On Wed, Jan 12, 2022 at 06:53:07PM -0300, Mauricio Faria de Oliveira wrote:
> Hi Minchan Kim,
>
> Thanks for handling the hard questions! :)
>
> On Wed, Jan 12, 2022 at 2:33 PM Minchan Kim <minchan@xxxxxxxxxx> wrote:
> >
> > On Wed, Jan 12, 2022 at 09:46:23AM +0800, Huang, Ying wrote:
> > > Yu Zhao <yuzhao@xxxxxxxxxx> writes:
> > >
> > > > On Wed, Jan 05, 2022 at 08:34:40PM -0300, Mauricio Faria de Oliveira wrote:
> > > >> diff --git a/mm/rmap.c b/mm/rmap.c
> > > >> index 163ac4e6bcee..8671de473c25 100644
> > > >> --- a/mm/rmap.c
> > > >> +++ b/mm/rmap.c
> > > >> @@ -1570,7 +1570,20 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> > > >>
> > > >> /* MADV_FREE page check */
> > > >> if (!PageSwapBacked(page)) {
> > > >> - if (!PageDirty(page)) {
> > > >> + int ref_count = page_ref_count(page);
> > > >> + int map_count = page_mapcount(page);
> > > >> +
> > > >> + /*
> > > >> + * The only page refs must be from the isolation
> > > >> + * (checked by the caller shrink_page_list() too)
> > > >> + * and one or more rmap's (dropped by discard:).
> > > >> + *
> > > >> + * Check the reference count before dirty flag
> > > >> + * with memory barrier; see __remove_mapping().
> > > >> + */
> > > >> + smp_rmb();
> > > >> + if ((ref_count - 1 == map_count) &&
> > > >> + !PageDirty(page)) {
> > > >> /* Invalidate as we cleared the pte */
> > > >> mmu_notifier_invalidate_range(mm,
> > > >> address, address + PAGE_SIZE);
> > > >
> > > > Out of curiosity, how does it work with COW in terms of reordering?
> > > > Specifically, it seems to me get_page() and page_dup_rmap() in
> > > > copy_present_pte() can happen in any order, and if page_dup_rmap()
> > > > is seen first, and direct io is holding a refcnt, this check can still
> > > > pass?
> > >
> > > I think that you are correct.
> > >
> > > After more thoughts, it appears very tricky to compare page count and
> > > map count. Even if we have added smp_rmb() between page_ref_count() and
> > > page_mapcount(), an interrupt may happen between them. During the
> > > interrupt, the page count and map count may be changed, for example,
> > > unmapped, or do_swap_page().
> >
> > Yeah, it happens but what specific problem are you concerning from the
> > count change under race? The fork case Yu pointed out was already known
> > for breaking DIO so user should take care not to fork under DIO(Please
> > look at O_DIRECT section in man 2 open). If you could give a specific
> > example, it would be great to think over the issue.
> >
> > I agree it's little tricky but it seems to be way other place has used
> > for a long time(Please look at write_protect_page in ksm.c).
>
> Ah, that's great to see it's being used elsewhere, for DIO particularly!
>
> > So, here what we missing is tlb flush before the checking.
>
> That shouldn't be required for this particular issue/case, IIUIC.
> One of the things we checked early on was disabling deferred TLB flush
> (similarly to what you've done), and it didn't help with the issue; also, the
> issue happens on uniprocessor mode too (thus no remote CPU involved.)
I guess you didn't try it with page_mapcount + 1 == page_count at tha
time? Anyway, I agree we don't need TLB flush here like KSM.
I think the reason KSM is doing TLB flush before the check it to
make sure trap trigger on the write from userprocess in other core.
However, this MADV_FREE case, HW already gaurantees the trap.
Please see below.
>
>
> >
> > Something like this.
> >
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index b0fd9dc19eba..b4ad9faa17b2 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1599,18 +1599,8 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
> >
> > /* MADV_FREE page check */
> > if (!PageSwapBacked(page)) {
> > - int refcount = page_ref_count(page);
> > -
> > - /*
> > - * The only page refs must be from the isolation
> > - * (checked by the caller shrink_page_list() too)
> > - * and the (single) rmap (dropped by discard:).
> > - *
> > - * Check the reference count before dirty flag
> > - * with memory barrier; see __remove_mapping().
> > - */
> > - smp_rmb();
> > - if (refcount == 2 && !PageDirty(page)) {
> > + if (!PageDirty(page) &&
> > + page_mapcount(page) + 1 == page_count(page)) {
>
> In the interest of avoiding a different race/bug, it seemed worth following the
> suggestion outlined in __remove_mapping(), i.e., checking PageDirty()
> after the page's reference count, with a memory barrier in between.
True so it means your patch as-is is good for me.
>
> I'm not familiar with the details of the original issue behind that code change,
> but it seemed to be possible here too, particularly as writes from user-space
> can happen asynchronously / after try_to_unmap_one() checked PTE clean
> and didn't set PageDirty, and if the page's PTE is present, there's no fault?
Yeah, it was discussed.
For clean pte, CPU has to fetch and update the actual pte entry, not TLB
so trap triggers for MADV_FREE page.
https://lkml.org/lkml/2015/4/15/565
https://lkml.org/lkml/2015/4/16/136
