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

[Heiko Carstens <hca@linux.ibm.com>]

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?