[v6,00/15] Fast kernel headers: split linux/mm.h

Message

Max Kellermann June 4, 2024, 9:08 a.m. UTC

This patch set aims to clean up the linux/mm.h header and reduce
dependencies on it by moving parts out.

The goal was to eliminate dependencies on linux/mm.h from other
popular headers such as highmem.h and dma-mapping.h, and I started by
checking which symbols were really used and moved those declarations
to separate slim headers.

This patch set borrows the name "fast kernel headers" from Ingo
Molnar's effort a few years ago.  While this kind of refactoring does
indeed improve build times because the amount of code that has to be
processed in each compilation unit is reduced, build speed is the
least important advantage.

Much more important is that this gives us greater confidence that the
code is correct: if we forget to include a header, it might (or might
not) already be included indirectly by some other header somewhere
down the large impenetrable include tree.  Chances are almost 100%
that linux/kernel.h or linux/mm.h gets included somewhere, which then
in turn includes everything else.  Now if this indirect include just
happens to be changed eventually, the build may suddenly fail at
remote places.  Worse, an #ifdef may silently produce different code.
Therefore, at each source file, all the includes that are needed
should be included directly (but not more).  This is only possible if
headers are small, and linux/mm.h and linux/kernel.h are large
offenders resisting the cleanup.

Therefore, splitting those is the first step towards leaner header
dependencies.  This patch set starts with linux/mm.h, and I am already
preparing another patch set addressing linux/kernel.h.

Some of these changes were part of a previous, bigger patch set
(https://lore.kernel.org/lkml/[e-mail]/)
but this patch set grew so large, it could not be reviewed.  This is
an attempt to break the patch set into smaller pieces.

---
v1 -> v2: added more explanations to commit messages; renamed several
  new headers from page_*.h to folio_*.h as suggested by Matthew
  Wilcox; rebase on linux-next; fix build failures on architectures
  um,nios2,hexagon by adding more missing includes
v2 -> v3: rebase on linux-next
v3 -> v4: rebase on linux-next; fix build failure on loongarch64
  (reported by kernel test robot); add missing includes to
  drivers/dma/bcm2835-dma.c and include/scsi/scsicam.h
v4 -> v5: rebase on linux-next; more text in cover letter as suggested
  by David Hildenbrand
v5 -> v6: rebase on linux-next d04466706db5...d97496ca23a2

Max Kellermann (15):
  drivers: add missing includes on linux/mm.h (and others)
  include/drm/drm_gem.h: add poll_table_struct forward declaration
  include/scsi/scsicam.h: forward-declare struct block_device
  linux/mm.h: move page_kasan_tag() to mm/page_kasan_tag.h
  linux/mm.h: move section functions to mm/page_section.h
  linux/mm.h: move page_address() and others to mm/page_address.h
  linux/mm.h: move folio_size(), ... to mm/folio_size.h
  linux/mm.h: move folio_next() to mm/folio_next.h
  linux/mm.h: move devmap-related declarations to mm/devmap_managed.h
  linux/mm.h: move usage count functions to mm/folio_usage.h
  linux/mm.h: move page_zone_id() and more to mm/folio_zone.h
  linux/mm.h: move pfmemalloc-related functions to pfmemalloc.h
  linux/mm.h: move is_vmalloc_addr() to mm/vmalloc_addr.h
  linux/mm.h: move high_memory to mm/high_memory.h
  include: reduce dependencies on linux/mm.h

