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Date:   Wed, 10 Aug 2022 21:44:55 +0500
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Cc:     usama.anjum@collabora.com,
        "kernel@collabora.com" <kernel@collabora.com>
Subject: Re: [PATCH 0/5] Add process_memwatch syscall
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To:     "Peter.Enderborg@sony.com" <Peter.Enderborg@sony.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>,
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        <linux-arch@vger.kernel.org>,
        "open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
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        <linux-kselftest@vger.kernel.org>,
        "krisman@collabora.com" <krisman@collabora.com>
References: <20220726161854.276359-1-usama.anjum@collabora.com>
 <456a8052-dfd2-e00c-6da7-fb5aa0c35f2c@sony.com>
From:   Muhammad Usama Anjum <usama.anjum@collabora.com>
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On 8/10/22 2:22 PM, Peter.Enderborg@sony.com wrote:
> On 7/26/22 18:18, Muhammad Usama Anjum wrote:
>> Hello,
>>
>> This patch series implements a new syscall, process_memwatch. Currently,
>> only the support to watch soft-dirty PTE bit is added. This syscall is
>> generic to watch the memory of the process. There is enough room to add
>> more operations like this to watch memory in the future.
>>
>> Soft-dirty PTE bit of the memory pages can be viewed by using pagemap
>> procfs file. The soft-dirty PTE bit for the memory in a process can be
>> cleared by writing to the clear_refs file. This series adds features that
>> weren't possible through the Proc FS interface.
>> - There is no atomic get soft-dirty PTE bit status and clear operation
>>   possible.
>> - The soft-dirty PTE bit of only a part of memory cannot be cleared.
>>
>> Historically, soft-dirty PTE bit tracking has been used in the CRIU
>> project. The Proc FS interface is enough for that as I think the process
>> is frozen. We have the use case where we need to track the soft-dirty
>> PTE bit for running processes. We need this tracking and clear mechanism
>> of a region of memory while the process is running to emulate the
>> getWriteWatch() syscall of Windows. This syscall is used by games to keep
>> track of dirty pages and keep processing only the dirty pages. This
>> syscall can be used by the CRIU project and other applications which
>> require soft-dirty PTE bit information.
>>
>> As in the current kernel there is no way to clear a part of memory (instead
>> of clearing the Soft-Dirty bits for the entire processi) and get+clear
>> operation cannot be performed atomically, there are other methods to mimic
>> this information entirely in userspace with poor performance:
>> - The mprotect syscall and SIGSEGV handler for bookkeeping
>> - The userfaultfd syscall with the handler for bookkeeping
>>
>>         long process_memwatch(int pidfd, unsigned long start, int len,
>>                               unsigned int flags, void *vec, int vec_len);
>>
>> 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
>>
> 
> Why can it not be done as a IOCTL?
It can be done as ioctl. I think this syscall can be used in future for
adding other operations like soft-dirty. This is why syscall has been added.

-- 
Muhammad Usama Anjum