Re: [PATCH v3] mm: Avoid unnecessary page fault retires on shared memory types

Heiko Carstens
2022-05-27T12:40:23Z

On Tue, May 24, 2022 at 07:45:31PM -0400, Peter Xu wrote:
> I observed that for each of the shared file-backed page faults, we're very
> likely to retry one more time for the 1st write fault upon no page. It's
> because we'll need to release the mmap lock for dirty rate limit purpose
> with balance_dirty_pages_ratelimited() (in fault_dirty_shared_page()).
>
> Then after that throttling we return VM_FAULT_RETRY.
>
> We did that probably because VM_FAULT_RETRY is the only way we can return
> to the fault handler at that time telling it we've released the mmap lock.
>
> However that's not ideal because it's very likely the fault does not need
> to be retried at all since the pgtable was well installed before the
> throttling, so the next continuous fault (including taking mmap read lock,
> walk the pgtable, etc.) could be in most cases unnecessary.
>
> It's not only slowing down page faults for shared file-backed, but also add
> more mmap lock contention which is in most cases not needed at all.
>
> To observe this, one could try to write to some shmem page and look at
> "pgfault" value in /proc/vmstat, then we should expect 2 counts for each
> shmem write simply because we retried, and vm event "pgfault" will capture
> that.
>
> To make it more efficient, add a new VM_FAULT_COMPLETED return code just to
> show that we've completed the whole fault and released the lock. It's also
> a hint that we should very possibly not need another fault immediately on
> this page because we've just completed it.
>
> This patch provides a ~12% perf boost on my aarch64 test VM with a simple
> program sequentially dirtying 400MB shmem file being mmap()ed and these are
> the time it needs:
>
> Before: 650.980 ms (+-1.94%)
> After: 569.396 ms (+-1.38%)
>
> I believe it could help more than that.
>
> We need some special care on GUP and the s390 pgfault handler (for gmap
> code before returning from pgfault), the rest changes in the page fault
> handlers should be relatively straightforward.
>
> Another thing to mention is that mm_account_fault() does take this new
> fault as a generic fault to be accounted, unlike VM_FAULT_RETRY.
>
> I explicitly didn't touch hmm_vma_fault() and break_ksm() because they do
> not handle VM_FAULT_RETRY even with existing code, so I'm literally keeping
> them as-is.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
...
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index e173b6187ad5..9503a7cfaf03 100644
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -339,6 +339,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
> unsigned long address;
> unsigned int flags;
> vm_fault_t fault;
> + bool need_unlock = true;
> bool is_write;
>
> tsk = current;
> @@ -433,6 +434,13 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
> goto out_up;
> goto out;
> }
> +
> + /* The fault is fully completed (including releasing mmap lock) */
> + if (fault & VM_FAULT_COMPLETED) {
> + need_unlock = false;
> + goto out_gmap;
> + }
> +
> if (unlikely(fault & VM_FAULT_ERROR))
> goto out_up;
>
> @@ -452,6 +460,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
> mmap_read_lock(mm);
> goto retry;
> }
> +out_gmap:
> if (IS_ENABLED(CONFIG_PGSTE) && gmap) {
> address = __gmap_link(gmap, current->thread.gmap_addr,
> address);
> @@ -466,7 +475,8 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
> }
> fault = 0;
> out_up:
> - mmap_read_unlock(mm);
> + if (need_unlock)
> + mmap_read_unlock(mm);
> out:

This seems to be incorrect. __gmap_link() requires the mmap_lock to be
held. Christian, Janosch, or David, could you please check?