Re: split_huge_page_to_list() races with page_mapcount() on migration entry in smaps code? [was: Re: [syzbot] kernel BUG in __page_mapcount]

[Jann Horn]

On Mon, Jun 7, 2021 at 8:03 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
> On Mon, Jun 07, 2021 at 07:27:23PM +0200, Jann Horn wrote:
> > === Short summary ===
> > I believe the issue here is a race between /proc/*/smaps and
> > split_huge_page_to_list():
> >
> > The codepath for /proc/*/smaps walks the pagetables and (e.g. in
> > smaps_account()) calls page_mapcount() not just on pages from normal
> > PTEs but also on migration entries (since commit b1d4d9e0cbd0a
> > "proc/smaps: carefully handle migration entries", from Linux v3.5).
> > page_mapcount() expects compound pages to be stable.
> >
> > The split_huge_page_to_list() path first protects the compound page by
> > locking it and replacing all its PTEs with migration entries (since
> > the THP rewrite in v4.5, I think?), then does the actual splitting
> > using __split_huge_page().
> >
> > So there's a mismatch of expectations here:
> > The smaps code expects that migration entries point to stable compound
> > pages, while the THP code expects that it's okay to split a compound
> > page while it has migration entries.
>
> Will it be a colossal performance penalty if we always get the page
> refcount after looking it up?  That will cause split_huge_page() to
> fail to split the page if it hits this race.

Hmm - but with that approach I'm not sure you could even easily take a
refcount on a page whose refcount may be frozen and which may be in
the middle of being shattered? get_page_unless_zero() is wrong because
you can't take references on tail pages, right? (Or can you?) And
try_get_page() is wrong because it bugs out if the refcount is zero -
and even if it didn't do that, you might end up holding a reference on
the head page while the page you're actually interested in is a tail
page?

I guess if it was really necessary, it'd be possible to do some kind
of retry thing that grabs a reference on the compound head, then
checks that the tail page is still associated with the compound head,
and if not, drops the compound head and tries again?