On 1/9/23 11:45 AM, Muhammad Usama Anjum wrote: > *Changes in v7:* > - Add uffd wp async > - Update the IOCTL to use uffd under the hood instead of soft-dirty > flags > > Stop using the soft-dirty flags for finding which pages have been > written to. It is too delicate and wrong as it shows more soft-dirty > pages than the actual soft-dirty pages. There is no interest in > correcting it [A][B] as this is how the feature was written years ago. > It shouldn't be updated to changed behaviour. Peter Xu has suggested > using the async version of the UFFD WP [C] as it is based inherently > on the PTEs. > > So in this patch series, I've added a new mode to the UFFD which is > asynchronous version of the write protect. When this variant of the > UFFD WP is used, the page faults are resolved automatically by the > kernel. The pages which have been written-to can be found by reading > pagemap file (!PM_UFFD_WP). This feature can be used successfully to > find which pages have been written to from the time the pages were > write protected. This works just like the soft-dirty flag without > showing any extra pages which aren't soft-dirty in reality. Any thoughts on this version are highly welcome. Please review. > > [A] https://lore.kernel.org/all/20221220162606.1595355-1-usama.anjum@xxxxxxxxxxxxx > [B] https://lore.kernel.org/all/20221122115007.2787017-1-usama.anjum@xxxxxxxxxxxxx > [C] https://lore.kernel.org/all/Y6Hc2d+7eTKs7AiH@x1n > > *Changes in v6:* > - Updated the interface and made cosmetic changes > > *Cover Letter in v5:* > Hello, > > This patch series implements IOCTL on the pagemap procfs file to get the > information about the page table entries (PTEs). The following operations > are supported in this ioctl: > - Get the information if the pages are soft-dirty, file mapped, present > or swapped. > - Clear the soft-dirty PTE bit of the pages. > - Get and clear the soft-dirty PTE bit of the pages atomically. > > Soft-dirty PTE bit of the memory pages can be read by using the 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. 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 here[1]. This series adds features that weren't > present earlier: > - 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 procfs interface is enough for finding the soft-dirty bit > status and clearing the soft-dirty bit of all the pages of a process. > We have the use case where we need to track the soft-dirty PTE bit for > only specific pages on demand. 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 to process only the dirty pages. > > The information related to pages if the page is file mapped, present and > swapped is required for the CRIU project[2][3]. The addition of the > required mask, any mask, excluded mask and return masks are also required > for the CRIU project[2]. > > The IOCTL returns the addresses of the pages which match the specific masks. > The page addresses are returned in struct page_region in a compact form. > The max_pages is needed to support a use case where user only wants to get > a specific number of pages. So there is no need to find all the pages of > interest in the range when max_pages is specified. The IOCTL returns when > the maximum number of the pages are found. The max_pages is optional. If > max_pages is specified, it must be equal or greater than the vec_size. > This restriction is needed to handle worse case when one page_region only > contains info of one page and it cannot be compacted. This is needed to > emulate the Windows getWriteWatch() syscall. > > Some non-dirty pages get marked as dirty because of the kernel's > internal activity (such as VMA merging as soft-dirty bit difference isn't > considered while deciding to merge VMAs). The dirty bit of the pages is > stored in the VMA flags and in the per page flags. If any of these two bits > are set, the page is considered to be soft dirty. Suppose you have cleared > the soft dirty bit of half of VMA which will be done by splitting the VMA > and clearing soft dirty bit flag in the half VMA and the pages in it. Now > kernel may decide to merge the VMAs again. So the half VMA becomes dirty > again. This splitting/merging costs performance. The application receives > a lot of pages which aren't dirty in reality but marked as dirty. > Performance is lost again here. Also sometimes user doesn't want the newly > allocated memory to be marked as dirty. PAGEMAP_NO_REUSED_REGIONS flag > solves both the problems. It is used to not depend on the soft dirty flag > in the VMA flags. So VMA splitting and merging doesn't happen. It only > depends on the soft dirty bit of the individual pages. Thus by using this > flag, there may be a scenerio such that the new memory regions which are > just created, doesn't look dirty when seen with the IOCTL, but look dirty > when seen from procfs. This seems okay as the user of this flag know the > implication of using it. > > [1] https://lore.kernel.org/lkml/54d4c322-cd6e-eefd-b161-2af2b56aae24@xxxxxxxxxxxxx/ > [2] https://lore.kernel.org/all/YyiDg79flhWoMDZB@xxxxxxxxx/ > [3] https://lore.kernel.org/all/20221014134802.1361436-1-mdanylo@xxxxxxxxxx/ > > Regards, > Muhammad Usama Anjum > > Muhammad Usama Anjum (4): > userfaultfd: Add UFFD WP Async support > userfaultfd: split mwriteprotect_range() > fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about > PTEs > selftests: vm: add pagemap ioctl tests > > fs/proc/task_mmu.c | 300 +++++++ > fs/userfaultfd.c | 161 ++-- > include/linux/userfaultfd_k.h | 10 + > include/uapi/linux/fs.h | 50 ++ > include/uapi/linux/userfaultfd.h | 6 + > mm/userfaultfd.c | 40 +- > tools/include/uapi/linux/fs.h | 50 ++ > tools/testing/selftests/vm/.gitignore | 1 + > tools/testing/selftests/vm/Makefile | 5 +- > tools/testing/selftests/vm/pagemap_ioctl.c | 884 +++++++++++++++++++++ > 10 files changed, 1424 insertions(+), 83 deletions(-) > create mode 100644 tools/testing/selftests/vm/pagemap_ioctl.c > -- BR, Muhammad Usama Anjum
