On 26.07.22 18:18, Muhammad Usama Anjum wrote: > Hello, Hi, > > 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. Such an interface might be easy to add, no? > - The soft-dirty PTE bit of only a part of memory cannot be cleared. Same. So I'm curious why we need a new syscall for that. > > 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 You write "poor performance". Did you actually implement a prototype using userfaultfd-wp? Can you share numbers for comparison? Adding an new syscall just for handling a corner case feature (soft-dirty, which we all love, of course) needs good justification. > > 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 Huh, why? VM_SOFTDIRTY is an internal implementation detail and should remain such. VM_SOFTDIRTY translates to "all pages in this VMA are soft-dirty". -- Thanks, David / dhildenb
