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From: Lance Yang <ioworker0@gmail.com>
Date: Mon, 3 Jun 2024 23:46:35 +0800
Message-ID: <CAK1f24=Xa-n2J4_sYL161PMB3hhJ8tcQtJ0qdV7ChPtjMTrPQg@mail.gmail.com>
Subject: Re: [PATCH v2 1/1] mm/mlock: implement folio_mlock_step() using folio_pte_batch()
To: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>, akpm@linux-foundation.org, ryan.roberts@arm.com, 
	21cnbao@gmail.com, baolin.wang@linux.alibaba.com, ziy@nvidia.com, 
	fengwei.yin@intel.com, ying.huang@intel.com, libang.li@antgroup.com, 
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
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On Mon, Jun 3, 2024 at 11:26=E2=80=AFPM David Hildenbrand <david@redhat.com=
> wrote:
>
> On 03.06.24 17:01, Matthew Wilcox wrote:
> > On Mon, Jun 03, 2024 at 04:56:05PM +0200, David Hildenbrand wrote:
> >> On 03.06.24 16:43, Matthew Wilcox wrote:
> >>> On Mon, Jun 03, 2024 at 10:07:45PM +0800, Lance Yang wrote:
> >>>> +++ b/mm/mlock.c
> >>>> @@ -307,26 +307,15 @@ void munlock_folio(struct folio *folio)
> >>>>    static inline unsigned int folio_mlock_step(struct folio *folio,
> >>>>                    pte_t *pte, unsigned long addr, unsigned long end=
)
> >>>>    {
> >>>> -  unsigned int count, i, nr =3D folio_nr_pages(folio);
> >>>> -  unsigned long pfn =3D folio_pfn(folio);
> >>>> +  const fpb_t fpb_flags =3D FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRT=
Y;
> >>>> +  unsigned int count =3D (end - addr) >> PAGE_SHIFT;
> >>>
> >>> This is a pre-existing bug, but ... what happens if you're on a 64-bi=
t
> >>> system and you mlock() a range that is exactly 2^44 bytes?  Seems to =
me
> >>> that count becomes 0.  Why not use an unsigned long here and avoid th=
e
> >>> problem entirely?
> >>>
> >>> folio_pte_batch() also needs to take an unsigned long max_nr in that
> >>> case, because you aren't restricting it to folio_nr_pages().
> >>
> >> Yeah, likely we should also take a look at other folio_pte_batch() use=
rs
> >> like copy_present_ptes() that pass the count as an int. Nothing should
> >> really be broken, but we might not batch as much as we could, which is
> >> unfortunate.
> >
> > You did include:
> >
> >          VM_WARN_ON_FOLIO(!folio_test_large(folio) || max_nr < 1, folio=
);
> >
> > so at the least we have a userspace-triggerable warning.
>
> Yes, and max_nr =3D=3D 0 would likely not be healthy to the system.
>
> But
>
> For copy_pte_range(), zap_pte_range() and the madvise users, we should
> always have:
>         next =3D pmd_addr_end(addr, end);
>
> and use "next" as the actual "end" -- not the VMA end. So "end - addr" =
=3D
> "next - addr" should never exceed a single PMD size.
>
>
> mlock_pte_range() is also called from walk_page_range(), which uses
>         next =3D pmd_addr_end(addr, end);
>
> So likely exceeding PMD size is not possible here and all is working as
> expected.

Thanks for clarifying!

I agree that currently all is fine, so perhaps we don't worry about that :)

>
> Will double check later.

I did a double-check and you're correct.

Thanks,
Lance

>
> --
> Cheers,
>
> David / dhildenb
>