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

Ingo Molnar
2022-05-27T10:46:54Z

* Peter Xu <peterx@redhat.com> wrote:

> 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%)

Nice!

> arch/x86/mm/fault.c | 4 ++++

Reviewed-by: Ingo Molnar <mingo@kernel.org>

Minor comment typo:

> + /*
> + * We should do the same as VM_FAULT_RETRY, but let's not
> + * return -EBUSY since that's not reflecting the reality on
> + * what has happened - we've just fully completed a page
> + * fault, with the mmap lock released. Use -EAGAIN to show
> + * that we want to take the mmap lock _again_.
> + */

s/reflecting the reality on what has happened
/reflecting the reality of what has happened

> ret = handle_mm_fault(vma, address, fault_flags, NULL);
> +
> + if (ret & VM_FAULT_COMPLETED) {
> + /*
> + * NOTE: it's a pity that we need to retake the lock here
> + * to pair with the unlock() in the callers. Ideally we
> + * could tell the callers so they do not need to unlock.
> + */
> + mmap_read_lock(mm);
> + *unlocked = true;
> + return 0;

Indeed that's a pity - I guess more performance could be gained here,
especially in highly parallel threaded workloads?

Thanks,

Ingo