Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
MIME-Version: 1.0
References: <20230109205336.3665937-1-surenb@google.com> <20230109205336.3665937-19-surenb@google.com>
 <Y8a9+ywh65fmuKvv@dhcp22.suse.cz> <CAG48ez2Adwqs8Vvm3YUKwpx8qzV1wWtnUdWVo1UphjzADjMZQQ@mail.gmail.com>
In-Reply-To: <CAG48ez2Adwqs8Vvm3YUKwpx8qzV1wWtnUdWVo1UphjzADjMZQQ@mail.gmail.com>
From:   Suren Baghdasaryan <surenb@google.com>
Date:   Tue, 17 Jan 2023 13:05:15 -0800
Message-ID: <CAJuCfpHrRAQBPD68PFr0wcbV4fYTEYgxFj0AeeOn6W1sszw9xw@mail.gmail.com>
Subject: Re: [PATCH 18/41] mm/khugepaged: write-lock VMA while collapsing a
 huge page
To:     Jann Horn <jannh@google.com>
Cc:     Michal Hocko <mhocko@suse.com>, akpm@linux-foundation.org,
        michel@lespinasse.org, jglisse@google.com, vbabka@suse.cz,
        hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net,
        willy@infradead.org, liam.howlett@oracle.com, peterz@infradead.org,
        ldufour@linux.ibm.com, laurent.dufour@fr.ibm.com,
        paulmck@kernel.org, luto@kernel.org, songliubraving@fb.com,
        peterx@redhat.com, david@redhat.com, dhowells@redhat.com,
        hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev,
        punit.agrawal@bytedance.com, lstoakes@gmail.com,
        peterjung1337@gmail.com, rientjes@google.com,
        axelrasmussen@google.com, joelaf@google.com, minchan@google.com,
        shakeelb@google.com, tatashin@google.com, edumazet@google.com,
        gthelen@google.com, gurua@google.com, arjunroy@google.com,
        soheil@google.com, hughlynch@google.com, leewalsh@google.com,
        posk@google.com, linux-mm@kvack.org,
        linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@android.com
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, Jan 17, 2023 at 12:28 PM Jann Horn <jannh@google.com> wrote:
>
> On Tue, Jan 17, 2023 at 4:25 PM Michal Hocko <mhocko@suse.com> wrote:
> > On Mon 09-01-23 12:53:13, Suren Baghdasaryan wrote:
> > > Protect VMA from concurrent page fault handler while collapsing a huge
> > > page. Page fault handler needs a stable PMD to use PTL and relies on
> > > per-VMA lock to prevent concurrent PMD changes. pmdp_collapse_flush(),
> > > set_huge_pmd() and collapse_and_free_pmd() can modify a PMD, which will
> > > not be detected by a page fault handler without proper locking.
> >
> > I am struggling with this changelog. Maybe because my recollection of
> > the THP collapsing subtleties is weak. But aren't you just trying to say
> > that the current #PF handling and THP collapsing need to be mutually
> > exclusive currently so in order to keep that assumption you have mark
> > the vma write locked?
> >
> > Also it is not really clear to me how that handles other vmas which can
> > share the same thp?
>
> It's not about the hugepage itself, it's about how the THP collapse
> operation frees page tables.
>
> Before this series, page tables can be walked under any one of the
> mmap lock, the mapping lock, and the anon_vma lock; so when khugepaged
> unlinks and frees page tables, it must ensure that all of those either
> are locked or don't exist. This series adds a fourth lock under which
> page tables can be traversed, and so khugepaged must also lock out that one.
>
> There is a codepath in khugepaged that iterates through all mappings
> of a file to zap page tables (retract_page_tables()), which locks each
> visited mm with mmap_write_trylock() and now also does
> vma_write_lock().
>
>
> I think one aspect of this patch that might cause trouble later on, if
> support for non-anonymous VMAs is added, is that retract_page_tables()
> now does vma_write_lock() while holding the mapping lock; the page
> fault handling path would probably take the locks the other way
> around, leading to a deadlock? So the vma_write_lock() in
> retract_page_tables() might have to become a trylock later on.
>
> Related: Please add the new VMA lock to the big lock ordering comments
> at the top of mm/rmap.c. (And maybe later mm/filemap.c, if/when you
> add file VMA support.)

Thanks for the clarifications and the warning. I'll add appropriate
comments and will take this deadlocking scenario into account when
later implementing support for file-backed page faults.