mm/vma: fix mmap_prepare file handling, remove file_doesnt_need_get

Patch series "mm: make VMA flag semantics explicit, eliminate VM_SPECIAL",
v6.

The VM_SPECIAL / VMA_SPECIAL_FLAGS mask conflates several unrelated
properties:

* Is this managed by core mm, or by a driver mapping MMIO, kernel-allocated
  pages, or even ordinary pages itself?
* Can it be expanded or merged?
* Is this a 'weird' case like mlock where migration might race and we
  'have' to set invalid flags to notify?
* Is it another 'weird' case where we just want to stop GUP from touching
  it?

Driver writers have often been confused about this, and who can blame them?

It also interacts badly with the eternal edgecase known as hugetlb - which
sets VMA_DONTEXPAND_BIT but doesn't also want to be treated like a
'special' flag.

Another issue is that we cannot make sensible assumptions about flag
use. It's not possible to assume VMA_IO_BIT means iommu because drivers
abuse it and mlock abuses it.

Special is also an overloaded term in mm. VDSO and VVAR mappings are also
called 'special' but they're special in a... special way.

Sometimes things are called special that are a subset of
VMA_SPECIAL_FLAGS (VMA_PFNMAP_BIT and VMA_MIXEDMAP_BIT for instance when it
comes to zapping or vm_normal_folio()).

There's a specific kind of special for THP too, which considers
PFN map, mixed map 'special' but DAX not.

It's all rather a mess.

This series brings some order to things by both limiting what drivers can
do with VMA flags and switching to using predicates that describe
behaviour, not arbitrary flags.

It establishes the invariant that mm-managed mappings may neither set
VMA_IO_BIT nor clear VMA_MAYWRITE_BIT in an mmap hook, enforcing this by
validating VMA state after every mmap and mmap_prepare hook.

It updates usbmon and sg to mmap_prepare in order to do so, adding a new
mmap action for mapping discontiguous kernel pages, and has hfi1 and the
ALSA PCM status page map their pages eagerly instead.

It also establishes the invariant that VMA_MIXEDMAP_BIT be set when mapping
kernel memory, something that is usually the case but happens not to be for
some users - specifically defio, cmt_speech, uprobes and the bpf arena, all
of which are updated to do the right thing.

It replaces VM_SPECIAL and arbitrary flag tests with predicates that say
what is actually being tested:

     vma_is_mm_managed()   Are the VMA's contents established and managed
                           by core mm, rather than a driver or other kernel
                           component?

      vma_is_mm_backed()   Is the VMA backed by memory core mm itself
                           faults in, rather than a driver or device?

  vma_is_fixed_mapping()   Is the VMA not permitted to be expanded or
                           merged?

         vma_can_merge()   Can the VMA be merged with a compatible
                           neighbour?

Remaining raw VMA_IO_BIT, VMA_PFNMAP_BIT and VMA_MIXEDMAP_BIT tests
scattered across mm are converted to predicates where they express one, and
left as explicit flag tests where they do not.

And also the opportunity is taken to eliminate THP's vma_is_special_huge()
which was an existing source of confusion.


This patch (of 39):

The map->file_doesnt_need_get flag is confusing and the existing
implementation has holes.

Drivers are permitted to change the owning file of a mapping. If they do
so, they are required to take a reference on that file.

The mmap() operation which ultimately invokes __mmap_region() is guaranteed
to drop the refcount for the original file the mapping was made under, but
this is not true for the replaced file.

This has been addressed so far by tracking map->file_doesnt_need_get, which
is rather poorly named and unfortunately fails to correctly track whether
or not an additional put were needed in a number of cases.

Make life easier by removing this flag, and instead drop the reference for
both mmap_prepare and the deprecated mmap callback in a new function
map_put_vm_file().

Track whether this needs to be done by aligning mmap_state with
vm_area_desc and store the original file in the map->file field, keeping
the updated file in map->vm_file.

In order to have the same behaviour for both types of hooks, only drop the
reference __mmap_new_file_vma() itself took in its error path, deferring
the replaced file's reference to map_put_vm_file().

To make this work correctly, map->vm_file has to be updated before any
error handling, so update __mmap_new_file_vma() and call_mmap_prepare() to
set this field first.

Also when mmap_prepare() changes the file and is then merged, the reference
count also must be decremented, so update the logic to call
map_put_vm_file() in this case too.

Also update __compat_vma_mmap() to manually perform this step for stacked
file systems using the compatibility layer, and update
compat_set_vma_from_desc() to replace vma_set_file() with a correct
refcount/file update.

No in-tree driver is impacted by the incorrect implementation of this
currently (no driver that does this is mergeable for one), so this does not
need to be a fix.

Link: https://lore.kernel.org/20261013-b4-mmap-prepare-vma-flag-sanify-v6-0-1aa3c9b139f9@kernel.org
Link: https://lore.kernel.org/20261013-b4-mmap-prepare-vma-flag-sanify-v6-1-1aa3c9b139f9@kernel.org
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Reviewed-by: Gregory Price (Meta) <gourry@gourry.net>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: David Hildenbrand <david@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Stephen Smalley <stephen.smalley.work@gmail.com>
Cc: Jaroslav Kysela <perex@perex.cz>
Cc: Takashi Iwai <tiwai@suse.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Eduard Zingerman <eddyz87@gmail.com>
Cc: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Nico Pache <nico.pache@linux.dev>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Barry Song <baohua@kernel.org>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Usama Arif <usama.arif@linux.dev>
Cc: Kiryl Shutsemau <kas@kernel.org>
Cc: Doug Gilbert <dgilbert@interlog.com>
Cc: James Bottomley <james.bottomley@hansenpartnership.com>
Cc: Martin K. Petersen <mkp@kernel.org>
Cc: Simona Vetter <simona@ffwll.ch>
Cc: Helge Deller <deller@gmx.de>
Cc: Sebastian Reichel <sre@kernel.org>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Harry Yoo <harry@kernel.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@gmail.com>
Cc: Will Deacon <will@kernel.org>
Cc: Aneesh Kumar K.V <aneesh.kumar@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Oliver Upton <oupton@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Anup Patel <anup@brainfault.org>
Cc: Paul Walmsley <pjw@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Janosch Frank <frankja@linux.ibm.com>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Chris Li <chrisl@kernel.org>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Youngjun Park <youngjun.park@lge.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Qi Zheng <qi.zheng@linux.dev>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Cc: Wei Xu <weixugc@google.com>
Cc: Chengming Zhou <chengming.zhou@linux.dev>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Xu Xin <xu.xin@linux.dev>
Cc: Ihor Solodrai <ihor.solodrai@linux.dev>
Cc: Bert Karwatzki <spasswolf@web.de>

0 files changed, 0 insertions, 0 deletions