Re: mm: delay rmap removal until after TLB flush

[Christian Borntraeger]

Am 02.11.22 um 10:14 schrieb Christian Borntraeger:
> Am 31.10.22 um 19:43 schrieb Linus Torvalds:
> > Updated subject line, and here's the link to the original discussion
> > for new people:
> >
> >      https://lore.kernel.org/all/B88D3073-440A-41C7-95F4-895D3F657EF2@xxxxxxxxx/
> >
> > On Mon, Oct 31, 2022 at 10:28 AM Linus Torvalds
> > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > > Ok. At that point we no longer have the pte or the virtual address, so
> > > it's not going to be exactly the same debug output.
> > >
> > > But I think it ends up being fairly natural to do
> > >
> > >          VM_WARN_ON_ONCE_PAGE(page_mapcount(page) < 0, page);
> > >
> > > instead, and I've fixed that last patch up to do that.
> >
> > Ok, so I've got a fixed set of patches based on the feedback from
> > PeterZ, and also tried to do the s390 updates for this blindly, and
> > pushed them out into a git branch:
> >
> >      https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/log/?h=mmu_gather-race-fix
> >
> > If people really want to see the patches in email again, I can do
> > that, but most of you already have, and the changes are either trivial
> > fixes or the s390 updates.
> >
> > For the s390 people that I've now added to the participant list maybe
> > the git tree is fine - and the fundamental explanation of the problem
> > is in that top-most commit (with the three preceding commits being
> > prep-work). Or that link to the thread about this all.
>
> Adding Gerald.

now the correct Gerald....
> > That top-most commit is also where I tried to fix things up for s390
> > that uses its own non-gathering TLB flush due to
> > CONFIG_MMU_GATHER_NO_GATHER.
> >
> > NOTE NOTE NOTE! Unlike my regular git branch, this one may end up
> > rebased etc for further comments and fixes. So don't consider that
> > stable, it's still more of an RFC branch.
> >
> > At a minimum I'll update it with Ack's etc, assuming I get those, and
> > my s390 changes are entirely untested and probably won't work.
> >
> > As far as I can tell, s390 doesn't actually *have* the problem that
> > causes this change, because of its synchronous TLB flush, but it
> > obviously needs to deal with the change of rmap zapping logic.
> >
> > Also added a few people who are explicitly listed as being mmu_gather
> > maintainers. Maybe people saw the discussion on the linux-mm list, but
> > let's make it explicit.
> >
> > Do people have any objections to this approach, or other suggestions?
> >
> > I do *not* consider this critical, so it's a "queue for 6.2" issue for me.
> >
> > It probably makes most sense to queue in the -MM tree (after the thing
> > is acked and people agree), but I can keep that branch alive too and
> > just deal with it all myself as well.
> >
> > Anybody?
> >
> >                       Linus
>
> It certainly needs a build fix for s390:
>
>
> In file included from kernel/sched/core.c:78:
> ./arch/s390/include/asm/tlb.h: In function '__tlb_remove_page_size':
> ./arch/s390/include/asm/tlb.h:50:17: error: implicit declaration of function 'page_zap_pte_rmap' [-Werror=implicit-function-declaration]
>     50 |                 page_zap_pte_rmap(page);
>        |                 ^~~~~~~~~~~~~~~~~