linux-kernel - Re: [PATCH v3 6/6] mm: khugepaged: set to next mm direct when mm has MMF_DISABLE_THP

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Message-ID: <CACZaFFM4mk71KDu9g6Fb8qBL3+5cMuZOfVUzQ+eb36q9BeKn8g@mail.gmail.com>
Date: Mon, 5 Jan 2026 10:06:57 +0800
From: Vernon Yang <vernon2gm@...il.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: lorenzo.stoakes@...cle.com, ziy@...dia.com, dev.jain@....com, 
	baohua@...nel.org, richard.weiyang@...il.com, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, david@...nel.org, 
	Vernon Yang <yanglincheng@...inos.cn>, akpm@...ux-foundation.org
Subject: Re: [PATCH v3 6/6] mm: khugepaged: set to next mm direct when mm has MMF_DISABLE_THP_COMPLETELY

On Sun, Jan 4, 2026 at 8:20 PM Lance Yang <lance.yang@...ux.dev> wrote:
>
> On 2026/1/4 13:41, Vernon Yang wrote:
> > When an mm with the MMF_DISABLE_THP_COMPLETELY flag is detected during
> > scanning, directly set khugepaged_scan.mm_slot to the next mm_slot,
> > reduce redundant operation.
> >
> > Signed-off-by: Vernon Yang <yanglincheng@...inos.cn>
> > ---
> >   mm/khugepaged.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 1ca034a5f653..d4ed0f397335 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -2541,7 +2541,7 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result,
> >        * Release the current mm_slot if this mm is about to die, or
> >        * if we scanned all vmas of this mm.
> >        */
> > -     if (hpage_collapse_test_exit(mm) || !vma) {
> > +     if (hpage_collapse_test_exit_or_disable(mm) || !vma) {
> >               /*
> >                * Make sure that if mm_users is reaching zero while
> >                * khugepaged runs here, khugepaged_exit will find
>
>
> Let's convert hpage_collapse_test_exit() in collect_mm_slot() as well,
> otherwise the mm_slot would not be freed and will be scanned again ...
>
> static void collect_mm_slot(struct mm_slot *slot)
> {
>         struct mm_struct *mm = slot->mm;
>
>         lockdep_assert_held(&khugepaged_mm_lock);
>
>         if (hpage_collapse_test_exit(mm)) { <-
>
>                 hash_del(&slot->hash);
>                 list_del(&slot->mm_node);
>
>                 mm_slot_free(mm_slot_cache, slot);
>                 mmdrop(mm);
>         }
> }

This patch just reduces redundant operation, For a detailed
discussion[1].

You already commit 5dad604809c5 ("mm/khugepaged: keep mm in mm_slot
without MMF_DISABLE_THP check"), I assume there is some problem here,
e.g. not can readd? data-race? etc. Can you explain the root cause? Thanks!

[1] https://lore.kernel.org/linux-mm/CACZaFFOvDad09MUopairAoAjZG6X5gffMaQbnfy0sCHGz8xSfg@mail.gmail.com

--
Thanks,
Vernon