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;
Date:   Thu, 23 Mar 2023 18:56:34 -0700 (PDT)
From:   Hugh Dickins <hughd@google.com>
To:     Song Liu <song@kernel.org>
cc:     Matthew Wilcox <willy@infradead.org>,
        Hugh Dickins <hughd@google.com>,
        Song Liu <songliubraving@fb.com>, Jiri Olsa <jolsa@kernel.org>,
        David Stevens <stevensd@chromium.org>, linux-mm@kvack.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Xu <peterx@redhat.com>,
        "Kirill A . Shutemov" <kirill@shutemov.name>,
        Yang Shi <shy828301@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Jiaqi Yan <jiaqiyan@google.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v5 3/3] mm/khugepaged: maintain page cache uptodate
 flag
In-Reply-To: <CAPhsuW5pTh2+S7VjRbRy=WUAb7-GLQpi6HKCW0ehxs5HAup_dQ@mail.gmail.com>
Message-ID: <b49b118d-893c-c749-61f6-9441539418a5@google.com>
References: <20230307052036.1520708-1-stevensd@google.com> <20230307052036.1520708-4-stevensd@google.com> <866d1a75-d462-563-dfd7-1aa2971a285b@google.com> <ZBzK99dSmbPB7vjA@casper.infradead.org>
 <CAPhsuW5pTh2+S7VjRbRy=WUAb7-GLQpi6HKCW0ehxs5HAup_dQ@mail.gmail.com>
MIME-Version: 1.0
Content-Type: multipart/mixed; boundary="-1463760895-38525656-1679623005=:6497"
Precedence: bulk

  This message is in MIME format.  The first part should be readable text,
  while the remaining parts are likely unreadable without MIME-aware tools.

---1463760895-38525656-1679623005=:6497
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: QUOTED-PRINTABLE

On Thu, 23 Mar 2023, Song Liu wrote:
> On Thu, Mar 23, 2023 at 2:56=E2=80=AFPM Matthew Wilcox <willy@infradead.o=
rg> wrote:
> >
> > On Thu, Mar 23, 2023 at 12:07:46PM -0700, Hugh Dickins wrote:
> > > On an earlier audit, for different reasons, I did also run across
> > > lib/buildid.c build_id_parse() using find_get_page() without checking
> > > PageUptodate() - looks as if it might do the wrong thing if it races
> > > with khugepaged collapsing text to huge, and should probably have a
> > > similar fix.
> >
> > That shouldn't be using find_get_page().  It should probably use
> > read_cache_folio() which will actually read in the data if it's not
> > present in the page cache, and return an ERR_PTR if the data couldn't
> > be read.
>=20
> build_id_parse() can be called from NMI, so I don't think we can let
> read_cache_folio() read-in the data.

Interesting.

This being the same Layering_Violation_ID which is asking for a home in
everyone's struct file?  (Okay, I'm being disagreeable, no need to answer!)

I think even the current find_get_page() is unsafe from NMI: imagine that
NMI interrupting a sequence (maybe THP collapse or splitting, maybe page
migration, maybe others) when the page refcount has been frozen to 0:
you'll just have to reboot the machine?

I guess the RCU-safety of find_get_page() implies that its XArray basics
are safe in NMI; but you need a low-level variant (xas_find()?) which
does none of the "goto retry"s, and fails immediately if anything is
wrong - including !PageUptodate.

Hugh
---1463760895-38525656-1679623005=:6497--