linux-kernel - Re: [PATCH v3 05/12] mm/gup: Drop folio_fast_pin

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Message-ID: <20240322122842.GH159172@nvidia.com>
Date: Fri, 22 Mar 2024 09:28:42 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: peterx@...hat.com
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org,
	Michael Ellerman <mpe@...erman.id.au>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Matthew Wilcox <willy@...radead.org>,
	Rik van Riel <riel@...riel.com>,
	Lorenzo Stoakes <lstoakes@...il.com>,
	Axel Rasmussen <axelrasmussen@...gle.com>,
	Yang Shi <shy828301@...il.com>, John Hubbard <jhubbard@...dia.com>,
	linux-arm-kernel@...ts.infradead.org,
	"Kirill A . Shutemov" <kirill@...temov.name>,
	Andrew Jones <andrew.jones@...ux.dev>,
	Vlastimil Babka <vbabka@...e.cz>, Mike Rapoport <rppt@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Muchun Song <muchun.song@...ux.dev>,
	Christoph Hellwig <hch@...radead.org>,
	linux-riscv@...ts.infradead.org,
	James Houghton <jthoughton@...gle.com>,
	David Hildenbrand <david@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Aneesh Kumar K . V" <aneesh.kumar@...nel.org>,
	Mike Kravetz <mike.kravetz@...cle.com>
Subject: Re: [PATCH v3 05/12] mm/gup: Drop folio_fast_pin_allowed() in hugepd
 processing

On Thu, Mar 21, 2024 at 06:07:55PM -0400, peterx@...hat.com wrote:
> From: Peter Xu <peterx@...hat.com>
> 
> Hugepd format for GUP is only used in PowerPC with hugetlbfs.  There are
> some kernel usage of hugepd (can refer to hugepd_populate_kernel() for
> PPC_8XX), however those pages are not candidates for GUP.
> 
> Commit a6e79df92e4a ("mm/gup: disallow FOLL_LONGTERM GUP-fast writing to
> file-backed mappings") added a check to fail gup-fast if there's potential
> risk of violating GUP over writeback file systems.  That should never apply
> to hugepd.  Considering that hugepd is an old format (and even
> software-only), there's no plan to extend hugepd into other file typed
> memories that is prone to the same issue.
> 
> Drop that check, not only because it'll never be true for hugepd per any
> known plan, but also it paves way for reusing the function outside
> fast-gup.
> 
> To make sure we'll still remember this issue just in case hugepd will be
> extended to support non-hugetlbfs memories, add a rich comment above
> gup_huge_pd(), explaining the issue with proper references.
> 
> Cc: Christoph Hellwig <hch@...radead.org>
> Cc: Lorenzo Stoakes <lstoakes@...il.com>
> Cc: Michael Ellerman <mpe@...erman.id.au>
> Cc: linuxppc-dev@...ts.ozlabs.org
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  mm/gup.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason