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 20:27:49 +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,
        Yang Shi <shy828301@gmail.com>
Subject: Re: [PATCH 3/3] mm/mmu_gather: send tlb_remove_table_smp_sync IPI
 only to CPUs in kernel mode
Message-ID: <20230406182749.GA405948@hirez.programming.kicks-ass.net>
References: <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>
 <20230406150213.GQ386572@hirez.programming.kicks-ass.net>
 <248392c0-52d1-d09d-75ec-9e930435c053@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <248392c0-52d1-d09d-75ec-9e930435c053@redhat.com>
Precedence: bulk

On Thu, Apr 06, 2023 at 05:51:52PM +0200, David Hildenbrand wrote:
> On 06.04.23 17:02, Peter Zijlstra wrote:

> > DavidH, what do you thikn about reviving Jann's patches here:
> > 
> >    https://bugs.chromium.org/p/project-zero/issues/detail?id=2365#c1
> > 
> > Those are far more invasive, but afaict they seem to do the right thing.
> > 
> 
> I recall seeing those while discussed on security@kernel.org. What we
> currently have was (IMHO for good reasons) deemed better to fix the issue,
> especially when caring about backports and getting it right.

Yes, and I think that was the right call. However, we can now revisit
without having the pressure of a known defect and backport
considerations.

> The alternative that was discussed in that context IIRC was to simply
> allocate a fresh page table, place the fresh page table into the list
> instead, and simply free the old page table (then using common machinery).
> 
> TBH, I'd wish (and recently raised) that we could just stop wasting memory
> on page tables for THPs that are maybe never going to get PTE-mapped ... and
> eventually just allocate on demand (with some caching?) and handle the
> places where we're OOM and cannot PTE-map a THP in some descend way.
> 
> ... instead of trying to figure out how to deal with these page tables we
> cannot free but have to special-case simply because of GUP-fast.

Not keeping them around sounds good to me, but I'm not *that* familiar
with the THP code, most of that happened after I stopped tracking mm. So
I'm not sure how feasible is it.

But it does look entirely feasible to rework this page-table freeing
along the lines Jann did.