linux-kernel - Re: [PATCH v3 0/5] add persistent huge zero folio support

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Message-ID: <hp2wzpu3bgwqyw6almor2x6exgx7t76kch4uec5fbh3xw6sy5w@p6bvhcdhpyea>
Date: Mon, 11 Aug 2025 12:12:20 +0200
From: "Pankaj Raghav (Samsung)" <kernel@...kajraghav.com>
To: David Hildenbrand <david@...hat.com>
Cc: Kiryl Shutsemau <kirill@...temov.name>, 
	Suren Baghdasaryan <surenb@...gle.com>, Ryan Roberts <ryan.roberts@....com>, 
	Baolin Wang <baolin.wang@...ux.alibaba.com>, Vlastimil Babka <vbabka@...e.cz>, Zi Yan <ziy@...dia.com>, 
	Mike Rapoport <rppt@...nel.org>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	Michal Hocko <mhocko@...e.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Nico Pache <npache@...hat.com>, Dev Jain <dev.jain@....com>, 
	"Liam R . Howlett" <Liam.Howlett@...cle.com>, Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org, 
	linux-mm@...ck.org, willy@...radead.org, Ritesh Harjani <ritesh.list@...il.com>, 
	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, 
	Pankaj Raghav <p.raghav@...sung.com>
Subject: Re: [PATCH v3 0/5] add persistent huge zero folio support

> > > > Add a config option PERSISTENT_HUGE_ZERO_FOLIO that will always allocate
> > > > the huge_zero_folio, and disable the shrinker so that huge_zero_folio is
> > > > never freed.
> > > > This makes using the huge_zero_folio without having to pass any mm struct and does
> > > > not tie the lifetime of the zero folio to anything, making it a drop-in
> > > > replacement for ZERO_PAGE.
> > > > 
> > > > I have converted blkdev_issue_zero_pages() as an example as a part of
> > > > this series. I also noticed close to 4% performance improvement just by
> > > > replacing ZERO_PAGE with persistent huge_zero_folio.
> > > > 
> > > > I will send patches to individual subsystems using the huge_zero_folio
> > > > once this gets upstreamed.
> > > > 
> > > > Looking forward to some feedback.
> > > 
> > > Why does it need to be compile-time? Maybe whoever needs huge zero page
> > > would just call get_huge_zero_page()/folio() on initialization to get it
> > > pinned?
> > 
> > That's what v2 did, and this way here is cleaner.
> 
> Sorry, RFC v2 I think. It got a bit confusing with series names/versions.
> 

Another reason we made it a compile time config is because not all
machines would want a PMD sized folio just for zeroing. For example,
Dave Hansen told in one of the early revisions that a small x86 VM would
not want this.

So it is a default N, and it will be an opt-in.

--
Pankaj