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From: Suren Baghdasaryan <surenb@google.com>
Date: Tue, 2 Jan 2024 15:16:20 -0800
Message-ID: <CAJuCfpG4tSPADrSpUCubsymoT_FWO4mONFODb2_sK4f-5RTY-A@mail.gmail.com>
Subject: Re: [PATCH 1/1] userfaultfd: fix move_pages_pte() splitting folio
 under RCU read lock
To: Peter Xu <peterx@redhat.com>
Cc: akpm@linux-foundation.org, viro@zeniv.linux.org.uk, brauner@kernel.org, 
	shuah@kernel.org, aarcange@redhat.com, lokeshgidra@google.com, 
	david@redhat.com, ryan.roberts@arm.com, hughd@google.com, mhocko@suse.com, 
	axelrasmussen@google.com, rppt@kernel.org, willy@infradead.org, 
	Liam.Howlett@oracle.com, jannh@google.com, zhangpeng362@huawei.com, 
	bgeffon@google.com, kaleshsingh@google.com, ngeoffray@google.com, 
	jdduke@google.com, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, 
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, 
	kernel-team@android.com
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On Tue, Jan 2, 2024 at 8:58=E2=80=AFAM Suren Baghdasaryan <surenb@google.co=
m> wrote:
>
> On Tue, Jan 2, 2024 at 1:00=E2=80=AFAM Peter Xu <peterx@redhat.com> wrote=
:
> >
> > On Fri, Dec 29, 2023 at 06:56:07PM -0800, Suren Baghdasaryan wrote:
> > > @@ -1078,9 +1078,14 @@ static int move_pages_pte(struct mm_struct *mm=
, pmd_t *dst_pmd, pmd_t *src_pmd,
> > >
> > >               /* at this point we have src_folio locked */
> > >               if (folio_test_large(src_folio)) {
> > > +                     /* split_folio() can block */
> > > +                     pte_unmap(&orig_src_pte);
> > > +                     pte_unmap(&orig_dst_pte);
> > > +                     src_pte =3D dst_pte =3D NULL;
> > >                       err =3D split_folio(src_folio);
> > >                       if (err)
> > >                               goto out;
> > > +                     goto retry;
> > >               }
> >
> > Do we also need to clear src_folio and src_folio_pte?  If the folio is =
a
> > thp, I think it means it's pte mapped here. Then after the split we may
> > want to fetch the small folio after the split, not the head one?
>
> I think we need to re-fetch the src_folio only if the src_addr falls
> into a non-head page. Looking at the __split_huge_page(), the head
> page is skipped in the last loop, so I think it should stay valid.
> That said, maybe it's just an implementation detail of the
> __split_huge_page() and I should not rely on that and refetch anyway?

I'll post a v2 with this fix and re-fetching the folio
unconditionally. We also don't need to reset src_folio_pte value
because it's used only if src_folio is not NULL.
Thanks for catching this, Peter!

>
> >
> > --
> > Peter Xu
> >