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 23:08:42 +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 10:56 PM David Hildenbrand <david@...hat.com> 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.

Could I change folio_pte_batch() to take an unsigned long max_nr?

Thanks,
Lance

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