Commit 6dc5ea16 authored by John Hubbard's avatar John Hubbard Committed by Linus Torvalds
Browse files

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.

Reported-by: default avatarQian Cai <cai@lca.pw>
Suggested-by: default avatarMatthew Wilcox <willy@infradead.org>
Signed-off-by: default avatarJohn Hubbard <jhubbard@nvidia.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Acked-by: default avatarVlastimil Babka <vbabka@suse.cz>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: William Kucharski <william.kucharski@oracle.com>
Link: http://lkml.kernel.org/r/20200804214807.169256-1-jhubbard@nvidia.com


Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 54a75157

include/linux/mm.h

+12 −2
Original line number Diff line number Diff line
@@ -779,6 +779,11 @@ static inline void *kvcalloc(size_t n, size_t size, gfp_t flags) 
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(struct page *page) 
{

	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_available(struct page *page) 
	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)

mm/debug.c

+3 −3
Original line number Diff line number Diff line
@@ -102,12 +102,12 @@ void __dump_page(struct page *page, const char *reason) 
		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))