 MAINTAINERS                                   |   1 +
 arch/arm/include/asm/memory.h                 |   4 +
 arch/arm/include/asm/pgtable.h                |   2 +
 arch/arm/mm/iomap.c                           |   3 +
 arch/csky/include/asm/page.h                  |   1 +
 arch/hexagon/include/asm/mem-layout.h         |   4 +
 arch/m68k/include/asm/page_mm.h               |   1 +
 arch/m68k/include/asm/pgtable_mm.h            |   1 +
 arch/parisc/include/asm/floppy.h              |   1 +
 arch/powerpc/include/asm/book3s/32/pgtable.h  |   4 +
 arch/powerpc/include/asm/nohash/32/pgtable.h  |   1 +
 arch/powerpc/include/asm/page.h               |   1 +
 arch/x86/include/asm/floppy.h                 |   1 +
 arch/x86/include/asm/pgtable_32_areas.h       |   4 +
 drivers/comedi/comedi_buf.c                   |   1 +
 .../qat/qat_common/adf_gen4_pm_debugfs.c      |   1 +
 drivers/dma/bcm2835-dma.c                     |   1 +
 drivers/dma/dma-axi-dmac.c                    |   1 +
 drivers/dma/sh/rcar-dmac.c                    |   1 +
 drivers/firmware/qcom/qcom_scm-legacy.c       |   1 +
 drivers/firmware/qcom/qcom_scm-smc.c          |   1 +
 drivers/firmware/raspberrypi.c                |   1 +
 drivers/iio/buffer/industrialio-buffer-dma.c  |   1 +
 drivers/iommu/iommufd/ioas.c                  |   2 +
 drivers/iommu/iommufd/selftest.c              |   1 +
 drivers/media/platform/mediatek/vpu/mtk_vpu.c |   1 +
 drivers/media/platform/ti/omap/omap_voutlib.c |   1 +
 drivers/misc/bcm-vk/bcm_vk_dev.c              |   1 +
 drivers/misc/fastrpc.c                        |   1 +
 drivers/misc/genwqe/card_dev.c                |   1 +
 drivers/misc/uacce/uacce.c                    |   1 +
 drivers/mtd/nand/onenand/onenand_samsung.c    |   1 +
 drivers/mtd/spi-nor/core.h                    |   2 +
 drivers/pci/p2pdma.c                          |   1 +
 drivers/pci/pci.c                             |   1 +
 drivers/remoteproc/mtk_scp.c                  |   1 +
 drivers/remoteproc/remoteproc_core.c          |   1 +
 drivers/soc/qcom/rmtfs_mem.c                  |   1 +
 drivers/spi/spi-aspeed-smc.c                  |   1 +
 drivers/spi/spi-bcm2835.c                     |   2 +
 drivers/spi/spi-intel.c                       |   1 +
 drivers/virtio/virtio_ring.c                  |   1 +
 include/drm/drm_file.h                        |   1 +
 include/linux/bio.h                           |   2 +
 include/linux/bpfptr.h                        |   1 -
 include/linux/dma-mapping.h                   |   1 +
 include/linux/highmem-internal.h              |   2 +
 include/linux/highmem.h                       |   4 +-
 include/linux/huge_mm.h                       |   2 +
 include/linux/iommu.h                         |   1 +
 include/linux/mm.h                            | 598 +-----------------
 include/linux/mm/devmap_managed.h             |  32 +
 include/linux/mm/folio_next.h                 |  27 +
 include/linux/mm/folio_size.h                 | 150 +++++
 include/linux/mm/folio_usage.h                | 182 ++++++
 include/linux/mm/folio_zone.h                 |  56 ++
 include/linux/mm/high_memory.h                |   7 +
 include/linux/mm/page_address.h               |  71 +++
 include/linux/mm/page_kasan_tag.h             |  66 ++
 include/linux/mm/page_section.h               |  23 +
 include/linux/mm/pfmemalloc.h                 |  52 ++
 include/linux/mm/vmalloc_addr.h               |  33 +
 include/linux/net.h                           |   2 +-
 include/linux/nvme-keyring.h                  |   2 +
 include/linux/oom.h                           |   2 +-
 include/linux/page-flags.h                    |   3 +
 include/linux/pagemap.h                       |   2 +-
 include/linux/scatterlist.h                   |   8 +-
 include/linux/skbuff.h                        |   4 +
 include/linux/vmstat.h                        |   2 +
 include/scsi/scsicam.h                        |   5 +
 kernel/dma/ops_helpers.c                      |   1 +
 kernel/dma/remap.c                            |   1 +
 kernel/rcu/rcutorture.c                       |   1 +
 lib/scatterlist.c                             |   1 +
 mm/dmapool.c                                  |   1 +
 76 files changed, 811 insertions(+), 595 deletions(-)
 create mode 100644 include/linux/mm/devmap_managed.h
 create mode 100644 include/linux/mm/folio_next.h
 create mode 100644 include/linux/mm/folio_size.h
 create mode 100644 include/linux/mm/folio_usage.h
 create mode 100644 include/linux/mm/folio_zone.h
 create mode 100644 include/linux/mm/high_memory.h
 create mode 100644 include/linux/mm/page_address.h
 create mode 100644 include/linux/mm/page_kasan_tag.h
 create mode 100644 include/linux/mm/page_section.h
 create mode 100644 include/linux/mm/pfmemalloc.h
 create mode 100644 include/linux/mm/vmalloc_addr.h

Comments

Matthew Wilcox June 4, 2024, 1:02 p.m. UTC | #1

On Tue, Jun 04, 2024 at 11:08:30AM +0200, Max Kellermann wrote:
> This patch set aims to clean up the linux/mm.h header and reduce
> dependencies on it by moving parts out.

I am not a fan of these patches.  They will make it harder to work on
the MM system.  We briefly discussed them at LSFMM and nobody was in
favour of them.  I'm afraid you're shouting into the wind.

Max Kellermann June 4, 2024, 1:22 p.m. UTC | #2

On Tue, Jun 4, 2024 at 3:02 PM Matthew Wilcox <[e-mail]> wrote:
> I am not a fan of these patches.  They will make it harder to work on
> the MM system.  We briefly discussed them at LSFMM and nobody was in
> favour of them.  I'm afraid you're shouting into the wind.

Thanks for letting me know, but I'm confused because similar patches
have been merged pretty often.

For a very weird example, look at commit adeb04362d74 ("kernel.h: Move
upper_*_bits() and lower_*_bits() to wordpart.h") which was submitted
on Feb 14th; and then look at
https://lore.kernel.org/lkml/[e-mail]/
- it's exactly the same patch, but I submitted it 5 days earlier, yet
the other one was merged.

Other recent examples (though without offending earlier patches from me):
- d186eb1ee885 ("cpumask: split out include/linux/cpumask_types.h")
- f7515d9fe8fc4 ("objtool: Add objtool_types.h")
- cb5a065b4ea9 ("headers/deps: mm: Split <linux/gfp_types.h> out of
<linux/gfp.h>")
- 50d91c765825 ("hrtimers: Split out hrtimer_types.h")
- 9983deb26d90 ("Split out irqflags_types.h")
- 6d5e9d636830 ("pid: Split out pid_types.h")
.... and so on ...

If there is a general agreement that nobody is in favor of these
patches, why are they merged when other people submit them?

Max