On Mon, May 30, 2022 at 02:34:50PM -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.
>
> Acked-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Acked-by: Vineet Gupta <vgupta@xxxxxxxxxx>
> Acked-by: Guo Ren <guoren@xxxxxxxxxx>
> Acked-by: Max Filippov <jcmvbkbc@xxxxxxxxx>
> Acked-by: Christian Borntraeger <borntraeger@xxxxxxxxxxxxx>
> Acked-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> (powerpc)
> Acked-by: Catalin Marinas <catalin.marinas@xxxxxxx>
> Reviewed-by: Alistair Popple <apopple@xxxxxxxxxx>
> Reviewed-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
For:
> diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
> index a062e07516dd..46cccd6bf705 100644
> --- a/arch/arm/mm/fault.c
> +++ b/arch/arm/mm/fault.c
> @@ -322,6 +322,10 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
> return 0;
> }
>
> + /* The fault is fully completed (including releasing mmap lock) */
> + if (fault & VM_FAULT_COMPLETED)
> + return 0;
> +
> if (!(fault & VM_FAULT_ERROR)) {
> if (fault & VM_FAULT_RETRY) {
> flags |= FAULT_FLAG_TRIED;
Acked-by: Russell King (Oracle) <rmk+kernel@xxxxxxxxxxxxxxx>
Thanks!
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