Re: [PATCH] mm: gup: fix the fast GUP race against THP collapse

[Yang Shi]

On Thu, Sep 1, 2022 at 11:39 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 02.09.22 01:50, Yang Shi wrote:
> > On Thu, Sep 1, 2022 at 4:26 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
> >>
> >> Hi, Yang,
> >>
> >> On Thu, Sep 01, 2022 at 03:27:07PM -0700, Yang Shi wrote:
> >>> Since general RCU GUP fast was introduced in commit 2667f50e8b81 ("mm:
> >>> introduce a general RCU get_user_pages_fast()"), a TLB flush is no longer
> >>> sufficient to handle concurrent GUP-fast in all cases, it only handles
> >>> traditional IPI-based GUP-fast correctly.
> >>
> >> If TLB flush (or, IPI broadcasts) used to work to protect against gup-fast,
> >> I'm kind of confused why it's not sufficient even if with RCU gup?  Isn't
> >> that'll keep working as long as interrupt disabled (which current fast-gup
> >> will still do)?
> >
> > Actually the wording was copied from David's commit log for his
> > PageAnonExclusive fix. My understanding is the IPI broadcast still
> > works, but it may not be supported by all architectures and not
> > preferred anymore. So we should avoid depending on IPI broadcast IIUC.
>
> Right. Not all architectures perform an IPI broadcast on TLB flush.
>
> IPI broadcasts will continue working until we use RCU instead of
> disabling local interrupts in GUP-fast.
>
>
> >>>    CPU A                                          CPU B
> >>> THP collapse                                     fast GUP
> >>>                                               gup_pmd_range() <-- see valid pmd
> >>>                                                   gup_pte_range() <-- work on pte
> >>> pmdp_collapse_flush() <-- clear pmd and flush
> >>> __collapse_huge_page_isolate()
> >>>     check page pinned <-- before GUP bump refcount
> >>>                                                       pin the page
> >>>                                                       check PTE <-- no change
> >>> __collapse_huge_page_copy()
> >>>     copy data to huge page
> >>>     ptep_clear()
> >>> install huge pmd for the huge page
> >>>                                                       return the stale page
> >>> discard the stale page
> >>>
> >>> The race could be fixed by checking whether PMD is changed or not after
> >>> taking the page pin in fast GUP, just like what it does for PTE.  If the
> >>> PMD is changed it means there may be parallel THP collapse, so GUP
> >>> should back off.
> >>
> >> Could the race also be fixed by impl pmdp_collapse_flush() correctly for
> >> the archs that are missing? Do you know which arch(s) is broken with it?
> >
> > Yes, and this was suggested by me in the first place, but per the
> > suggestion from John and David, this is not the preferred way. I think
> > it is because:
> >
> > Firstly, using IPI to serialize against fast GUP is not recommended
> > anymore since fast GUP does check PTE then back off so we should avoid
> > it.
> > Secondly, if checking PMD then backing off could solve the problem,
> > why do we still need broadcast IPI? It doesn't sound performant.
>
> I'd say, using an IPI is the old-styled way of doing things. Sure, using
> an IPI broadcast will work (and IMHO it's a lot easier to
> not-get-wrong). But it somewhat contradicts to the new way of doing things.
>
> >>
> >> It's just not clear to me whether this patch is an optimization or a fix,
> >> if it's a fix whether the IPI broadcast in ppc pmdp_collapse_flush() would
> >> still be needed.
> >
> > It is a fix and the fix will make IPI broadcast not useful anymore.
>
> I'd wonder how "easy" adding the IPI broadcast would be -- IOW, if the
> IPI fix has a real advantage.

Not sure either, but I guess calling a dummy function via IPI
broadcast should just work. Powepc does so.

>
>
> --
> Thanks,
>
> David / dhildenb
>