linux-kernel - Re: [PATCH mm-unstable v1 5/5] mm: multi-gen LRU: use mmu_notifier_test_clear

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAOUHufb4yFPJ8bLt-YRC7eMAyT2PMA_JF82Z412+O=79edsuwQ@mail.gmail.com>
Date:   Thu, 23 Feb 2023 13:48:59 -0700
From:   Yu Zhao <yuzhao@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Michael Larabel <michael@...haellarabel.com>,
        kvmarm@...ts.linux.dev, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org, x86@...nel.org,
        linux-mm@...gle.com
Subject: Re: [PATCH mm-unstable v1 5/5] mm: multi-gen LRU: use mmu_notifier_test_clear_young()

On Thu, Feb 23, 2023 at 1:29 PM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Thu, Feb 23, 2023, Yu Zhao wrote:
> > On Thu, Feb 23, 2023 at 12:58 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > >
> > > On Thu, Feb 23, 2023, Yu Zhao wrote:
> > > > On Thu, Feb 23, 2023 at 12:11 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > > > >
> > > > > On Thu, Feb 23, 2023, Yu Zhao wrote:
> > > > > > > As alluded to in patch 1, unless batching the walks even if KVM does _not_ support
> > > > > > > a lockless walk is somehow _worse_ than using the existing mmu_notifier_clear_flush_young(),
> > > > > > > I think batching the calls should be conditional only on LRU_GEN_SPTE_WALK.  Or
> > > > > > > if we want to avoid batching when there are no mmu_notifier listeners, probe
> > > > > > > mmu_notifiers.  But don't call into KVM directly.
> > > > > >
> > > > > > I'm not sure I fully understand. Let's present the problem on the MM
> > > > > > side: assuming KVM supports lockless walks, batching can still be
> > > > > > worse (very unlikely), because GFNs can exhibit no memory locality at
> > > > > > all. So this option allows userspace to disable batching.
> > > > >
> > > > > I'm asking the opposite.  Is there a scenario where batching+lock is worse than
> > > > > !batching+lock?  If not, then don't make batching depend on lockless walks.
> > > >
> > > > Yes, absolutely. batching+lock means we take/release mmu_lock for
> > > > every single PTE in the entire VA space -- each small batch contains
> > > > 64 PTEs but the entire batch is the whole KVM.
> > >
> > > Who is "we"?
> >
> > Oops -- shouldn't have used "we".
> >
> > > I don't see anything in the kernel that triggers walking the whole
> > > VMA, e.g. lru_gen_look_around() limits the walk to a single PMD.  I feel like I'm
> > > missing something...
> >
> > walk_mm() -> walk_pud_range() -> walk_pmd_range() -> walk_pte_range()
> > -> test_spte_young() -> mmu_notifier_test_clear_young().
> >
> > MGLRU takes two passes: during the first pass, it sweeps entire VA
> > space on each MM (per MM/KVM); during the second pass, it uses the rmap on each
> > folio (per folio).
>
> Ah.  IIUC, userspace can use LRU_GEN_SPTE_WALK to control whether or not to walk
> secondary MMUs, and the kernel further restricts LRU_GEN_SPTE_WALK to secondary
> MMUs that implement a lockless walk.  And if the answer is "no", secondary MMUs
> are simply not consulted.

Sorry for the bad naming -- probably LRU_GEN_SPTE_BATCH_WALK would be
less confusing.

MGLRU always consults the secondary MMU for each page it's going to
reclaim (during the second pass), i.e., it checks the A-bit in the
SPTE mapping a page (by the rmap) it plans to reclaim so that it won't
take a hot page away from KVM.

If the lockless walk is supported, MGLRU doesn't need to work at page
granularity: (physical) pages on the LRU list may have nothing in
common (e.g., from different processes), checking their PTEs/SPTEs one
by one is expensive. Instead, it sweeps the entire KVM spaces in the
first pass and checks the *adjacent SPTEs* of a page it plans to
reclaim in the second pass. Both rely on the *assumption* there would
be some spatial locality to exploit. This assumption can be wrong, and
LRU_GEN_SPTE_WALK disables it.

> If that's correct, then the proper way to handle this is by extending mmu_notifier_ops
> to query (a) if there's at least one register listeners that implements
> test_clear_young() and (b) if all registered listeners that implement test_clear_young()
> support lockless walks.  That avoids direct dependencies on KVM, and avoids making
> assumptions that may not always hold true, e.g. that KVM is the only mmu_notifier
> user that supports the young APIs.
>
> P.S. all of this info absolutely belongs in documentation and/or changelogs.

Will do.