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

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Date:   Fri, 28 Oct 2022 19:20:40 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Mike Kravetz <mike.kravetz@...cle.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 Fri, Oct 28, 2022 at 02:17:01PM -0700, Mike Kravetz wrote:
> 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.

How about special case hugetlb as you mentioned?  If I'm not wrong, it
should be 3 lines change:

---8<---
diff --git a/mm/memory.c b/mm/memory.c
index c5599a9279b1..0a1632e44571 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -1706,11 +1706,13 @@ void zap_page_range(struct vm_area_struct *vma, unsigned long start,
        lru_add_drain();
        mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma, vma->vm_mm,
                                start, start + size);
+       if (is_vm_hugetlb_page(vma))
+               adjust_range_if_pmd_sharing_possible(vma, &range.start, &range.end);
        tlb_gather_mmu(&tlb, vma->vm_mm);
        update_hiwater_rss(vma->vm_mm);
        mmu_notifier_invalidate_range_start(&range);
        do {
-               unmap_single_vma(&tlb, vma, start, range.end, NULL);
+               unmap_single_vma(&tlb, vma, start, start + size, NULL);
        } while ((vma = mas_find(&mas, end - 1)) != NULL);
        mmu_notifier_invalidate_range_end(&range);
        tlb_finish_mmu(&tlb);
---8<---

As zap_page_range() is already vma-oriented anyway.  But maybe I missed
something important?

-- 
Peter Xu