Re: [RFC PATCH RESEND 00/28] per-VMA locks proposal

[Kent Overstreet <kent.overstreet@linux.dev>]

On Thu, Sep 01, 2022 at 10:34:48AM -0700, Suren Baghdasaryan wrote:
> Resending to fix the issue with the In-Reply-To tag in the original
> submission at [4].
> 
> This is a proof of concept for per-vma locks idea that was discussed
> during SPF [1] discussion at LSF/MM this year [2], which concluded with
> suggestion that “a reader/writer semaphore could be put into the VMA
> itself; that would have the effect of using the VMA as a sort of range
> lock. There would still be contention at the VMA level, but it would be an
> improvement.” This patchset implements this suggested approach.
> 
> When handling page faults we lookup the VMA that contains the faulting
> page under RCU protection and try to acquire its lock. If that fails we
> fall back to using mmap_lock, similar to how SPF handled this situation.
> 
> One notable way the implementation deviates from the proposal is the way
> VMAs are marked as locked. Because during some of mm updates multiple
> VMAs need to be locked until the end of the update (e.g. vma_merge,
> split_vma, etc). Tracking all the locked VMAs, avoiding recursive locks
> and other complications would make the code more complex. Therefore we
> provide a way to "mark" VMAs as locked and then unmark all locked VMAs
> all at once. This is done using two sequence numbers - one in the
> vm_area_struct and one in the mm_struct. VMA is considered locked when
> these sequence numbers are equal. To mark a VMA as locked we set the
> sequence number in vm_area_struct to be equal to the sequence number
> in mm_struct. To unlock all VMAs we increment mm_struct's seq number.
> This allows for an efficient way to track locked VMAs and to drop the
> locks on all VMAs at the end of the update.

I like it - the sequence numbers are a stroke of genuius. For what it's doing
the patchset seems almost small.

Two complaints so far:
 - I don't like the vma_mark_locked() name. To me it says that the caller
   already took or is taking the lock and this function is just marking that
   we're holding the lock, but it's really taking a different type of lock. But
   this function can block, it really is taking a lock, so it should say that.
   
   This is AFAIK a new concept, not sure I'm going to have anything good either,
   but perhaps vma_lock_multiple()?

 - I don't like the #ifdef and the separate fallback path in the fault handlers.

   Can we make find_and_lock_anon_vma() do the right thing, and not fail unless
   e.g. there isn't a vma at that address? Just have it wait for vm_lock_seq to
   change and then retry if needed.