linux-kernel - Re: [PATCH v2 22/37] mm/cma: refuse handing out non-contiguous page ranges

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Message-ID: <6ec933b1-b3f7-41c0-95d8-e518bb87375e@redhat.com>
Date: Tue, 9 Sep 2025 11:55:51 +0200
From: David Hildenbrand <david@...hat.com>
To: linux-kernel@...r.kernel.org, Andrew Morton <akpm@...uxfoundation.org>
Cc: Alexandru Elisei <alexandru.elisei@....com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Alexander Potapenko <glider@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Brendan Jackman <jackmanb@...gle.com>, Christoph Lameter <cl@...two.org>,
 Dennis Zhou <dennis@...nel.org>, Dmitry Vyukov <dvyukov@...gle.com>,
 dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
 iommu@...ts.linux.dev, io-uring@...r.kernel.org,
 Jason Gunthorpe <jgg@...dia.com>, Jens Axboe <axboe@...nel.dk>,
 Johannes Weiner <hannes@...xchg.org>, John Hubbard <jhubbard@...dia.com>,
 kasan-dev@...glegroups.com, kvm@...r.kernel.org,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>, linux-arm-kernel@...s.com,
 linux-arm-kernel@...ts.infradead.org, linux-crypto@...r.kernel.org,
 linux-ide@...r.kernel.org, linux-kselftest@...r.kernel.org,
 linux-mips@...r.kernel.org, linux-mmc@...r.kernel.org, linux-mm@...ck.org,
 linux-riscv@...ts.infradead.org, linux-s390@...r.kernel.org,
 linux-scsi@...r.kernel.org, Marco Elver <elver@...gle.com>,
 Marek Szyprowski <m.szyprowski@...sung.com>, Michal Hocko <mhocko@...e.com>,
 Mike Rapoport <rppt@...nel.org>, Muchun Song <muchun.song@...ux.dev>,
 netdev@...r.kernel.org, Oscar Salvador <osalvador@...e.de>,
 Peter Xu <peterx@...hat.com>, Robin Murphy <robin.murphy@....com>,
 Suren Baghdasaryan <surenb@...gle.com>, Tejun Heo <tj@...nel.org>,
 virtualization@...ts.linux.dev, Vlastimil Babka <vbabka@...e.cz>,
 wireguard@...ts.zx2c4.com, x86@...nel.org, Zi Yan <ziy@...dia.com>
Subject: Re: [PATCH v2 22/37] mm/cma: refuse handing out non-contiguous page
 ranges

On 01.09.25 17:03, David Hildenbrand wrote:
> Let's disallow handing out PFN ranges with non-contiguous pages, so we
> can remove the nth-page usage in __cma_alloc(), and so any callers don't
> have to worry about that either when wanting to blindly iterate pages.
> 
> This is really only a problem in configs with SPARSEMEM but without
> SPARSEMEM_VMEMMAP, and only when we would cross memory sections in some
> cases.
> 
> Will this cause harm? Probably not, because it's mostly 32bit that does
> not support SPARSEMEM_VMEMMAP. If this ever becomes a problem we could
> look into allocating the memmap for the memory sections spanned by a
> single CMA region in one go from memblock.
> 
> Reviewed-by: Alexandru Elisei <alexandru.elisei@....com>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---

@Andrew, the following fixup on top. I'm still cross-compiling it, but
at the time you read this mail my cross compiles should have been done.

 From cbfa2763e1820b917ce3430f45e5f3a55eb2970f Mon Sep 17 00:00:00 2001
From: David Hildenbrand <david@...hat.com>
Date: Tue, 9 Sep 2025 05:50:13 -0400
Subject: [PATCH] fixup: mm/cma: refuse handing out non-contiguous page ranges

Apparently we can have NUMMU configs with SPARSEMEM enabled.

Signed-off-by: David Hildenbrand <david@...hat.com>
---
  mm/util.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/util.c b/mm/util.c
index 248f877f629b6..6c1d64ed02211 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1306,6 +1306,7 @@ unsigned int folio_pte_batch(struct folio *folio, pte_t *ptep, pte_t pte,
  {
  	return folio_pte_batch_flags(folio, NULL, ptep, &pte, max_nr, 0);
  }
+#endif /* CONFIG_MMU */

  #if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
  /**
@@ -1342,4 +1343,3 @@ bool page_range_contiguous(const struct page *page, unsigned long nr_pages)
  }
  EXPORT_SYMBOL(page_range_contiguous);
  #endif
-#endif /* CONFIG_MMU */
-- 
2.50.1

-- 
Cheers

David / dhildenb