linux-kernel - Re: [PATCH v7 2/4] mm/rmap: add helper to restart pgtable walk on changes

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Date: Thu, 13 Jun 2024 16:54:39 +0800
From: Lance Yang <ioworker0@...il.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org, sj@...nel.org, 
	baolin.wang@...ux.alibaba.com, maskray@...gle.com, ziy@...dia.com, 
	ryan.roberts@....com, 21cnbao@...il.com, mhocko@...e.com, 
	fengwei.yin@...el.com, zokeefe@...gle.com, shy828301@...il.com, 
	xiehuan09@...il.com, libang.li@...group.com, wangkefeng.wang@...wei.com, 
	songmuchun@...edance.com, peterx@...hat.com, minchan@...nel.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 2/4] mm/rmap: add helper to restart pgtable walk on changes

On Thu, Jun 13, 2024 at 4:30 PM David Hildenbrand <david@...hat.com> wrote:
>
> On 10.06.24 14:02, Lance Yang wrote:
> > Introduce the page_vma_mapped_walk_restart() helper to handle scenarios
> > where the page table walk needs to be restarted due to changes in the page
> > table, such as when a PMD is split. It releases the PTL held during the
> > previous walk and resets the state, allowing a new walk to start at the
> > current address stored in pvmw->address.
> >
> > Suggested-by: David Hildenbrand <david@...hat.com>
> > Signed-off-by: Lance Yang <ioworker0@...il.com>
> > ---
> >   include/linux/rmap.h | 22 ++++++++++++++++++++++
> >   1 file changed, 22 insertions(+)
> >
> > diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> > index 7229b9baf20d..5f18509610cc 100644
> > --- a/include/linux/rmap.h
> > +++ b/include/linux/rmap.h
> > @@ -710,6 +710,28 @@ static inline void page_vma_mapped_walk_done(struct page_vma_mapped_walk *pvmw)
> >               spin_unlock(pvmw->ptl);
> >   }
> >
> > +/**
> > + * page_vma_mapped_walk_restart - Restart the page table walk.
> > + * @pvmw: Pointer to struct page_vma_mapped_walk.
> > + *
> > + * It restarts the page table walk when changes occur in the page
> > + * table, such as splitting a PMD. Ensures that the PTL held during
> > + * the previous walk is released and resets the state to allow for
> > + * a new walk starting at the current address stored in pvmw->address.
> > + */
> > +static inline void
> > +page_vma_mapped_walk_restart(struct page_vma_mapped_walk *pvmw)
> > +{
> > +     WARN_ON_ONCE(!pvmw->pmd);
>
> Can we have this more general, like
>
> WARN_ON_ONCE(!pvmw->pmd && !pvmw->pte);

Sure, let’s make it more general.

>
> And then setting both to NULL below?
>
>
> > +     WARN_ON_ONCE(!pvmw->ptl);
>
> This is confusing: you check for ptl below. What would be clearer is
>
> if (likely(pvmw->ptl))
>         spin_unlock(pvmw->ptl);
> else
>         WARN_ON_ONCE(1);

Will adjust as you suggested, thanks!

>
>
> > +
> > +     if (pvmw->ptl)
> > +             spin_unlock(pvmw->ptl);
> > +
> > +     pvmw->ptl = NULL;
> > +     pvmw->pmd = NULL;
> > +}
> > +
> >   bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw);
> >
> >   /*
>
> I'd suggest squashing that into the next patch.

Got it.

Thanks,
Lance

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