Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20211022074619.57355-1-andrea.righi@canonical.com>
 <YXKdRDKp+l6lis/R@casper.infradead.org> <CAHbLzkqFpADQmrPq572M-y53ChJzFJ+uDOHUzzeRFUTv0acq9A@mail.gmail.com>
In-Reply-To: <CAHbLzkqFpADQmrPq572M-y53ChJzFJ+uDOHUzzeRFUTv0acq9A@mail.gmail.com>
From:   Yang Shi <shy828301@gmail.com>
Date:   Fri, 22 Oct 2021 11:13:53 -0700
Message-ID: <CAHbLzkqFnQub71so5Qpi-UrNt_VVUfX82Gz2FTqx48qa-M_UKw@mail.gmail.com>
Subject: Re: [PATCH] mm: fix sleeping copy_huge_page called from atomic context
To:     Matthew Wilcox <willy@infradead.org>,
        Hugh Dickins <hughd@google.com>
Cc:     Andrea Righi <andrea.righi@canonical.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        Minchan Kim <minchan@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Fri, Oct 22, 2021 at 10:38 AM Yang Shi <shy828301@gmail.com> wrote:
>
> On Fri, Oct 22, 2021 at 4:16 AM Matthew Wilcox <willy@infradead.org> wrote:
> >
> > On Fri, Oct 22, 2021 at 09:46:19AM +0200, Andrea Righi wrote:
> > > copy_huge_page() can be called with mapping->private_lock held from
> > > __buffer_migrate_page() -> migrate_page_copy(), so it is not safe to
> > > do a cond_resched() in this context.
> > >
> > > Introduce migrate_page_copy_nowait() and copy_huge_page_nowait()
> > > variants that can be used from an atomic context.
> >
> > I think this is a consequence of THPs being created when they should not
> > be.  This is the wrong way to fix this problem; and I suspect it may
> > already be fixed at least in -mm.  We should have taken this path:
> >
> >         if (!page_has_buffers(page))
> >                 return migrate_page(mapping, newpage, page, mode);
> >
> > but since we didn't, we can infer that there's a THP which has buffers
> > (this should never occur).  It's the same root cause as the invalidatepage
> > problem, just with a very different signature.
>
> Yeah, exactly. And I replied to that syzbot report a few days ago
> (https://lore.kernel.org/linux-mm/CAHbLzkoFaowaG8AU6tg_WMPdjcAdyE+Wafs7TJz1Z23TRg_d8A@mail.gmail.com/)
> with the same conclusion.
>
> I'm not sure why Hugh didn't submit his patch, maybe he was waiting
> for the test result from the bug reporter of that invalidatepage
> issue? It should be fine, the fix is quite straightforward IMHO.

Anyway if Hugh doesn't have time to do it, I could prepare the patch
for formal review.

>
> >