On 8/10/22 2:22 PM, Peter.Enderborg@xxxxxxxx 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
