From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Jason Gunthorpe <jgg@nvidia.com>,
"peterx@redhat.com" <peterx@redhat.com>
Cc: "linux-mm@kvack.org" <linux-mm@kvack.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
James Houghton <jthoughton@google.com>,
David Hildenbrand <david@redhat.com>,
"Kirill A . Shutemov" <kirill@shutemov.name>,
Yang Shi <shy828301@gmail.com>,
"linux-riscv@lists.infradead.org"
<linux-riscv@lists.infradead.org>,
Andrew Morton <akpm@linux-foundation.org>,
"Aneesh Kumar K . V" <aneesh.kumar@kernel.org>,
Rik van Riel <riel@surriel.com>,
Andrea Arcangeli <aarcange@redhat.com>,
Axel Rasmussen <axelrasmussen@google.com>,
Mike Rapoport <rppt@kernel.org>,
John Hubbard <jhubbard@nvidia.com>,
Vlastimil Babka <vbabka@suse.cz>,
Michael Ellerman <mpe@ellerman.id.au>,
Andrew Jones <andrew.jones@linux.dev>,
"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
Mike Kravetz <mike.kravetz@oracle.com>,
Muchun Song <muchun.song@linux.dev>,
"linux-arm-kernel@lists.infradead.org"
<linux-arm-kernel@lists.infradead.org>,
Christoph Hellwig <hch@infradead.org>,
Lorenzo Stoakes <lstoakes@gmail.com>,
Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v2 06/13] mm/gup: Drop folio_fast_pin_allowed() in hugepd processing
Date: Tue, 16 Jan 2024 06:30:39 +0000 [thread overview]
Message-ID: <c60c9d88-33aa-4312-a23c-20206e503b6e@csgroup.eu> (raw)
In-Reply-To: <20240115183748.GR734935@nvidia.com>
Le 15/01/2024 à 19:37, Jason Gunthorpe a écrit :
> On Wed, Jan 03, 2024 at 05:14:16PM +0800, peterx@redhat.com wrote:
>> From: Peter Xu <peterx@redhat.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.
>
> I didn't dig into the ppc stuff too deeply, but this looks to me like
> it is the same thing as ARM's contig bits?
>
> ie a chunk of PMD/etc entries are all managed together as though they
> are a virtual larger entry and we use the hugepte_addr_end() stuff to
> iterate over each sub entry.
As far as I understand ARM's contig stuff, hugepd on powerpc is
something different.
hugepd is a page directory dedicated to huge pages, where you have huge
pages listed instead of regular pages. For instance, on powerpc 32 with
each PGD entries covering 4Mbytes, a regular page table has 1024 PTEs. A
hugepd for 512k is a page table with 8 entries.
And for 8Mbytes entries, the hugepd is a page table with only one entry.
And 2 consecutive PGS entries will point to the same hugepd to cover the
entire 8Mbytes.
>
> But WHY is GUP doing this or caring about this? GUP should have no
> problem handling the super-size entry (eg 8M on nohash) as a single
> thing. It seems we only lack an API to get this out of the arch code?
>
> It seems to me we should see ARM and PPC agree on what the API is for
> this and then get rid of hugepd by making both use the same page table
> walker API. Is that too hopeful?
Can't see the similarity between ARM contig PTE and PPC huge page
directories.
>
>> 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.
>
> I didn't see any other caller of this function in this series? When
> does this re-use happen??
>
> Jason
Christophe
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next prev parent reply other threads:[~2024-01-16 6:31 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-03 9:14 [PATCH v2 00/13] mm/gup: Unify hugetlb, part 2 peterx
2024-01-03 9:14 ` [PATCH v2 01/13] mm/Kconfig: CONFIG_PGTABLE_HAS_HUGE_LEAVES peterx
2024-01-15 17:37 ` Jason Gunthorpe
2024-01-22 8:25 ` Peter Xu
2024-01-03 9:14 ` [PATCH v2 02/13] mm/hugetlb: Declare hugetlbfs_pagecache_present() non-static peterx
2024-01-03 9:14 ` [PATCH v2 03/13] mm: Provide generic pmd_thp_or_huge() peterx
2024-01-15 17:55 ` Jason Gunthorpe
2024-02-21 9:37 ` Peter Xu
2024-02-21 12:57 ` Jason Gunthorpe
2024-02-22 8:04 ` Peter Xu
2024-01-03 9:14 ` [PATCH v2 04/13] mm: Make HPAGE_PXD_* macros even if !THP peterx
2024-01-15 17:59 ` Jason Gunthorpe
2024-01-03 9:14 ` [PATCH v2 05/13] mm: Introduce vma_pgtable_walk_{begin|end}() peterx
2024-01-03 9:14 ` [PATCH v2 06/13] mm/gup: Drop folio_fast_pin_allowed() in hugepd processing peterx
2024-01-15 18:37 ` Jason Gunthorpe
2024-01-16 6:30 ` Christophe Leroy [this message]
2024-01-16 12:31 ` Jason Gunthorpe
[not found] ` <44e450cb-5d3f-407e-97a3-024eb936f74b@csgroup.eu>
2024-01-17 13:22 ` Jason Gunthorpe
2024-01-18 15:15 ` Ryan Roberts
2024-02-21 11:55 ` Peter Xu
2024-01-03 9:14 ` [PATCH v2 07/13] mm/gup: Refactor record_subpages() to find 1st small page peterx
2024-01-15 18:38 ` Jason Gunthorpe
2024-01-03 9:14 ` [PATCH v2 08/13] mm/gup: Handle hugetlb for no_page_table() peterx
2024-01-15 18:39 ` Jason Gunthorpe
2024-01-03 9:14 ` [PATCH v2 09/13] mm/gup: Cache *pudp in follow_pud_mask() peterx
2024-01-15 18:41 ` Jason Gunthorpe
2024-01-03 9:14 ` [PATCH v2 10/13] mm/gup: Handle huge pud for follow_pud_mask() peterx
2024-01-15 18:49 ` Jason Gunthorpe
2024-02-21 11:49 ` Peter Xu
2024-01-03 9:14 ` [PATCH v2 11/13] mm/gup: Handle huge pmd for follow_pmd_mask() peterx
2024-01-15 18:51 ` Jason Gunthorpe
2024-01-03 9:14 ` [PATCH v2 12/13] mm/gup: Handle hugepd for follow_page() peterx
2024-01-03 9:14 ` [PATCH v2 13/13] mm/gup: Handle hugetlb in the generic follow_page_mask code peterx
2024-01-03 11:14 ` [PATCH v2 00/13] mm/gup: Unify hugetlb, part 2 Christophe Leroy
2024-01-08 7:27 ` Peter Xu
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