Re: [PATCH v3 1/2] mm/uffd: UFFD_FEATURE_WP_UNPOPULATED

[Peter Xu <peterx@redhat.com>]

On Tue, Mar 07, 2023 at 05:11:07PM +0100, David Hildenbrand wrote:
> On 06.03.23 22:39, Peter Xu wrote:
> 
> Note that I wodnered for a second if we'd call it "UFFD_FEATURE_WP_MISSING"
> instead (similar to the definition of MISSING uffd that triggers when we
> have nothing mapped).

I'll be fine with either WP_UNPOPULATED or WP_ZEROPAGE, but hope to avoid
MISSING as that's definitely a keyword in uffd which can be confusing.

> 
> Just a thought.
> 
> [...]
> 
> > 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?

> case of background snapshots we'll be reading all VM memory I think ... but
> we could optimize in QEMU by consulting the pagemap if there is anything
> mapped at all, and not read zeros in that case [an optimization brought up
> several times already].
> 
> I am not sure yet if we want to change the QEMU implementation. But anyhow,
> that's a different discussion.

The rest comments all look good here, I'll fix and resend, thanks.

-- 
Peter Xu