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

[Matthew Wilcox]

On Fri, Aug 07, 2020 at 05:35:04PM +0300, Kirill A. Shutemov wrote:
> 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().

[...]
> > +static inline int head_mapcount(struct page *head)
> > +{
> 
> Do we want VM_BUG_ON_PAGE(!PageHead(head), head) here?

Well, no.  That was the point of the bug report -- by the time we called
compound_mapcount, the page was no longer a head page.

> > 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.

OK, point taken.  I might go for head_compound_mapcount()?  Or as I
originally suggested, just opencoding it like we do in __page_mapcount().