linux-kernel - Re: [RFC PATCH 0/4] KVM: ioctl for populating guest

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Message-ID: <f55d56d7-0ab9-495f-96bf-9bf642a9762d@redhat.com>
Date: Wed, 20 Nov 2024 17:44:03 +0100
From: David Hildenbrand <david@...hat.com>
To: kalyazin@...zon.com, pbonzini@...hat.com, corbet@....net,
 kvm@...r.kernel.org, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Cc: jthoughton@...gle.com, brijesh.singh@....com, michael.roth@....com,
 graf@...zon.de, jgowans@...zon.com, roypat@...zon.co.uk, derekmn@...zon.com,
 nsaenz@...zon.es, xmarcalx@...zon.com,
 Sean Christopherson <seanjc@...gle.com>, linux-mm@...ck.org
Subject: Re: [RFC PATCH 0/4] KVM: ioctl for populating guest_memfd

>> No, I can't immediately see why it shouldn't work.  My main concern
>> would probably still be about the latency of the population stage as I
>> can't see why it would improve compared to what we have now, because my
>   > feeling is this is linked with the sharedness property of guest_memfd.
> 
> If the problem is the "pagecache" overhead, then yes, it will be a
> harder nut to crack. But maybe there are some low-hanging fruits to
> optimize? Finding the main cause for the added overhead would be
> interesting.

Can you compare uffdio_copy() when using anonymous memory vs. shmem? 
That's likely the best we could currently achieve with guest_memfd.

There is the tools/testing/selftests/mm/uffd-stress benchmark, not sure 
if that is of any help; it SEGFAULTS for me right now with a (likely) 
division by 0.

-- 
Cheers,

David / dhildenb