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From: Matthew Wilcox <willy@infradead.org>
To: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Cc: Hongru Zhang <zhanghongru06@gmail.com>,
	akpm@linux-foundation.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, baohua@kernel.org,
	surenb@google.com, david@kernel.org, liam@infradead.org,
	mhocko@suse.com, rppt@kernel.org, shakeel.butt@linux.dev,
	vbabka@kernel.org, zhaonanzhe@xiaomi.com, linux@armlinux.org.uk,
	catalin.marinas@arm.com, will@kernel.org, mark.rutland@arm.com,
	linux-arm-kernel@lists.infradead.org, chenhuacai@kernel.org,
	kernel@xen0n.name, loongarch@lists.linux.dev,
	maddy@linux.ibm.com, mpe@ellerman.id.au, npiggin@gmail.com,
	chleroy@kernel.org, linuxppc-dev@lists.ozlabs.org,
	pjw@kernel.org, palmer@dabbelt.com, aou@eecs.berkeley.edu,
	alex@ghiti.fr, linux-riscv@lists.infradead.org,
	agordeev@linux.ibm.com, gerald.schaefer@linux.ibm.com,
	hca@linux.ibm.com, gor@linux.ibm.com, borntraeger@linux.ibm.com,
	svens@linux.ibm.com, linux-s390@vger.kernel.org,
	dave.hansen@linux.intel.com, luto@kernel.org,
	peterz@infradead.org, tglx@kernel.org, mingo@redhat.com,
	bp@alien8.de, x86@kernel.org, hpa@zytor.com,
	Hongru Zhang <zhanghongru@xiaomi.com>
Subject: Re: [PATCH v6] mm: retry page faults once under the per-VMA lock
Date: Sun, 27 Sep 2026 23:55:01 +0100	[thread overview]
Message-ID: <armexcXDiG8ryZd6@casper.infradead.org> (raw)
In-Reply-To: <arI_w2wTfA00Mgy9@gremlin>

On Tue, Sep 22, 2026 at 09:55:23AM +0100, Lorenzo Stoakes (ARM) wrote:
> On Mon, Sep 21, 2026 at 07:57:23PM +0100, Matthew Wilcox wrote:
> > more to say on this in two weeks at Plumbers.
> 
> I look forward to it :)

So while doing my slides, I realised that what we need to avoid doing
is (a) sleeping while holding the mmap_lock (b) returning RETRY while
holding the VMA lock

And that turns out to be as simple as this patch:

diff --git a/include/linux/mm.h b/include/linux/mm.h
index dd09c438fa23..94ed2333f8d8 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -723,6 +723,8 @@ enum {
  */
 static inline bool fault_flag_allow_retry_first(enum fault_flag flags)
 {
+	if (flags & FAULT_FLAG_VMA_LOCK)
+		return false;
 	return (flags & FAULT_FLAG_ALLOW_RETRY) &&
 	    (!(flags & FAULT_FLAG_TRIED));
 }

OK, this is a hack.  The function is spectacularly badly named, and
needs to be renamed before a patch can go upstream.  But this should
fix the contention on mmap_lock.

Could somebody try it?  I've verified it boots and runs some userspace
fine, but I don't have the workload to test the contention.

_______________________________________________
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linux-riscv@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-riscv

