Re: [PATCH v2] mm, dump_page: do not crash with bad compound_mapcount()

[Kirill A. Shutemov]

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@xxxxxx>
> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
> ---
> 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@xxxxxxxxxx/
> 
> 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