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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [RFC PATCH 13/37] mm: implement speculative handling in __handle_mm_fault().
Date:   Thu, 29 Apr 2021 11:04:56 -0700
Message-Id: <C02655BC-F722-4EAD-B93E-D890A2DEC05A@amacapital.net>
References: <20210429161234.GG1847222@casper.infradead.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Michel Lespinasse <michel@lespinasse.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Laurent Dufour <ldufour@linux.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Michal Hocko <mhocko@suse.com>,
        Rik van Riel <riel@surriel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Joel Fernandes <joelaf@google.com>,
        Rom Lemarchand <romlem@google.com>,
        Linux-Kernel <linux-kernel@vger.kernel.org>
In-Reply-To: <20210429161234.GG1847222@casper.infradead.org>
To:     Matthew Wilcox <willy@infradead.org>
Precedence: bulk


> On Apr 29, 2021, at 9:12 AM, Matthew Wilcox <willy@infradead.org> wrote:
>=20
> =EF=BB=BFOn Wed, Apr 28, 2021 at 05:05:17PM -0700, Andy Lutomirski wrote:
>>> On Wed, Apr 28, 2021 at 5:02 PM Michel Lespinasse <michel@lespinasse.org=
> wrote:
>>> Thanks Paul for confirming / clarifying this. BTW, it would be good to
>>> add this to the rcu header files, just so people have something to
>>> reference to when they depend on such behavior (like fast GUP
>>> currently does).
>>=20
>> Or, even better, fast GUP could add an explicit RCU read lock.
>>=20
>>>=20
>>> Going back to my patch. I don't need to protect against THP splitting
>>> here, as I'm only handling the small page case. So when
>>> MMU_GATHER_RCU_TABLE_FREE is enabled, I *think* I could get away with
>>> using only an rcu read lock, instead of disabling interrupts which
>>> implicitly creates the rcu read lock. I'm not sure which way to go -
>>> fast GUP always disables interrupts regardless of the
>>> MMU_GATHER_RCU_TABLE_FREE setting, and I think there is a case to be
>>> made for following the fast GUP stes rather than trying to be smarter.
>>=20
>> How about adding some little helpers:
>>=20
>> lockless_page_walk_begin();
>>=20
>> lockless_page_walk_end();
>>=20
>> these turn into RCU read locks if MMU_GATHER_RCU_TABLE_FREE and into
>> irqsave otherwise.  And they're somewhat self-documenting.
>=20
> One of the worst things we can do while holding a spinlock is take a
> cache miss because we then delay for several thousand cycles to wait for
> the cache line.  That gives every other CPU a really long opportunity
> to slam into the spinlock and things go downhill fast at that point.
> We've even seen patches to do things like read A, take lock L, then read
> A to avoid the cache miss while holding the lock.
>=20
> What sort of performance effect would it have to free page tables
> under RCU for all architectures?  It's painful on s390 & powerpc because
> different tables share the same struct page, but I have to believe that's
> a solvable problem.

The IPI locking mechanism is entirely useless on any architecture that wants=
 to do paravirt shootdowns, so this seems like a good strategy to me.=