linux-kernel - Re: [PATCH] hugetlb: unshare some PMDs when splitting VMAs

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Date:   Wed, 4 Jan 2023 19:34:00 +0000
From:   James Houghton <jthoughton@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] hugetlb: unshare some PMDs when splitting VMAs

> > @@ -4828,6 +4830,23 @@ static int hugetlb_vm_op_split(struct vm_area_struct *vma, unsigned long addr)
> >  {
> >       if (addr & ~(huge_page_mask(hstate_vma(vma))))
> >               return -EINVAL;
> > +
> > +     /* We require PUD_SIZE VMA alignment for PMD sharing. */
>
> I can get the point, but it reads slightly awkward.  How about:
>
>         /*
>          * If the address to split can be in the middle of a shared pmd
>          * range, unshare before split the vma.
>          */
>

How about:

/*
 * PMD sharing is only possible for PUD_SIZE-aligned address ranges
 * in HugeTLB VMAs. If we will lose PUD_SIZE alignment due to this split,
 * unshare PMDs in the PUD_SIZE interval surrounding addr now.
 */

> I remember you had a helper to check pmd sharing possibility.  Can use here
> depending on whether that existed in the code base or in your hgm series
> (or just pick that up with this one?).

Right, it introduces `pmd_sharing_possible` but I don't think it helps here.

>
> > +     if (addr & ~PUD_MASK) {
> > +             /*
> > +              * hugetlb_vm_op_split is called right before we attempt to
> > +              * split the VMA. We will need to unshare PMDs in the old and
> > +              * new VMAs, so let's unshare before we split.
> > +              */
> > +             unsigned long floor = addr & PUD_MASK;
> > +             unsigned long ceil = floor + PUD_SIZE;
> > +
> > +             if (floor < vma->vm_start || ceil >= vma->vm_end)
>
> s/>=/>/?

Indeed, thanks.

>
> > +                     /* PMD sharing is already impossible. */
> > +                     return 0;
>
> IMHO slightly cleaner to write in the reversed way and let it fall through:
>
>   if (floor >= vma->vm_start && ceil <= vma->vm_end)
>       hugetlb_unshare_pmds(vma, floor, ceil);

Will do.

>
> Thanks,
>

Thanks Peter!