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charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 3A4D5140002 X-Stat-Signature: kpckkwbebjko9ms38qjifh3tuyzjccr4 X-Rspam-User: X-HE-Tag: 1757664267-327344 X-HE-Meta: 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 bfB0Yfd2 yXNlnWOa9ALGeI1GPEb2cO8NcKnw2zp2RZDC/0jzGnVijT6AzQFB4QyWLY/qZVSnJiXvsYzw1GkABwDy2slLC/1Ez8g== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: >> >> struct lazy_mmu_state { >>     uint8_t enabled_count; >>     bool paused; > > Looking at the arm64 implementation, I'm thinking: instead of the paused > member, how about a PF_LAZY_MMU task flag? It would be set when lazy_mmu > is actually enabled (i.e. inside an enter()/leave() section, and not > inside a pause()/resume() section). This way, architectures could use > that flag directly to tell if lazy_mmu is enabled instead of reinventing > the wheel, all in slightly different ways. Namely: > > * arm64 uses a thread flag (TIF_LAZY_MMU) - this is trivially replaced > with PF_LAZY_MMU > * powerpc and sparc use batch->active where batch is a per-CPU variable; > I expect this can also be replaced with PF_LAZY_MMU > * x86/xen is more complex as it has xen_lazy_mode which tracks both > LAZY_MMU and LAZY_CPU modes. I'd probably leave that one alone, unless a > Xen expert is motivated to refactor it. > > With that approach, the implementation of arch_enter() and arch_leave() > becomes very simple (no tracking of lazy_mmu status) on arm64, powerpc > and sparc. > > (Of course we could also have an "enabled" member in lazy_mmu_state > instead of PF_LAZY_MMU, there is no functional difference.) > No strong opinion, but to me it feels like PF_LAZY_MMU is rather "the effective state when combining nested+paused", and might complicate the code + sanity checks? So we could maintain that in addition fairly easily of course from the core instead of letting archs do that manually. I would probably have to see the end result to judge whether removing the "paused" bool makes things look more complicated or not. >> } >> >> c) With that config, common-code lazy_mmu_*() functions implement the >> updating of the lazy_mmu_state in task_struct and call into arch code >> on the transition from 0->1, 1->0 etc. > > Indeed, this is how I thought about it. There is actually quite a lot > that can be moved to the generic functions: > * Updating lazy_mmu_state > * Sanity checks on lazy_mmu_state (e.g. underflow/overflow) > * Bailing out if in_interrupt() (not done consistently across arch's at > the moment) > >> >> Maybe that can be done through exiting >> arch_enter_lazy_mmu_mode()/arch_leave_lazy_mmu_mode() callbacks, maybe >> we need more. I feel like >> we might be able to implement that through the existing helpers. > > We might want to rename them to align with the new generic helpers, but > yes otherwise the principle should remain unchanged. > > In fact, we will also need to revive arch_flush_lazy_mmu_mode(). That's okay if it's all hidden behaind a sane core API. > Indeed, > in the nested situation, we need the following arch calls: > > enter() -> arch_enter() >     enter() -> [nothing] >     leave() -> arch_flush() > leave() -> arch_leave() > > leave() must always flush whatever arch state was batched, as may be > expected by the caller. > > How does all that sound? I am no expert on the "always flush when leaving", but it sounds reasonable to me. Which arch operations would you call from pause() continue() ? >> And on top of the proposal above we will have some >> >> struct arch_lazy_mmu_state; >> >> define by the architecture (could be an empty struct on most). >> >> We can store that inside "struct lazy_mmu_state;" or if we ever have >> to, start returning only that from the enable/disable etc. functions. > > I'm not sure we'd want to mix those styles (task_struct member + local > variable), that's adding complexity without much upside... Also having a > local variable at every nesting level only makes sense if we have an > arch callback regardless of nesting level, which is unnecessary in this > proposed API. Yes, that was rather a "if we ever really run out of space we could look into that", I am not a fan of it obviously. -- Cheers David / dhildenb