linux-kernel - Re: [PATCH 0/5] add STATIC_PMD_ZERO

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Message-ID: <vmc7bu6muygheuepfltjvbbio6gvjemxostq4rjum66s4ok2f7@x7l3y7ot7mf4>
Date: Mon, 16 Jun 2025 12:49:27 +0200
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: David Hildenbrand <david@...hat.com>
Cc: Dave Hansen <dave.hansen@...el.com>, 
	Pankaj Raghav <p.raghav@...sung.com>, Suren Baghdasaryan <surenb@...gle.com>, 
	Ryan Roberts <ryan.roberts@....com>, Mike Rapoport <rppt@...nel.org>, Michal Hocko <mhocko@...e.com>, 
	Thomas Gleixner <tglx@...utronix.de>, Nico Pache <npache@...hat.com>, Dev Jain <dev.jain@....com>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>, 
	"H . Peter Anvin" <hpa@...or.com>, Vlastimil Babka <vbabka@...e.cz>, Zi Yan <ziy@...dia.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, "Liam R . Howlett" <Liam.Howlett@...cle.com>, 
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	willy@...radead.org, x86@...nel.org, linux-block@...r.kernel.org, 
	linux-fsdevel@...r.kernel.org, "Darrick J . Wong" <djwong@...nel.org>, mcgrof@...nel.org, 
	gost.dev@...sung.com, hch@....de
Subject: Re: [PATCH 0/5] add STATIC_PMD_ZERO_PAGE config option

> > > 
> > > The mm is a nice convenient place to stick an mm but there are other
> > > ways to keep an efficient refcount around. For instance, you could just
> > > bump a per-cpu refcount and then have the shrinker sum up all the
> > > refcounts to see if there are any outstanding on the system as a whole.
> > > 
> > > I understand that the current refcounts are tied to an mm, but you could
> > > either replace the mm-specific ones or add something in parallel for
> > > when there's no mm.
> > 
> > But the whole idea of allocating a static PMD page for sane
> > architectures like x86 started with the intent of avoiding the refcounts and
> > shrinker.
> > 
> > This was the initial feedback I got[2]:
> > 
> > I mean, the whole thing about dynamically allocating/freeing it was for
> > memory-constrained systems. For large systems, we just don't care.
> 
> For non-mm usage we can just use the folio refcount. The per-mm refcounts
> are all combined into a single folio refcount. The way the global variable
> is managed based on per-mm refcounts is the weird thing.
> 
> In some corner cases we might end up having multiple instances of huge zero
> folios right now. Just imagine:
> 
> 1) Allocate huge zero folio during read fault
> 2) vmsplice() it
> 3) Unmap the huge zero folio
> 4) Shrinker runs and frees it
> 5) Repeat with 1)
> 
> As long as the folio is vmspliced(), it will not get actually freed ...
> 
> I would hope that we could remove the shrinker completely, and simply never
> free the huge zero folio once allocated. Or at least, only free it once it
> is actually no longer used.
> 

Thanks for the explanation, David.

But I am still a bit confused on how to proceed with these patches.

So IIUC, our eventual goal is to get rid of the shrinker.

But do we still want to add a static PMD page in the .bss or do we take
an alternate approach here?

--
Pankaj