Re: [PATCH v4 1/5] mm: Make per-VMA locks available universally

On 8/6/26 22:05, Suren Baghdasaryan wrote:

From: Dave Hansen <dave.hansen@linux.intel.com>

The per-VMA locks have been around for several years. They've had some
bugs worked out of them and have seen quite wide use. However, they
are still only available when architectures explicitly enable them.
Remove the conditional compilation around the per-VMA locks, making
them available on all architectures and configs.

The approach up to now seemed to be to add ARCH_SUPPORTS_PER_VMA_LOCK
when the architecture started using per-VMA locks in the fault
handler. But, contrary to the naming, the Kconfig option does not
really indicate whether the architecture supports per-VMA locks or
not. It is more of a marker for whether the architecture is likely to
benefit from per-VMA locks.

To me, the most important thing side-effect of universal availability
is letting per-VMA locks be used in SMP=n configs. This lets us use
per-VMA locking in all x86 code without fallbacks.

Overall, this just generally makes the kernel simpler. Just look at
the diffstat. It also opens the door to users that want to use the
per-VMA locks in common code. Doing *that* brings additional
simplifications.

The downside of this is adding some fields to vm_area_struct and
mm_struct. There are likely ways to optimize this, especially for
things like SMP=n configs. For now, do the simplest thing: use the
same implementation everywhere.

== Considerations for NOMMU config ==

NOMMU systems do not write-lock VMAs, therefore read-locking a VMA
would always succeed unless VMA is detached. Therefore for NOMMU
config we make vma_mark_attached() a NOOP, which keeps VMAs always in
detached state. This causes VMA read-locking to always fail and the
caller falls back to locking mmap_lock.

The following functions will have a different implementation in NOMMU
config:

- vma_mark_attached(), vma_mark_detached() are made NOOPs, keeping VMAs
always in a detached state and preventing assertions and refcount
underflows;

- vma_start_write(), vma_start_write_killable() are made NOOPs to avoid
warnings in __vma_start_write() due to VMAs being detached.
These functions are not used in NOMMU code but __vma_start_write()
is an exported function, therefore might be used by drivers.

- vma_assert_attached() is made NOOP because it's reachable from NOMMU
code via split_vma()->vma_iter_store_new()->vma_iter_store_overwrite();

- vma_assert_write_locked() is asserting vma->vm_mm is write-locked;

The following functions work for both MMU and NOMMU configs:

- vma_lock_init() performs the same initialization as for MMU config;

- mm_lock_seqcount_init(), mm_lock_seqcount_begin(), mm_lock_seqcount_end()
are called from mmap_write_{lock|unlock} and update mm_lock_seq correctly.

- mmap_lock_speculate_try_begin(), mmap_lock_speculate_retry() work as is
because mm_lock_seq is updated correctly;

- vma_start_read(), vma_start_read_locked() will always fail because VMAs
are always detached;

- vma_end_read() will never be called because vma_start_read() never
succeeds;

- vma_is_attached() always return false because VMAs are always detached;

- vma_assert_detached() will never trigger because VMAs are never attached;

Changes in the following files are not affecting NOMMU config:

task_mmu.c - not compiled when CONFIG_MMU=n;
pagewalk.c - not compiled when CONFIG_MMU=n;
userfaultfd.c - not compiled when CONFIG_MMU=n (CONFIG_USERFAULTFD depends
on CONFIG_MMU);

The following changes are made to keep NOMMU config working like before:

stack_map_lock_vma() - keeps mmap_lock in NOMMU config;
bpf_iter_task_vma_new() - bails out in NOMMU config;

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: "Liam R. Howlett" <redacted>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: linux-mm@kvack.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Todd Kjos <tkjos@android.com>
Cc: Christian Brauner <christian@brauner.io>
Cc: Carlos Llamas <cmllamas@google.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: David Ahern <dsahern@kernel.org>
Cc: netdev@vger.kernel.org

Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>