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mm, dump_page: do not crash with bad compound_mapcount()

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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: Qian Cai <cai@lca.pw>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Vlastimil 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: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
John Hubbard 2020-08-06 23:19:51 -07:00 committed by Linus Torvalds
parent 54a75157d9
commit 6dc5ea16c8
2 changed files with 15 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
include/linux/mm.h
View File

@ -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)

6
mm/debug.c
View File

@ -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))