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? > 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 process_memwatch() syscall 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. > > Regards, > Muhammad Usama Anjum > > Muhammad Usama Anjum (5): > fs/proc/task_mmu: make functions global to be used in other files > mm: Implement process_memwatch syscall > mm: wire up process_memwatch syscall for x86 > selftests: vm: add process_memwatch syscall tests > mm: add process_memwatch syscall documentation > > Documentation/admin-guide/mm/soft-dirty.rst | 48 +- > arch/x86/entry/syscalls/syscall_32.tbl | 1 + > arch/x86/entry/syscalls/syscall_64.tbl | 1 + > fs/proc/task_mmu.c | 84 +-- > include/linux/mm_inline.h | 99 +++ > include/linux/syscalls.h | 3 +- > include/uapi/asm-generic/unistd.h | 5 +- > include/uapi/linux/memwatch.h | 12 + > kernel/sys_ni.c | 1 + > mm/Makefile | 2 +- > mm/memwatch.c | 285 ++++++++ > tools/include/uapi/asm-generic/unistd.h | 5 +- > .../arch/x86/entry/syscalls/syscall_64.tbl | 1 + > tools/testing/selftests/vm/.gitignore | 1 + > tools/testing/selftests/vm/Makefile | 2 + > tools/testing/selftests/vm/memwatch_test.c | 635 ++++++++++++++++++ > 16 files changed, 1098 insertions(+), 87 deletions(-) > create mode 100644 include/uapi/linux/memwatch.h > create mode 100644 mm/memwatch.c > create mode 100644 tools/testing/selftests/vm/memwatch_test.c >
