From: John Hubbard <jhubbard@xxxxxxxxxx> Subject: mm, dump_page: do not crash with bad compound_mapcount() If a compound page is being split while dump_page() is being run on that page, we can end up calling compound_mapcount() on a page that is no longer compound. This leads to a crash (already seen at least once in the field), due to the VM_BUG_ON_PAGE() assertion inside compound_mapcount(). (The above is from Matthew Wilcox's analysis of Qian Cai's bug report.) A similar problem is possible, via compound_pincount() instead of compound_mapcount(). In order to avoid this kind of crash, make dump_page() slightly more robust, by providing a pair of simpler routines that don't contain assertions: head_mapcount() and head_pincount(). For debug tools, we don't want to go *too* far in this direction, but this is a simple small fix, and the crash has already been seen, so it's a good trade-off. Link: http://lkml.kernel.org/r/20200804214807.169256-1-jhubbard@xxxxxxxxxx Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx> Reported-by: Qian Cai <cai@xxxxxx> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx> Acked-by: Vlastimil Babka <vbabka@xxxxxxx> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx> Cc: William Kucharski <william.kucharski@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- include/linux/mm.h | 14 ++++++++++++-- mm/debug.c | 6 +++--- 2 files changed, 15 insertions(+), 5 deletions(-) --- a/include/linux/mm.h~mm-dump_page-do-not-crash-with-bad-compound_mapcount +++ a/include/linux/mm.h @@ -779,6 +779,11 @@ static inline void *kvcalloc(size_t n, s extern void kvfree(const void *addr); extern void kvfree_sensitive(const void *addr, size_t len); +static inline int head_mapcount(struct page *head) +{ + return atomic_read(compound_mapcount_ptr(head)) + 1; +} + /* * Mapcount of compound page as a whole, does not include mapped sub-pages. * @@ -788,7 +793,7 @@ static inline int compound_mapcount(stru { VM_BUG_ON_PAGE(!PageCompound(page), page); page = compound_head(page); - return atomic_read(compound_mapcount_ptr(page)) + 1; + return head_mapcount(page); } /* @@ -901,11 +906,16 @@ static inline bool hpage_pincount_availa return PageCompound(page) && compound_order(page) > 1; } +static inline int head_pincount(struct page *head) +{ + return atomic_read(compound_pincount_ptr(head)); +} + static inline int compound_pincount(struct page *page) { VM_BUG_ON_PAGE(!hpage_pincount_available(page), page); page = compound_head(page); - return atomic_read(compound_pincount_ptr(page)); + return head_pincount(page); } static inline void set_compound_order(struct page *page, unsigned int order) --- a/mm/debug.c~mm-dump_page-do-not-crash-with-bad-compound_mapcount +++ a/mm/debug.c @@ -102,12 +102,12 @@ void __dump_page(struct page *page, cons if (hpage_pincount_available(page)) { pr_warn("head:%p order:%u compound_mapcount:%d compound_pincount:%d\n", head, compound_order(head), - compound_mapcount(head), - compound_pincount(head)); + head_mapcount(head), + head_pincount(head)); } else { pr_warn("head:%p order:%u compound_mapcount:%d\n", head, compound_order(head), - compound_mapcount(head)); + head_mapcount(head)); } } if (PageKsm(page)) _
