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

On 8/6/26 22:05, Suren Baghdasaryan wrote:
> From: Dave Hansen <[email protected]>
> 
> 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 <[email protected]>
> Signed-off-by: Suren Baghdasaryan <[email protected]>
> Cc: Suren Baghdasaryan <[email protected]>
> Cc: Andrew Morton <[email protected]>
> Cc: "Liam R. Howlett" <[email protected]>
> Cc: Lorenzo Stoakes <[email protected]>
> Cc: Vlastimil Babka <[email protected]>
> Cc: Shakeel Butt <[email protected]>
> Cc: [email protected]
> Cc: Greg Kroah-Hartman <[email protected]>
> Cc: Arve Hjønnevåg <[email protected]>
> Cc: Todd Kjos <[email protected]>
> Cc: Christian Brauner <[email protected]>
> Cc: Carlos Llamas <[email protected]>
> Cc: Alice Ryhl <[email protected]>
> Cc: "David S. Miller" <[email protected]>
> Cc: David Ahern <[email protected]>
> Cc: [email protected]

Acked-by: Vlastimil Babka (SUSE) <[email protected]>