@ static inline void page_mapcount_reset(struct page *page)
 
 int __page_mapcount(struct page *page);
 
+/*
+ * Mapcount of 0-order page, for sub-page includes compound_mapcount().
+ *
+ * Result is undefined for pages which cannot be mapped into userspace.
+ * For example SLAB or special types of pages. See function page_has_type().
+ * They use this place in struct page differently.
+ */
 static inline int page_mapcount(struct page *page)
 {
-	VM_BUG_ON_PAGE(PageSlab(page), page);
-
 	if (unlikely(PageCompound(page)))
 		return __page_mapcount(page);
 	return atomic_read(&page->_mapcount) + 1;