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    SubjectRe: [PATCH v2] mm, dump_page: do not crash with bad compound_mapcount()
    On Tue, Aug 04, 2020 at 02:48:07PM -0700, John Hubbard wrote:
    > 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().

    I find naming misleading. head_mapcount() and head_pincount() sounds like
    a mapcount/pincount of the head page, but it's not. It's mapcount and
    pincount of the compound page.

    Maybe compound_mapcount_head() and compound_pincoun_head()? Or
    __compound_mapcount() and __compound_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>
    > Cc: Vlastimil Babka <vbabka@suse.cz>
    > Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    > Signed-off-by: John Hubbard <jhubbard@nvidia.com>
    > ---
    > Hi,
    >
    > I'm assuming that a fix is not required for -stable, but let me know if
    > others feel differently. The dump_page() code has changed a lot in that
    > area.
    >
    > Changes since v1 [1]:
    >
    > 1) Rebased onto mmotm
    >
    > 2) Used a simpler head_*count() approach.
    >
    > 3) Added Matthew's Suggested-by: tag
    >
    > 4) Support pincount as well as mapcount.
    >
    > [1] https://lore.kernel.org/linux-mm/20200804183943.1244828-1-jhubbard@nvidia.com/
    >
    > thanks,
    > John Hubbard
    >
    > include/linux/mm.h | 14 ++++++++++++--
    > mm/debug.c | 6 +++---
    > 2 files changed, 15 insertions(+), 5 deletions(-)
    >
    > diff --git a/include/linux/mm.h b/include/linux/mm.h
    > index 4f12b2465e80..8ab941cf73f4 100644
    > --- a/include/linux/mm.h
    > +++ b/include/linux/mm.h
    > @@ -776,6 +776,11 @@ 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)
    > +{

    Do we want VM_BUG_ON_PAGE(!PageHead(head), head) here?

    > + return atomic_read(compound_mapcount_ptr(head)) + 1;
    > +}
    > +
    > /*
    > * Mapcount of compound page as a whole, does not include mapped sub-pages.
    > *
    > @@ -785,7 +790,7 @@ 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);
    > }
    >
    > /*
    > @@ -898,11 +903,16 @@ static inline bool hpage_pincount_available(struct page *page)
    > return PageCompound(page) && compound_order(page) > 1;
    > }
    >
    > +static inline int head_pincount(struct page *head)
    > +{

    Ditto.

    > + 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)
    > diff --git a/mm/debug.c b/mm/debug.c
    > index c27fff1e3ca8..69b60637112b 100644
    > --- a/mm/debug.c
    > +++ b/mm/debug.c
    > @@ -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))
    > --
    > 2.28.0
    >

    --
    Kirill A. Shutemov

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