[PATCH 00/13] fs/dax: Fix FS DAX page reference counts

Alistair Popple
2024-06-27T00:54:39Z

FS DAX pages have always maintained their own page reference counts
without following the normal rules for page reference counting. In
particular pages are considered free when the refcount hits one rather
than zero and refcounts are not added when mapping the page.

Tracking this requires special PTE bits (PTE_DEVMAP) and a secondary
mechanism for allowing GUP to hold references on the page (see
get_dev_pagemap). However there doesn't seem to be any reason why FS
DAX pages need their own reference counting scheme.

By treating the refcounts on these pages the same way as normal pages
we can remove a lot of special checks. In particular pXd_trans_huge()
becomes the same as pXd_leaf(), although I haven't made that change
here. It also frees up a valuable SW define PTE bit on architectures
that have devmap PTE bits defined.

It also almost certainly allows further clean-up of the devmap managed
functions, but I have left that as a future improvment.

This is an update to the original RFC rebased onto v6.10-rc5. Unlike
the original RFC it passes the same number of ndctl test suite
(https://github.com/pmem/ndctl) tests as my current development
environment does without these patches.

I am not intimately familiar with the FS DAX code so would appreciate
some careful review there. In particular I have not given any thought
at all to CONFIG_FS_DAX_LIMITED.

Signed-off-by: Alistair Popple <apopple@nvidia.com>

Alistair Popple (13):
mm/gup.c: Remove redundant check for PCI P2PDMA page
pci/p2pdma: Don't initialise page refcount to one
fs/dax: Refactor wait for dax idle page
fs/dax: Add dax_page_free callback
mm: Allow compound zone device pages
mm/memory: Add dax_insert_pfn
huge_memory: Allow mappings of PUD sized pages
huge_memory: Allow mappings of PMD sized pages
gup: Don't allow FOLL_LONGTERM pinning of FS DAX pages
fs/dax: Properly refcount fs dax pages
huge_memory: Remove dead vmf_insert_pXd code
mm: Remove pXX_devmap callers
mm: Remove devmap related functions and page table bits

Documentation/mm/arch_pgtable_helpers.rst | 6 +-
arch/arm64/Kconfig | 1 +-
arch/arm64/include/asm/pgtable-prot.h | 1 +-
arch/arm64/include/asm/pgtable.h | 24 +--
arch/powerpc/Kconfig | 1 +-
arch/powerpc/include/asm/book3s/64/hash-4k.h | 6 +-
arch/powerpc/include/asm/book3s/64/hash-64k.h | 7 +-
arch/powerpc/include/asm/book3s/64/pgtable.h | 52 +----
arch/powerpc/include/asm/book3s/64/radix.h | 14 +-
arch/powerpc/mm/book3s64/hash_pgtable.c | 3 +-
arch/powerpc/mm/book3s64/pgtable.c | 8 +-
arch/powerpc/mm/book3s64/radix_pgtable.c | 5 +-
arch/powerpc/mm/pgtable.c | 2 +-
arch/x86/Kconfig | 1 +-
arch/x86/include/asm/pgtable.h | 50 +----
arch/x86/include/asm/pgtable_types.h | 5 +-
drivers/dax/device.c | 12 +-
drivers/dax/super.c | 2 +-
drivers/gpu/drm/nouveau/nouveau_dmem.c | 2 +-
drivers/nvdimm/pmem.c | 9 +-
drivers/pci/p2pdma.c | 4 +-
fs/dax.c | 204 +++++++---------
fs/ext4/inode.c | 5 +-
fs/fuse/dax.c | 4 +-
fs/fuse/virtio_fs.c | 8 +-
fs/userfaultfd.c | 2 +-
fs/xfs/xfs_inode.c | 4 +-
include/linux/dax.h | 11 +-
include/linux/huge_mm.h | 17 +-
include/linux/memremap.h | 23 +-
include/linux/migrate.h | 2 +-
include/linux/mm.h | 40 +---
include/linux/page-flags.h | 6 +-
include/linux/pfn_t.h | 20 +--
include/linux/pgtable.h | 21 +--
include/linux/rmap.h | 14 +-
lib/test_hmm.c | 2 +-
mm/Kconfig | 4 +-
mm/debug_vm_pgtable.c | 59 +-----
mm/gup.c | 178 +--------------
mm/hmm.c | 12 +-
mm/huge_memory.c | 248 +++++++------------
mm/internal.h | 2 +-
mm/khugepaged.c | 2 +-
mm/mapping_dirty_helpers.c | 4 +-
mm/memory-failure.c | 6 +-
mm/memory.c | 114 ++++++---
mm/memremap.c | 38 +---
mm/migrate_device.c | 6 +-
mm/mlock.c | 2 +-
mm/mm_init.c | 5 +-
mm/mprotect.c | 2 +-
mm/mremap.c | 5 +-
mm/page_vma_mapped.c | 5 +-
mm/pgtable-generic.c | 7 +-
mm/rmap.c | 48 ++++-
mm/swap.c | 2 +-
mm/userfaultfd.c | 2 +-
mm/vmscan.c | 5 +-
59 files changed, 485 insertions(+), 869 deletions(-)

base-commit: f2661062f16b2de5d7b6a5c42a9a5c96326b8454
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git-series 0.9.1