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;
From:   Gabriel Krisman Bertazi <krisman@collabora.com>
To:     Peter.Enderborg@sony.com
Cc:     Muhammad Usama Anjum <usama.anjum@collabora.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Arnd Bergmann <arnd@arndb.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Shuah Khan <shuah@kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        "open list:PROC FILESYSTEM" <linux-fsdevel@vger.kernel.org>,
        "open list:ABI/API" <linux-api@vger.kernel.org>,
        "open list:GENERIC INCLUDE/ASM HEADER FILES" 
        <linux-arch@vger.kernel.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
        "open list:PERFORMANCE EVENTS SUBSYSTEM" 
        <linux-perf-users@vger.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        "kernel@collabora.com" <kernel@collabora.com>
Subject: Re: [PATCH 0/5] Add process_memwatch syscall
References: <20220726161854.276359-1-usama.anjum@collabora.com>
        <456a8052-dfd2-e00c-6da7-fb5aa0c35f2c@sony.com>
Date:   Wed, 10 Aug 2022 12:53:07 -0400
In-Reply-To: <456a8052-dfd2-e00c-6da7-fb5aa0c35f2c@sony.com> (Peter's message
        of "Wed, 10 Aug 2022 09:22:02 +0000")
Message-ID: <87tu6kghvw.fsf@collabora.com>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.1 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

"Peter.Enderborg@sony.com" <Peter.Enderborg@sony.com> writes:
>>
>> This syscall can be used by the CRIU project and other applications which
>> require soft-dirty PTE bit information. The following operations are
>> supported in this syscall:
>> - Get the pages that are soft-dirty.
>> - Clear the pages which are soft-dirty.
>> - The optional flag to ignore the VM_SOFTDIRTY and only track per page
>> soft-dirty PTE bit
>>

Hi Peter,

(For context, I wrote a previous version of this patch and have been
working with Usama on the current patch).

> Why can it not be done as a IOCTL?

Considering an ioctl is basically a namespaced syscall with extra-steps,
surely we can do it :) There are a few reasons we haven't, though:

1) ioctl auditing/controling is much harder than syscall

2) There is a concern for performance, since this might be executed
frequently by windows applications running over wine.  There is an extra
cost with unnecessary copy_[from/to]_user that we wanted to avoid, even
though we haven't measured.

3) I originally wrote this at the time process_memadvise was merged.  I
felt it fits the same kind of interface exposed by
process_memadvise/process_mrelease,  recently merged.

4) Not obvious whether the ioctl would be against pagemap/clear_refs.
Neither file name describes both input and output semantics.

Obviously, all of those reasons can be worked around, and we can turn
this into an ioctl.

Thanks,

-- 
Gabriel Krisman Bertazi