linux-kernel - Re: [PATCH v2 1/1] mm/mlock: implement folio_mlock_step() using folio_pte

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Date: Tue, 4 Jun 2024 09:00:45 +1200
From: Barry Song <21cnbao@...il.com>
To: Lance Yang <ioworker0@...il.com>
Cc: David Hildenbrand <david@...hat.com>, Matthew Wilcox <willy@...radead.org>, akpm@...ux-foundation.org, 
	ryan.roberts@....com, baolin.wang@...ux.alibaba.com, ziy@...dia.com, 
	fengwei.yin@...el.com, ying.huang@...el.com, libang.li@...group.com, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] mm/mlock: implement folio_mlock_step() using folio_pte_batch()

On Tue, Jun 4, 2024 at 3:46 AM Lance Yang <ioworker0@...il.com> wrote:
>
> On Mon, Jun 3, 2024 at 11:26 PM David Hildenbrand <david@...hat.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 = folio_nr_pages(folio);
> > >>>> -  unsigned long pfn = folio_pfn(folio);
> > >>>> +  const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
> > >>>> +  unsigned int count = (end - addr) >> PAGE_SHIFT;
> > >>>
> > >>> This is a pre-existing bug, but ... what happens if you're on a 64-bit
> > >>> 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 the
> > >>> 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() users
> > >> 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 == 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 = pmd_addr_end(addr, end);
> >
> > and use "next" as the actual "end" -- not the VMA end. So "end - addr" =
> > "next - addr" should never exceed a single PMD size.
> >
> >
> > mlock_pte_range() is also called from walk_page_range(), which uses
> >         next = 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 :)

I agree with this point.

These functions are all scanning PTEs under a PMD. Any value exceeding
the PTE entries of one PMD has been a bug of callers but not the callee.

>
> >
> > Will double check later.
>
> I did a double-check and you're correct.
>
> Thanks,
> Lance
>
> >
> > --
> > Cheers,
> >
> > David / dhildenb
> >

Thanks
Barry