WARNING: multiple messages have this Message-ID (diff)
From: Matthew Wilcox <willy@infradead.org>
To: "Lorenzo Stoakes (ARM)" <ljs@kernel.org>
Cc: Hongru Zhang <zhanghongru06@gmail.com>,
	akpm@linux-foundation.org, linux-mm@kvack.org,
	linux-kernel@vger.kernel.org, baohua@kernel.org,
	surenb@google.com, david@kernel.org, liam@infradead.org,
	mhocko@suse.com, rppt@kernel.org, shakeel.butt@linux.dev,
	vbabka@kernel.org, zhaonanzhe@xiaomi.com, linux@armlinux.org.uk,
	catalin.marinas@arm.com, will@kernel.org, mark.rutland@arm.com,
	linux-arm-kernel@lists.infradead.org, chenhuacai@kernel.org,
	kernel@xen0n.name, loongarch@lists.linux.dev,
	maddy@linux.ibm.com, mpe@ellerman.id.au, npiggin@gmail.com,
	chleroy@kernel.org, linuxppc-dev@lists.ozlabs.org,
	pjw@kernel.org, palmer@dabbelt.com, aou@eecs.berkeley.edu,
	alex@ghiti.fr, linux-riscv@lists.infradead.org,
	agordeev@linux.ibm.com, gerald.schaefer@linux.ibm.com,
	hca@linux.ibm.com, gor@linux.ibm.com, borntraeger@linux.ibm.com,
	svens@linux.ibm.com, linux-s390@vger.kernel.org,
	dave.hansen@linux.intel.com, luto@kernel.org,
	peterz@infradead.org, tglx@kernel.org, mingo@redhat.com,
	bp@alien8.de, x86@kernel.org, hpa@zytor.com,
	Hongru Zhang <zhanghongru@xiaomi.com>
Subject: Re: [PATCH v6] mm: retry page faults once under the per-VMA lock
Date: Sun, 27 Sep 2026 23:55:01 +0100	[thread overview]
Message-ID: <armexcXDiG8ryZd6@casper.infradead.org> (raw)
In-Reply-To: <arI_w2wTfA00Mgy9@gremlin>

On Tue, Sep 22, 2026 at 09:55:23AM +0100, Lorenzo Stoakes (ARM) wrote:
> On Mon, Sep 21, 2026 at 07:57:23PM +0100, Matthew Wilcox wrote:
> > more to say on this in two weeks at Plumbers.
> 
> I look forward to it :)

So while doing my slides, I realised that what we need to avoid doing
is (a) sleeping while holding the mmap_lock (b) returning RETRY while
holding the VMA lock

And that turns out to be as simple as this patch:

diff --git a/include/linux/mm.h b/include/linux/mm.h
index dd09c438fa23..94ed2333f8d8 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -723,6 +723,8 @@ enum {
  */
 static inline bool fault_flag_allow_retry_first(enum fault_flag flags)
 {
+	if (flags & FAULT_FLAG_VMA_LOCK)
+		return false;
 	return (flags & FAULT_FLAG_ALLOW_RETRY) &&
 	    (!(flags & FAULT_FLAG_TRIED));
 }

OK, this is a hack.  The function is spectacularly badly named, and
needs to be renamed before a patch can go upstream.  But this should
fix the contention on mmap_lock.

Could somebody try it?  I've verified it boots and runs some userspace
fine, but I don't have the workload to test the contention.


  reply	other threads:[~2026-09-27 22:55 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2026-09-11  2:56 [PATCH v6] mm: retry page faults once under the per-VMA lock Hongru Zhang
2026-09-11  2:56 ` Hongru Zhang
2026-09-11  3:07 ` sashiko-bot
2026-09-15 13:05 ` Xueyuan Chen
2026-09-15 13:05   ` Xueyuan Chen
2026-09-21 17:54 ` Lorenzo Stoakes (ARM)
2026-09-21 17:54   ` Lorenzo Stoakes (ARM)
2026-09-21 18:57   ` Matthew Wilcox
2026-09-21 18:57     ` Matthew Wilcox
2026-09-22  8:55     ` Lorenzo Stoakes (ARM)
2026-09-22  8:55       ` Lorenzo Stoakes (ARM)
2026-09-27 22:55       ` Matthew Wilcox [this message]
2026-09-27 22:55         ` Matthew Wilcox
2026-09-28  2:48         ` Barry Song
2026-09-28  2:48           ` Barry Song
2026-10-01 21:12           ` Matthew Wilcox
2026-10-01 21:12             ` Matthew Wilcox
2026-10-02 13:12             ` Lorenzo Stoakes (ARM)
2026-10-02 13:12               ` Lorenzo Stoakes (ARM)
2026-10-02 19:30             ` Barry Song
2026-10-02 19:30               ` Barry Song
2026-09-23  9:53   ` Hongru Zhang

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