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

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Date: Mon, 3 Jun 2024 23:46:35 +0800
From: Lance Yang <ioworker0@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: Matthew Wilcox <willy@...radead.org>, akpm@...ux-foundation.org, ryan.roberts@....com, 
	21cnbao@...il.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 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 :)

>
> Will double check later.

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

Thanks,
Lance

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