Messages in this thread | | | Date | Fri, 7 Aug 2020 19:48:05 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCH v2] mm, dump_page: do not crash with bad compound_mapcount() |
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On Fri, Aug 07, 2020 at 04:10:29PM +0100, Matthew Wilcox wrote: > 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.
Right. VM_BUG_ON_PAGE(PageTail(head), head)?
> > > 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().
I'm fine either way.
-- Kirill A. Shutemov
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