linux-kernel - Re: [PATCH v2] hugetlb: don't delete vma_lock in hugetlb MADV

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Date:   Fri, 28 Oct 2022 14:17:01 -0700
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Naoya Horiguchi <naoya.horiguchi@...ux.dev>,
        David Hildenbrand <david@...hat.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mina Almasry <almasrymina@...gle.com>,
        Rik van Riel <riel@...riel.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Wei Chen <harperchen1110@...il.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2] hugetlb: don't delete vma_lock in hugetlb
 MADV_DONTNEED processing

On 10/28/22 12:13, Peter Xu wrote:
> On Fri, Oct 28, 2022 at 08:23:25AM -0700, Mike Kravetz wrote:
> > On 10/26/22 21:12, Peter Xu wrote:
> > > On Wed, Oct 26, 2022 at 04:54:01PM -0700, Mike Kravetz wrote:
> > > > On 10/26/22 17:42, Peter Xu wrote:
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index c7105ec6d08c..d8b4d7e56939 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -790,7 +790,10 @@ static int madvise_free_single_vma(struct vm_area_struct *vma,
> >  static long madvise_dontneed_single_vma(struct vm_area_struct *vma,
> >  					unsigned long start, unsigned long end)
> >  {
> > -	zap_page_range(vma, start, end - start);
> > +	if (!is_vm_hugetlb_page(vma))
> > +		zap_page_range(vma, start, end - start);
> > +	else
> > +		clear_hugetlb_page_range(vma, start, end);
> 
> With the new ZAP_FLAG_UNMAP flag, clear_hugetlb_page_range() can be dropped
> completely?  As zap_page_range() won't be with ZAP_FLAG_UNMAP so we can
> identify things?
> 
> IIUC that's the major reason why I thought the zap flag could be helpful..

Argh.  I went to drop clear_hugetlb_page_range() but there is one issue.
In zap_page_range() the MMU_NOTIFY_CLEAR notifier is certainly called.
However, we really need to have a 'adjust_range_if_pmd_sharing_possible'
call in there because the 'range' may be part of a shared pmd.  :(

I think we need to either have a separate routine like clear_hugetlb_page_range
that sets up the appropriate range, or special case hugetlb in zap_page_range.
What do you think?
I think clear_hugetlb_page_range is the least bad of the two options.
-- 
Mike Kravetz