linux-kernel - Re: [PATCH v3 1/2] mm/uffd: UFFD_FEATURE_WP

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Date:   Wed, 8 Mar 2023 16:57:55 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Paul Gofman <pgofman@...eweavers.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 1/2] mm/uffd: UFFD_FEATURE_WP_UNPOPULATED

>>
>>> With WP_UNPOPUATED, application like QEMU can avoid pre-read faults all the
>>> memory before wr-protect during taking a live snapshot.  Quotting from
>>> Muhammad's test result here [3] based on a simple program [4]:
>>>
>>>     (1) With huge page disabled
>>>     echo madvise > /sys/kernel/mm/transparent_hugepage/enabled
>>>     ./uffd_wp_perf
>>>     Test DEFAULT: 4
>>>     Test PRE-READ: 1111453 (pre-fault 1101011)
>>>     Test MADVISE: 278276 (pre-fault 266378)
>>>     Test WP-UNPOPULATE: 11712
>>>
>>>     (2) With Huge page enabled
>>>     echo always > /sys/kernel/mm/transparent_hugepage/enabled
>>>     ./uffd_wp_perf
>>>     Test DEFAULT: 4
>>>     Test PRE-READ: 22521 (pre-fault 22348)
>>>     Test MADVISE: 4909 (pre-fault 4743)
>>>     Test WP-UNPOPULATE: 14448
>>>
>>> There'll be a great perf boost for no-thp case, while for thp enabled with
>>> extreme case of all-thp-zero WP_UNPOPULATED can be slower than MADVISE, but
>>> that's low possibility in reality, also the overhead was not reduced but
>>> postponed until a follow up write on any huge zero thp, so potentitially it
>>
>> s/potentitially/potentially/
>>
>>> is faster by making the follow up writes slower.
>>
>> What I realized, interrestingly not only the writes, but also the reads. In
> 
> Curious why reading a zeropage would be a problem?

Oh, my thinking was that with markers you postpone placing the shared 
zeropage. So the next read access will require a pagefault to map the 
shared zeropage. Your v1 would have performed best in that case I guess.

-- 
Thanks,

David / dhildenb