Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
Date:   Mon, 30 May 2022 23:26:09 +0100
From:   "Russell King (Oracle)" <linux@armlinux.org.uk>
To:     Peter Xu <peterx@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Ingo Molnar <mingo@redhat.com>,
        Vasily Gorbik <gor@linux.ibm.com>,
        linuxppc-dev@lists.ozlabs.org, Stafford Horne <shorne@gmail.com>,
        "David S . Miller" <davem@davemloft.net>,
        Johannes Berg <johannes@sipsolutions.net>,
        Brian Cain <bcain@quicinc.com>, x86@kernel.org,
        linux-parisc@vger.kernel.org, Richard Henderson <rth@twiddle.net>,
        Christian Borntraeger <borntraeger@linux.ibm.com>,
        Richard Weinberger <richard@nod.at>,
        linux-hexagon@vger.kernel.org,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Janosch Frank <frankja@linux.ibm.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Anton Ivanov <anton.ivanov@cambridgegreys.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        linux-ia64@vger.kernel.org, Borislav Petkov <bp@alien8.de>,
        Sven Schnelle <svens@linux.ibm.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        "James E . J . Bottomley" <James.Bottomley@hansenpartnership.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Alistair Popple <apopple@nvidia.com>,
        Jonas Bonn <jonas@southpole.se>, sparclinux@vger.kernel.org,
        linux-csky@vger.kernel.org, Will Deacon <will@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>, linux-um@lists.infradead.org,
        Michal Simek <monstr@monstr.eu>,
        Matt Turner <mattst88@gmail.com>,
        linux-m68k@lists.linux-m68k.org, Paul Mackerras <paulus@samba.org>,
        linux-xtensa@linux-xtensa.org,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        David Hildenbrand <david@redhat.com>,
        openrisc@lists.librecores.org,
        linux-arm-kernel@lists.infradead.org,
        Nicholas Piggin <npiggin@gmail.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Stefan Kristiansson <stefan.kristiansson@saunalahti.fi>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
        Chris Zankel <chris@zankel.net>,
        Hugh Dickins <hughd@google.com>,
        Vineet Gupta <vgupta@kernel.org>,
        Dinh Nguyen <dinguyen@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Rich Felker <dalias@libc.org>,
        "H . Peter Anvin" <hpa@zytor.com>, linux-s390@vger.kernel.org,
        linux-sh@vger.kernel.org, linux-riscv@lists.infradead.org,
        Heiko Carstens <hca@linux.ibm.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Andy Lutomirski <luto@kernel.org>,
        Max Filippov <jcmvbkbc@gmail.com>, Guo Ren <guoren@kernel.org>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        linux-snps-arc@lists.infradead.org, linux-alpha@vger.kernel.org,
        linux-mips@vger.kernel.org, Helge Deller <deller@gmx.de>,
        Yoshinori Sato <ysato@users.sourceforge.jp>,
        Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH v5] mm: Avoid unnecessary page fault retires on shared
 memory types
Message-ID: <YpVEgWHzzH3ZtVzA@shell.armlinux.org.uk>
References: <20220530183450.42886-1-peterx@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20220530183450.42886-1-peterx@redhat.com>
Sender: Russell King (Oracle) <linux@armlinux.org.uk>
Precedence: bulk

On Mon, May 30, 2022 at 02:34:50PM -0400, Peter Xu wrote:
> I observed that for each of the shared file-backed page faults, we're very
> likely to retry one more time for the 1st write fault upon no page.  It's
> because we'll need to release the mmap lock for dirty rate limit purpose
> with balance_dirty_pages_ratelimited() (in fault_dirty_shared_page()).
> 
> Then after that throttling we return VM_FAULT_RETRY.
> 
> We did that probably because VM_FAULT_RETRY is the only way we can return
> to the fault handler at that time telling it we've released the mmap lock.
> 
> However that's not ideal because it's very likely the fault does not need
> to be retried at all since the pgtable was well installed before the
> throttling, so the next continuous fault (including taking mmap read lock,
> walk the pgtable, etc.) could be in most cases unnecessary.
> 
> It's not only slowing down page faults for shared file-backed, but also add
> more mmap lock contention which is in most cases not needed at all.
> 
> To observe this, one could try to write to some shmem page and look at
> "pgfault" value in /proc/vmstat, then we should expect 2 counts for each
> shmem write simply because we retried, and vm event "pgfault" will capture
> that.
> 
> To make it more efficient, add a new VM_FAULT_COMPLETED return code just to
> show that we've completed the whole fault and released the lock.  It's also
> a hint that we should very possibly not need another fault immediately on
> this page because we've just completed it.
> 
> This patch provides a ~12% perf boost on my aarch64 test VM with a simple
> program sequentially dirtying 400MB shmem file being mmap()ed and these are
> the time it needs:
> 
>   Before: 650.980 ms (+-1.94%)
>   After:  569.396 ms (+-1.38%)
> 
> I believe it could help more than that.
> 
> We need some special care on GUP and the s390 pgfault handler (for gmap
> code before returning from pgfault), the rest changes in the page fault
> handlers should be relatively straightforward.
> 
> Another thing to mention is that mm_account_fault() does take this new
> fault as a generic fault to be accounted, unlike VM_FAULT_RETRY.
> 
> I explicitly didn't touch hmm_vma_fault() and break_ksm() because they do
> not handle VM_FAULT_RETRY even with existing code, so I'm literally keeping
> them as-is.
> 
> Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> Acked-by: Vineet Gupta <vgupta@kernel.org>
> Acked-by: Guo Ren <guoren@kernel.org>
> Acked-by: Max Filippov <jcmvbkbc@gmail.com>
> Acked-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
> Acked-by: Catalin Marinas <catalin.marinas@arm.com>
> Reviewed-by: Alistair Popple <apopple@nvidia.com>
> Reviewed-by: Ingo Molnar <mingo@kernel.org>
> Signed-off-by: Peter Xu <peterx@redhat.com>

For:

> diff --git a/arch/arm/mm/fault.c b/arch/arm/mm/fault.c
> index a062e07516dd..46cccd6bf705 100644
> --- a/arch/arm/mm/fault.c
> +++ b/arch/arm/mm/fault.c
> @@ -322,6 +322,10 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs)
>  		return 0;
>  	}
>  
> +	/* The fault is fully completed (including releasing mmap lock) */
> +	if (fault & VM_FAULT_COMPLETED)
> +		return 0;
> +
>  	if (!(fault & VM_FAULT_ERROR)) {
>  		if (fault & VM_FAULT_RETRY) {
>  			flags |= FAULT_FLAG_TRIED;

Acked-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>

Thanks!

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!