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:   Wed, 5 Jul 2023 22:27:56 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Peter Xu <peterx@redhat.com>
Cc:     Suren Baghdasaryan <surenb@google.com>,
        David Hildenbrand <david@redhat.com>,
        akpm@linux-foundation.org, jirislaby@kernel.org,
        jacobly.alt@gmail.com, holger@applied-asynchrony.com,
        hdegoede@redhat.com, michel@lespinasse.org, jglisse@google.com,
        mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org,
        mgorman@techsingularity.net, dave@stgolabs.net,
        liam.howlett@oracle.com, peterz@infradead.org,
        ldufour@linux.ibm.com, paulmck@kernel.org, mingo@redhat.com,
        will@kernel.org, luto@kernel.org, songliubraving@fb.com,
        dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de,
        kent.overstreet@linux.dev, punit.agrawal@bytedance.com,
        lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com,
        chriscli@google.com, axelrasmussen@google.com, joelaf@google.com,
        minchan@google.com, rppt@kernel.org, jannh@google.com,
        shakeelb@google.com, tatashin@google.com, edumazet@google.com,
        gthelen@google.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH v3 2/2] mm: disable CONFIG_PER_VMA_LOCK until its fixed
Message-ID: <ZKXgXFfrBzUjfkDN@casper.infradead.org>
References: <20230705171213.2843068-1-surenb@google.com>
 <20230705171213.2843068-3-surenb@google.com>
 <3cdaa7d4-1293-3806-05ce-6b7fc4382458@redhat.com>
 <CAJuCfpGTNF9BWBxZoqYKSDrtq=iJoN1n8oTc=Yu0pPzW8cs8rQ@mail.gmail.com>
 <ZKXRsQC8ufiebDGu@x1n>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <ZKXRsQC8ufiebDGu@x1n>
Precedence: bulk

On Wed, Jul 05, 2023 at 04:25:21PM -0400, Peter Xu wrote:
> There'll still try to be a final fix, am I right?  As IIRC allowing page
> faults during fork() is one of the major goals of vma lock.

Good grief, no.  Why would we want to optimise something that happens
so rarely?  The goal is, as usual, more performance.  Satisfying page
faults while mmap()/munmap()/mprotect() are happening is worthwhile.
Those happen a lot more than fork().

In this case though, there's also a priority-inversion problem that
we're trying to solve where process A (high priority) calls mmap() while
process B (low priority) is reading /proc/$pid/smaps and now (because
rwsems are fair), none of process A's other threads can satisy any page
faults until process B is scheduled.

Where on earth did you get the idea that we cared even a little bit
about the performance of page fault during fork()?