linux-kernel - Re: [PATCH v10 07/13] khugepaged: skip collapsing mTHP to smaller orders

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Message-ID: <a345af33-ddc4-47a4-ad26-65495e671964@lucifer.local>
Date: Fri, 22 Aug 2025 11:33:44 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Dev Jain <dev.jain@....com>
Cc: Nico Pache <npache@...hat.com>, linux-mm@...ck.org,
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Subject: Re: [PATCH v10 07/13] khugepaged: skip collapsing mTHP to smaller
 orders

On Thu, Aug 21, 2025 at 06:03:52PM +0530, Dev Jain wrote:
>
> On 21/08/25 5:35 pm, Lorenzo Stoakes wrote:
> >
> >
> > > ---
> > >   mm/khugepaged.c | 9 +++++++++
> > >   1 file changed, 9 insertions(+)
> > >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 1ad7e00d3fd6..6a4cf7e4a7cc 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -611,6 +611,15 @@ static int __collapse_huge_page_isolate(struct vm_area_struct *vma,
> > >   		folio = page_folio(page);
> > >   		VM_BUG_ON_FOLIO(!folio_test_anon(folio), folio);
> > >
> > > +		/*
> > > +		 * TODO: In some cases of partially-mapped folios, we'd actually
> > > +		 * want to collapse.
> > > +		 */
> > Not a fan of adding todo's in code, they have a habit of being left forever. I'd
> > maybe put a more written  out comment something similar to the commit message.
>
> I had suggested to add in https://lore.kernel.org/all/20250211111326.14295-10-dev.jain@arm.com/
> from the get go, but then we decided to leave it for later. So rest assured this TODO won't
> be left forever : )

:)

I think it's better, despite Steven's sage words :P, to put something a little
more meaningful here like 'currently we don't blah blah because blah blah'
without the TODO ;)

But obviously this isn't a substantially pressing issue... :>)

Cheers, Lorenzo