[PATCH v2 00/37] mm: remove nth_page()

David Hildenbrand
2025-11-18T18:38:08Z

This is based on mm-unstable.

I will only CC non-MM folks on the cover letter and the respective patch
to not flood too many inboxes (the lists receive all patches).

--

As discussed recently with Linus, nth_page() is just nasty and we would
like to remove it.

To recap, the reason we currently need nth_page() within a folio is because
on some kernel configs (SPARSEMEM without SPARSEMEM_VMEMMAP), the
memmap is allocated per memory section.

While buddy allocations cannot cross memory section boundaries, hugetlb
and dax folios can.

So crossing a memory section means that "page++" could do the wrong thing.
Instead, nth_page() on these problematic configs always goes from
page->pfn, to the go from (++pfn)->page, which is rather nasty.

Likely, many people have no idea when nth_page() is required and when
it might be dropped.

We refer to such problematic PFN ranges and "non-contiguous pages".
If we only deal with "contiguous pages", there is not need for nth_page().

Besides that "obvious" folio case, we might end up using nth_page()
within CMA allocations (again, could span memory sections), and in
one corner case (kfence) when processing memblock allocations (again,
could span memory sections).

So let's handle all that, add sanity checks, and remove nth_page().

Patch #1 -> #5 : stop making SPARSEMEM_VMEMMAP user-selectable + cleanups
Patch #6 -> #13 : disallow folios to have non-contiguous pages
Patch #14 -> #20 : remove nth_page() usage within folios
Patch #22 : disallow CMA allocations of non-contiguous pages
Patch #23 -> #33 : sanity+check + remove nth_page() usage within SG entry
Patch #34 : sanity-check + remove nth_page() usage in
unpin_user_page_range_dirty_lock()
Patch #35 : remove nth_page() in kfence
Patch #36 : adjust stale comment regarding nth_page
Patch #37 : mm: remove nth_page()

A lot of this is inspired from the discussion at [1] between Linus, Jason
and me, so cudos to them.

[1] https://lore.kernel.org/all/CAHk-=wiCYfNp4AJLBORU-c7ZyRBUp66W2-Et6cdQ4REx-GyQ_A@mail.gmail.com/T/#u

v1 -> v2:
* "fs: hugetlbfs: cleanup folio in adjust_range_hwpoison()"
-> Add comment for loop and remove comment of function regarding
copy_page_to_iter().
* Various smaller patch description tweaks I am not going to list for my
sanity
* "mips: mm: convert __flush_dcache_pages() to
__flush_dcache_folio_pages()"
-> Fix flush_dcache_page()
-> Drop "extern"
* "mm/gup: remove record_subpages()"
-> Added
* "mm/hugetlb: check for unreasonable folio sizes when registering hstate"
-> Refine comment
* "mm/cma: refuse handing out non-contiguous page ranges"
-> Add comment above loop
* "mm/page_alloc: reject unreasonable folio/compound page sizes in
alloc_contig_range_noprof()"
-> Added comment above check
* "mm/gup: drop nth_page() usage in unpin_user_page_range_dirty_lock()"
-> Refined comment

RFC -> v1:
* "wireguard: selftests: remove CONFIG_SPARSEMEM_VMEMMAP=y from qemu kernel
config"
-> Mention that it was never really relevant for the test
* "mm/mm_init: make memmap_init_compound() look more like
prep_compound_page()"
-> Mention the setup of page links
* "mm: limit folio/compound page sizes in problematic kernel configs"
-> Improve comment for PUD handling, mentioning hugetlb and dax
* "mm: simplify folio_page() and folio_page_idx()"
-> Call variable "n"
* "mm/hugetlb: cleanup hugetlb_folio_init_tail_vmemmap()"
-> Keep __init_single_page() and refer to the usage of
memblock_reserved_mark_noinit()
* "fs: hugetlbfs: cleanup folio in adjust_range_hwpoison()"
* "fs: hugetlbfs: remove nth_page() usage within folio in
adjust_range_hwpoison()"
-> Separate nth_page() removal from cleanups
-> Further improve cleanups
* "io_uring/zcrx: remove nth_page() usage within folio"
-> Keep the io_copy_cache for now and limit to nth_page() removal
* "mm/gup: drop nth_page() usage within folio when recording subpages"
-> Cleanup record_subpages as bit
* "mm/cma: refuse handing out non-contiguous page ranges"
-> Replace another instance of "pfn_to_page(pfn)" where we already have
the page
* "scatterlist: disallow non-contigous page ranges in a single SG entry"
-> We have to EXPORT the symbol. I thought about moving it to mm_inline.h,
but I really don't want to include that in include/linux/scatterlist.h
* "ata: libata-eh: drop nth_page() usage within SG entry"
* "mspro_block: drop nth_page() usage within SG entry"
* "memstick: drop nth_page() usage within SG entry"
* "mmc: drop nth_page() usage within SG entry"
-> Keep PAGE_SHIFT
* "scsi: scsi_lib: drop nth_page() usage within SG entry"
* "scsi: sg: drop nth_page() usage within SG entry"
-> Split patches, Keep PAGE_SHIFT
* "crypto: remove nth_page() usage within SG entry"
-> Keep PAGE_SHIFT
* "kfence: drop nth_page() usage"
-> Keep modifying i and use "start_pfn" only instead

