linux-kernel - Re: [PATCH] KVM: x86: Fix rmap allocation for very large memslots

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Message-ID: <YfRkivAI2P6urdfn@google.com>
Date:   Fri, 28 Jan 2022 21:47:54 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Michal Hocko <mhocko@...e.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Fix rmap allocation for very large memslots

On Fri, Jan 28, 2022, Maciej S. Szmigiero wrote:
> From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
> 
> Commit 7661809d493b ("mm: don't allow oversized kvmalloc() calls") has
> forbidden using kvmalloc() to make allocations larger than INT_MAX (2 GiB).
> 
> Unfortunately, adding a memslot exceeding 1 TiB in size will result in rmap
> code trying to make an allocation exceeding this limit.
> Besides failing this allocation, such operation will also trigger a
> WARN_ON_ONCE() added by the aforementioned commit.
> 
> Since we probably still want to use kernel slab for small rmap allocations
> let's only redirect such oversized allocations to vmalloc.
> 
> A possible alternative would be to add some kind of a __GFP_LARGE flag to
> skip the INT_MAX check behind kvmalloc(), however this will impact the
> common kernel memory allocation code, not just KVM.

Paolo has a cleaner fix for this[1][2], but it appears to have stalled out somewhere.

Paolo???

[1] https://lore.kernel.org/all/20211015165519.135670-1-pbonzini@redhat.com
[2] https://lore.kernel.org/all/20211016064302.165220-1-pbonzini@redhat.com