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Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.13.0 Cc: usama.anjum@collabora.com, kernel@collabora.com, Gabriel Krisman Bertazi , David Hildenbrand , Peter Enderborg , Greg KH Subject: Re: [PATCH v3 0/4] Implement IOCTL to get and clear soft dirty PTE Content-Language: en-US To: Jonathan Corbet , Alexander Viro , Andrew Morton , Shuah Khan , "open list:DOCUMENTATION" , open list , "open list:PROC FILESYSTEM" , "open list:MEMORY MANAGEMENT" , "open list:KERNEL SELFTEST FRAMEWORK" References: <20220826064535.1941190-1-usama.anjum@collabora.com> From: Muhammad Usama Anjum In-Reply-To: <20220826064535.1941190-1-usama.anjum@collabora.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1662543646; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; 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spf=pass (imf29.hostedemail.com: domain of usama.anjum@collabora.com designates 46.235.227.172 as permitted sender) smtp.mailfrom=usama.anjum@collabora.com; dmarc=pass (policy=none) header.from=collabora.com X-Rspamd-Server: rspam09 X-HE-Tag: 1662543646-310286 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 8/26/22 11:45 AM, Muhammad Usama Anjum wrote: > > Hello, > > This patch series implements a new ioctl on the pagemap proc fs file to > get, clear and perform both get and clear at the same time atomically on > the specified range of the memory. > > Soft-dirty PTE bit of the memory pages can be viewed by using pagemap > procfs file. The soft-dirty PTE bit for the whole memory range of the > process can be cleared by writing to the clear_refs file. This series > adds features that weren't present earlier. > - There is no atomic get soft-dirty PTE bit status and clear operation > present. > - 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 the 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 > new ioctl 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 process) 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 > Some benchmarks can be seen [1]. > > This ioctl can be used by the CRIU project and other applications which > require soft-dirty PTE bit information. The following operations are > supported in this ioctl: > - 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 Thoughts? > > There are two decisions which have been taken about how to get the output > from the syscall. > - Return offsets of the pages from the start in the vec > - Stop execution when vec is filled with dirty pages > These two arguments doesn't follow the mincore() philosophy where the > output array corresponds to the address range in one to one fashion, hence > the output buffer length isn't passed and only a flag is set if the page > is present. This makes mincore() easy to use with less control. We are > passing the size of the output array and putting return data consecutively > which is offset of dirty pages from the start. The user can convert these > offsets back into the dirty page addresses easily. Suppose, the user want > to get first 10 dirty pages from a total memory of 100 pages. He'll > allocate output buffer of size 10 and the ioctl will abort after finding the > 10 pages. This behaviour is needed to support Windows' getWriteWatch(). The > behaviour like mincore() can be achieved by passing output buffer of 100 > size. This interface can be used for any desired behaviour. > > [1] https://lore.kernel.org/lkml/54d4c322-cd6e-eefd-b161-2af2b56aae24@collabora.com/ > > Regards, > Muhammad Usama Anjum > > Muhammad Usama Anjum (4): > fs/proc/task_mmu: update functions to clear the soft-dirty PTE bit > fs/proc/task_mmu: Implement IOCTL to get and clear soft dirty PTE bit > selftests: vm: add pagemap ioctl tests > mm: add documentation of the new ioctl on pagemap > > Documentation/admin-guide/mm/soft-dirty.rst | 42 +- > fs/proc/task_mmu.c | 342 ++++++++++- > include/uapi/linux/fs.h | 23 + > tools/include/uapi/linux/fs.h | 23 + > tools/testing/selftests/vm/.gitignore | 1 + > tools/testing/selftests/vm/Makefile | 2 + > tools/testing/selftests/vm/pagemap_ioctl.c | 649 ++++++++++++++++++++ > 7 files changed, 1050 insertions(+), 32 deletions(-) > create mode 100644 tools/testing/selftests/vm/pagemap_ioctl.c > -- Muhammad Usama Anjum