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

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Message-ID: <Yw+uASvgWU5+9PvI@xz-m1.local>
Date:   Wed, 31 Aug 2022 14:52:49 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Yang Shi <shy828301@...il.com>
Cc:     David Hildenbrand <david@...hat.com>, 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>,
        "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 10:55:43AM -0700, 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

Maybe the pmd level tlb flush is still needed, but on pte level it's
optional (where we can rely on fast-gup rechecking on the pte change)?

-- 
Peter Xu