Re: [v3 PATCH 2/2] mm: thp: check page_mapped instead of page_mapcount for split

[Hugh Dickins]

On Wed, 26 May 2021, Yang Shi wrote:
> On Tue, May 25, 2021 at 4:58 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> > On Tue, 25 May 2021, Yang Shi wrote:
> > >
> > > We should be able to make dump_page() print total mapcount, right? The
> > > dump_page() should be just called in some error paths so taking some
> > > extra overhead to dump more information seems harmless, or am I
> > > missing something? Of course, this can be done in a separate patch.
> >
> > I didn't want to ask that of you, but yes, if you're willing to add
> > total_mapcount() into dump_page(), I think that would be ideal; and
> > could be helpful for other cases too.
> >
> > Looking through total_mapcount(), I think it's safe to call from
> > dump_page() - I always worry about extending crash info with
> > something that depends on a maybe-corrupted pointer which would
> > generate a further crash and either recurse or truncate the output -
> > but please check that carefully.
> 
> Yes, it is possible. If the THP is being split, some VM_BUG_* might be
> triggered if total_mapcount() is called. But it is still feasible to
> print total mapcount as long as we implement a more robust version for
> dump_page().

Oh dear. I think the very last thing the kernel needs is yet another
subtly different variant of *mapcount*().

Do you have a specific VM_BUG_* in mind there?  Of course there's
the VM_BUG_ON_PAGE(PageTail) at the start of it, and you'd want to
print total_mapcount(head) to avoid that one.

Looks like __dump_page() is already careful about "head", checking
whether "page" is within the expected bounds.  Of course, once we're
in serious VM_WARN territory, there might be races which could flip
fields midway: PageTail set by the time it reaches total_mapcount()?
Narrow the race (rather like it does with PageSlab) by testing
PageTail immediately before calling total_mapcount(head)?

Hugh