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@xxxxxxxxxxxxx/ > > 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