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: "Liam R. Howlett" <Liam.Howlett@oracle.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Robin Murphy <robin.murphy@arm.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Marco Elver <elver@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Brendan Jackman <jackmanb@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Dennis Zhou <dennis@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Cc: Christoph Lameter <cl@gentwo.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: x86@kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mips@vger.kernel.org
Cc: linux-s390@vger.kernel.org
Cc: linux-crypto@vger.kernel.org
Cc: linux-ide@vger.kernel.org
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-mmc@vger.kernel.org
Cc: linux-arm-kernel@axis.com
Cc: linux-scsi@vger.kernel.org
Cc: kvm@vger.kernel.org
Cc: virtualization@lists.linux.dev
Cc: linux-mm@kvack.org
Cc: io-uring@vger.kernel.org
Cc: iommu@lists.linux.dev
Cc: kasan-dev@googlegroups.com
Cc: wireguard@lists.zx2c4.com
Cc: netdev@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: linux-riscv@lists.infradead.org

David Hildenbrand (37):
mm: stop making SPARSEMEM_VMEMMAP user-selectable
arm64: Kconfig: drop superfluous "select SPARSEMEM_VMEMMAP"
s390/Kconfig: drop superfluous "select SPARSEMEM_VMEMMAP"
x86/Kconfig: drop superfluous "select SPARSEMEM_VMEMMAP"
wireguard: selftests: remove CONFIG_SPARSEMEM_VMEMMAP=y from qemu
kernel config
mm/page_alloc: reject unreasonable folio/compound page sizes in
alloc_contig_range_noprof()
mm/memremap: reject unreasonable folio/compound page sizes in
memremap_pages()
mm/hugetlb: check for unreasonable folio sizes when registering hstate
mm/mm_init: make memmap_init_compound() look more like
prep_compound_page()
mm: sanity-check maximum folio size in folio_set_order()
mm: limit folio/compound page sizes in problematic kernel configs
mm: simplify folio_page() and folio_page_idx()
mm/hugetlb: cleanup hugetlb_folio_init_tail_vmemmap()
mm/mm/percpu-km: drop nth_page() usage within single allocation
fs: hugetlbfs: remove nth_page() usage within folio in
adjust_range_hwpoison()
fs: hugetlbfs: cleanup folio in adjust_range_hwpoison()
mm/pagewalk: drop nth_page() usage within folio in folio_walk_start()
mm/gup: drop nth_page() usage within folio when recording subpages
mm/gup: remove record_subpages()
io_uring/zcrx: remove nth_page() usage within folio
mips: mm: convert __flush_dcache_pages() to
__flush_dcache_folio_pages()
mm/cma: refuse handing out non-contiguous page ranges
dma-remap: drop nth_page() in dma_common_contiguous_remap()
scatterlist: disallow non-contigous page ranges in a single SG entry
ata: libata-sff: drop nth_page() usage within SG entry
drm/i915/gem: drop nth_page() usage within SG entry
mspro_block: drop nth_page() usage within SG entry
memstick: drop nth_page() usage within SG entry
mmc: drop nth_page() usage within SG entry
scsi: scsi_lib: drop nth_page() usage within SG entry
scsi: sg: drop nth_page() usage within SG entry
vfio/pci: drop nth_page() usage within SG entry
crypto: remove nth_page() usage within SG entry
mm/gup: drop nth_page() usage in unpin_user_page_range_dirty_lock()
kfence: drop nth_page() usage
block: update comment of "struct bio_vec" regarding nth_page()
mm: remove nth_page()

arch/arm64/Kconfig | 1 -
arch/mips/include/asm/cacheflush.h | 11 +++--
arch/mips/mm/cache.c | 8 ++--
arch/s390/Kconfig | 1 -
arch/x86/Kconfig | 1 -
crypto/ahash.c | 4 +-
crypto/scompress.c | 8 ++--
drivers/ata/libata-sff.c | 6 +--
drivers/gpu/drm/i915/gem/i915_gem_pages.c | 2 +-
drivers/memstick/core/mspro_block.c | 3 +-
drivers/memstick/host/jmb38x_ms.c | 3 +-
drivers/memstick/host/tifm_ms.c | 3 +-
drivers/mmc/host/tifm_sd.c | 4 +-
drivers/mmc/host/usdhi6rol0.c | 4 +-
drivers/scsi/scsi_lib.c | 3 +-
drivers/scsi/sg.c | 3 +-
drivers/vfio/pci/pds/lm.c | 3 +-
drivers/vfio/pci/virtio/migrate.c | 3 +-
fs/hugetlbfs/inode.c | 36 +++++---------
include/crypto/scatterwalk.h | 4 +-
include/linux/bvec.h | 7 +--
include/linux/mm.h | 48 +++++++++++++++----
include/linux/page-flags.h | 5 +-
include/linux/scatterlist.h | 3 +-
io_uring/zcrx.c | 4 +-
kernel/dma/remap.c | 2 +-
mm/Kconfig | 3 +-
mm/cma.c | 39 +++++++++------
mm/gup.c | 36 +++++++-------
mm/hugetlb.c | 22 +++++----
mm/internal.h | 1 +
mm/kfence/core.c | 12 +++--
mm/memremap.c | 3 ++
mm/mm_init.c | 15 +++---
mm/page_alloc.c | 10 +++-
mm/pagewalk.c | 2 +-
mm/percpu-km.c | 2 +-
mm/util.c | 36 ++++++++++++++
tools/testing/scatterlist/linux/mm.h | 1 -
.../selftests/wireguard/qemu/kernel.config | 1 -
40 files changed, 217 insertions(+), 146 deletions(-)

base-commit: b73c6f2b5712809f5f386780ac46d1d78c31b2e6
--
2.50.1