linux-kernel - Re: [RFC PATCH 0/5] Separate compound page from folio

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Message-ID: <20260204184911.GI3931454@nvidia.com>
Date: Wed, 4 Feb 2026 14:49:11 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Zi Yan <ziy@...dia.com>
Cc: Balbir Singh <balbirs@...dia.com>, David Hildenbrand <david@...nel.org>,
	Matthew Wilcox <willy@...radead.org>,
	Alistair Popple <apopple@...dia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Michal Hocko <mhocko@...e.com>, Jens Axboe <axboe@...nel.dk>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Nico Pache <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
	Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
	Lance Yang <lance.yang@...ux.dev>,
	Muchun Song <muchun.song@...ux.dev>,
	Oscar Salvador <osalvador@...e.de>,
	Brendan Jackman <jackmanb@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, io-uring@...r.kernel.org
Subject: Re: [RFC PATCH 0/5] Separate compound page from folio

On Wed, Feb 04, 2026 at 01:29:45PM -0500, Zi Yan wrote:
> > For in-tree users, I am converting them all in this series.
> 
> Considering a recent report[1], where drivers/scsi/sg.c allocates compound
> pages with __GFP_COMP and maps them into userspace via sg_vma_fault(),
> I guess almost all __GFP_COMP users are really using folios instead of
> compound pages.

This would be my guess, and it seems like a good cleanup to make them
actually create fully proper folios if they are being mmaped.

I suspect the only places not using "folios" are frozen page users (or
places yet to be converted to frozen pages)

Which is back to my previous remarks that having a good definition for
what struct page memory is allowed to be used by a frozen page user
would be helpful. If the mm can retain some of the tail page memory
for itself you don't need to make several of the changes here.

Jason