Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 6 Apr 2023 17:06:31 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     Marcelo Tosatti <mtosatti@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Yair Podemsky <ypodemsk@redhat.com>, linux@armlinux.org.uk,
        mpe@ellerman.id.au, npiggin@gmail.com, christophe.leroy@csgroup.eu,
        hca@linux.ibm.com, gor@linux.ibm.com, agordeev@linux.ibm.com,
        borntraeger@linux.ibm.com, svens@linux.ibm.com,
        davem@davemloft.net, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org,
        hpa@zytor.com, will@kernel.org, aneesh.kumar@linux.ibm.com,
        akpm@linux-foundation.org, arnd@arndb.de, keescook@chromium.org,
        paulmck@kernel.org, jpoimboe@kernel.org, samitolvanen@google.com,
        ardb@kernel.org, juerg.haefliger@canonical.com,
        rmk+kernel@armlinux.org.uk, geert+renesas@glider.be,
        tony@atomide.com, linus.walleij@linaro.org,
        sebastian.reichel@collabora.com, nick.hawkins@hpe.com,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org,
        sparclinux@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-mm@kvack.org, vschneid@redhat.com, dhildenb@redhat.com,
        alougovs@redhat.com, jannh@google.com
Subject: Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp_sync IPI
 only to CPUs in kernel mode
Message-ID: <20230406150631.GR386572@hirez.programming.kicks-ass.net>
References: <20230404134224.137038-1-ypodemsk@redhat.com>
 <20230404134224.137038-4-ypodemsk@redhat.com>
 <ZC1Q7uX4rNLg3vEg@lothringen>
 <ZC1XD/sEJY+zRujE@lothringen>
 <ZC3P3Ds/BIcpRNGr@tpad>
 <20230405195226.GB365912@hirez.programming.kicks-ass.net>
 <ZC69Wmqjdwk+I8kn@tpad>
 <20230406132928.GM386572@hirez.programming.kicks-ass.net>
 <20230406140423.GA386634@hirez.programming.kicks-ass.net>
 <1654e2d5-5a32-a253-e335-0ee42f69f5ef@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1654e2d5-5a32-a253-e335-0ee42f69f5ef@redhat.com>
Precedence: bulk

On Thu, Apr 06, 2023 at 04:42:02PM +0200, David Hildenbrand wrote:
> On 06.04.23 16:04, Peter Zijlstra wrote:
> > On Thu, Apr 06, 2023 at 03:29:28PM +0200, Peter Zijlstra wrote:
> > > On Thu, Apr 06, 2023 at 09:38:50AM -0300, Marcelo Tosatti wrote:
> > > 
> > > > > To actually hit this path you're doing something really dodgy.
> > > > 
> > > > Apparently khugepaged is using the same infrastructure:
> > > > 
> > > > $ grep tlb_remove_table khugepaged.c
> > > > 	tlb_remove_table_sync_one();
> > > > 	tlb_remove_table_sync_one();
> > > > 
> > > > So just enabling khugepaged will hit that path.
> > > 
> > > Urgh, WTF..
> > > 
> > > Let me go read that stuff :/
> > 
> > At the very least the one on collapse_and_free_pmd() could easily become
> > a call_rcu() based free.
> > 
> > I'm not sure I'm following what collapse_huge_page() does just yet.
> 
> It wants to replace a leaf page table by a THP (Transparent Huge Page mapped
> by a PMD). So we want to rip out a leaf page table while other code
> (GUP-fast) might still be walking it. 

Right, I got that far.

> In contrast to freeing the page table,
> we put it into a list where it can be reuse when having to PTE-map a THP
> again.

Yeah, this is the bit I couldn't find, that code is a bit of a maze.

> Now, similar to after freeing the page table, someone else could reuse that
> page table and modify it.

So ideally we'll RCU free the page instead of sticking it on that list.