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

[Matthew Wilcox]

On Thu, Aug 06, 2020 at 05:53:10PM +0200, Vlastimil Babka wrote:
> On 8/6/20 5:39 PM, Matthew Wilcox wrote:
> >> >> +++ b/mm/huge_memory.c
> >> >> @@ -2125,7 +2125,7 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> >> >>  	 * Set PG_double_map before dropping compound_mapcount to avoid
> >> >>  	 * false-negative page_mapped().
> >> >>  	 */
> >> >> -	if (compound_mapcount(page) > 1 && !TestSetPageDoubleMap(page)) {
> >> >> +	if (head_mapcount(page) > 1 && !TestSetPageDoubleMap(page)) {
> >> > 
> >> > I'm a little nervous about this one.  The page does actually come from
> >> > pmd_page(), and today that's guaranteed to be a head page.  But I'm
> >> > not convinced that's going to still be true in twenty years.  With the
> >> > current THP patchset, I won't allocate pages larger than PMD order, but
> >> > I can see there being interest in tracking pages in chunks larger than
> >> > 2MB in the future.  And then pmd_page() might well return a tail page.
> >> > So it might be a good idea to not convert this one.
> >> 
> >> Hmm the function converts the compound mapcount of the whole page to a
> >> HPAGE_PMD_NR of base pages. If suddenly the compound page was bigger than a pmd,
> >> then I guess this wouldn't work properly anymore without changes anyway?
> >> Maybe we could stick something like VM_BUG_ON(PageTransHuge(page)) there as
> >> "enforced documentation" for now?
> > 
> > I think it would work as-is.  But also I may have totally misunderstood it.
> > I'll write this declaratively and specifically for x86 (PMD order is 9)
> > ... tell me when I've made a mistake ;-)
> > 
> > This function is for splitting the PMD.  We're leaving the underlying
> > page intact and just changing the page table.  So if, say, we have an
> > underlying 4MB page (and maybe the pages are mapped as PMDs in this
> > process), we might get subpage number 512 of this order-10 page.  We'd
> > need to check the DoubleMap bit on subpage 1, and the compound_mapcount
> > also stored in page 1, but we'd only want to spread the mapcount out
> > over the 512 subpages from 512-1023; we wouldn't want to spread it out
> > over 0-511 because they aren't affected by this particular PMD.
> 
> Yeah, and then we decrease the compound mapcount, which is a counter of "how
> many times is this compound page mapped as a whole". But we only removed (the
> second) half of the compound mapping, so imho that would be wrong?

I'd expect that count to be incremented by 1 for each PMD that it's
mapped to?  ie change the definition of that counter slightly.

> > Having to reason about stuff like this is why I limited the THP code to
> > stop at PMD order ... I don't want to make my life even more complicated
> > than I have to!
> 
> Kirill might correct me but I'd expect the THP code right now has baked in many
> assumptions about THP pages being exactly HPAGE_PMD_ORDER large?

There are somewhat fewer places that make that assumption after applying
the ~80 patches here ... http://git.infradead.org/users/willy/pagecache.git

I have mostly not touched the anonymous THPs (obviously some of the code
paths are shared), although both Kirill & I think there's a win to be
had there too.