On Mon, Jun 07, 2021 at 09:55:09PM +0200, Jann Horn wrote: > 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? Right; that's how get_user_page_fast() works -- see try_get_compound_head(). If it can't get the reference, it just fails. I suspect for smaps, we can just choose to not count the page. It'll be an inaccuracy in the stats, but I don't think that's a big deal.
