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From: Hugh Dickins <hughd@google.com>
To: Jann Horn <jannh@google.com>
Cc: Hugh Dickins <hughd@google.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Christoph Hellwig <hch@lst.de>,
	kernel list <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	"Eric W . Biederman" <ebiederm@xmission.com>,
	Oleg Nesterov <oleg@redhat.com>
Subject: Re: [PATCH v5 7/7] mm: Remove the now-unnecessary mmget_still_valid() hack
Date: Mon, 31 Aug 2020 13:36:16 -0700 (PDT)	[thread overview]
Message-ID: <alpine.LSU.2.11.2008311326540.3602@eggly.anvils> (raw)
In-Reply-To: <CAG48ez15Zxbkjv0WRChZZZ6F78pFVXPTu_Bn1Pqaxx=7Gk1gUg@mail.gmail.com>

On Mon, 31 Aug 2020, Jann Horn wrote:
> On Mon, Aug 31, 2020 at 8:07 AM Hugh Dickins <hughd@google.com> wrote:
> > On Thu, 27 Aug 2020, Jann Horn wrote:
> >
> > > The preceding patches have ensured that core dumping properly takes the
> > > mmap_lock. Thanks to that, we can now remove mmget_still_valid() and all
> > > its users.
> >
> > Hi Jann, while the only tears to be shed over losing mmget_still_valid()
> > will be tears of joy, I think you need to explain why you believe it's
> > safe to remove the instance in mm/khugepaged.c: which you'll have found
> > I moved just recently, to cover an extra case (sorry for not Cc'ing you).
> >
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -431,7 +431,7 @@ static void insert_to_mm_slots_hash(struct mm_struct *mm,
> > >
> > >  static inline int khugepaged_test_exit(struct mm_struct *mm)
> > >  {
> > > -     return atomic_read(&mm->mm_users) == 0 || !mmget_still_valid(mm);
> > > +     return atomic_read(&mm->mm_users) == 0;
> > >  }
> > >
> > >  static bool hugepage_vma_check(struct vm_area_struct *vma,
> >
> > The movement (which you have correctly followed) was in
> > bbe98f9cadff ("khugepaged: khugepaged_test_exit() check mmget_still_valid()")
> > but the "pmd .. physical page 0" issue is explained better in its parent
> > 18e77600f7a1 ("khugepaged: retract_page_tables() remember to test exit")
> >
> > I think your core dumping is still reading the page tables without
> > holding mmap_lock
> 
> Where? get_dump_page() takes mmap_lock now:
> <https://lore.kernel.org/lkml/20200827114932.3572699-7-jannh@google.com/>

Right, sorry for the noise, that's precisely what 6/7 is all about,
and properly declares itself there in its Subject - I plead that I
got distracted by the vma snapshot part of the series, and paid too
little attention before bleating.

Looks good to me - thanks.

> 
> I don't think there should be any paths into __get_user_pages() left
> that don't hold the mmap_lock. Actually, we should probably try
> sticking mmap_assert_locked() in there now as a follow-up?

Maybe: I haven't given it thought, to be honest.

Hugh

  reply	other threads:[~2020-08-31 20:36 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-27 11:49 [PATCH v5 0/7] Fix ELF / FDPIC ELF core dumping, and use mmap_lock properly in there Jann Horn
2020-08-27 11:49 ` [PATCH v5 1/7] binfmt_elf_fdpic: Stop using dump_emit() on user pointers on !MMU Jann Horn
2020-08-27 11:49 ` [PATCH v5 2/7] coredump: Let dump_emit() bail out on short writes Jann Horn
2020-08-27 11:49 ` [PATCH v5 3/7] coredump: Refactor page range dumping into common helper Jann Horn
2020-08-27 11:49 ` [PATCH v5 4/7] coredump: Rework elf/elf_fdpic vma_dump_size() " Jann Horn
2020-08-27 11:49 ` [PATCH v5 5/7] binfmt_elf, binfmt_elf_fdpic: Use a VMA list snapshot Jann Horn
2020-08-27 11:49 ` [PATCH v5 6/7] mm/gup: Take mmap_lock in get_dump_page() Jann Horn
2020-08-27 17:13   ` Linus Torvalds
2020-08-27 11:49 ` [PATCH v5 7/7] mm: Remove the now-unnecessary mmget_still_valid() hack Jann Horn
2020-08-31  6:06   ` Hugh Dickins
2020-08-31  9:58     ` Jann Horn
2020-08-31 20:36       ` Hugh Dickins [this message]
2020-08-31 21:30       ` Hugh Dickins
2020-08-27 17:15 ` [PATCH v5 0/7] Fix ELF / FDPIC ELF core dumping, and use mmap_lock properly in there Linus Torvalds

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