linux-kernel - Re: [PATCH v1] mm/ksm: update stale comment in write_protect

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Date:   Wed, 31 Aug 2022 12:43:31 -0700
From:   Yang Shi <shy828301@...il.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, Peter Xu <peterx@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v1] mm/ksm: update stale comment in write_protect_page()

On Wed, Aug 31, 2022 at 12:36 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 31.08.22 21:34, Yang Shi wrote:
> > On Wed, Aug 31, 2022 at 12:15 PM David Hildenbrand <david@...hat.com> wrote:
> >>
> >> On 31.08.22 21:08, Yang Shi wrote:
> >>> On Wed, Aug 31, 2022 at 11:29 AM David Hildenbrand <david@...hat.com> wrote:
> >>>>
> >>>> On 31.08.22 19:55, Yang Shi wrote:
> >>>>> On Wed, Aug 31, 2022 at 1:30 AM David Hildenbrand <david@...hat.com> wrote:
> >>>>>>
> >>>>>> The comment is stale, because a TLB flush is no longer sufficient and
> >>>>>> required to synchronize against concurrent GUP-fast. This used to be true
> >>>>>> in the past, whereby a TLB flush would have implied an IPI on architectures
> >>>>>> that support GUP-fast, resulting in GUP-fast that disables local interrupts
> >>>>>> from completing before completing the flush.
> >>>>>
> >>>>> Hmm... it seems there might be problem for THP collapse IIUC. THP
> >>>>> collapse clears and flushes pmd before doing anything on pte and
> >>>>> relies on interrupt disable of fast GUP to serialize against fast GUP.
> >>>>> But if TLB flush is no longer sufficient, then we may run into the
> >>>>> below race IIUC:
> >>>>>
> >>>>>          CPU A                                                CPU B
> >>>>> THP collapse                                             fast GUP
> >>>>>
> >>>>> gup_pmd_range() <-- see valid pmd
> >>>>>
> >>>>> gup_pte_range() <-- work on pte
> >>>>> clear pmd and flush TLB
> >>>>> __collapse_huge_page_isolate()
> >>>>>     isolate page <-- before GUP bump refcount
> >>>>>
> >>>>>    pin the page
> >>>>> __collapse_huge_page_copy()
> >>>>>     copy data to huge page
> >>>>>     clear pte (don't flush TLB)
> >>>>> Install huge pmd for huge page
> >>>>>
> >>>>> return the obsolete page
> >>>>
> >>>> Hm, the is_refcount_suitable() check runs while the PTE hasn't been
> >>>> cleared yet. And we don't check if the PMD changed once we're in
> >>>> gup_pte_range().
> >>>
> >>> Yes
> >>>
> >>>>
> >>>> The comment most certainly should be stale as well -- unless there is
> >>>> some kind of an implicit IPI broadcast being done.
> >>>>
> >>>> 2667f50e8b81 mentions: "The RCU page table free logic coupled with an
> >>>> IPI broadcast on THP split (which is a rare event), allows one to
> >>>> protect a page table walker by merely disabling the interrupts during
> >>>> the walk."
> >>>>
> >>>> I'm not able to quickly locate that IPI broadcast -- maybe there is one
> >>>> being done here (in collapse) as well?
> >>>
> >>> The TLB flush may call IPI. I'm supposed it is arch dependent, right?
> >>> Some do use IPI, some may not.
> >>
> >> Right, and the whole idea of the RCU GUP-fast was to support
> >> architectures that don't do it. x86-64 does it. IIRC, powerpc doesn't do
> >> it -- but maybe it does so for PMDs?
> >
> > It looks powerpc does issue IPI for pmd flush. But arm64 doesn't IIRC.
> >
> > So maybe we should implement pmdp_collapse_flush() for those arches to
> > issue IPI.
>
> ... or find another way to detect and handle this in GUP-fast?
>
> Not sure if, for handling PMDs, it could be sufficient to propagate the
> pmdp pointer + value and double check that the values didn't change.

Should work too, right before pinning the page.

pmdp_collapse_flush() is actually just called by khugepaged, so arch
specific implementation should not be a problem and we avoid making
gup fast more complicated.

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