On Mon, Apr 17, 2023 at 08:00:48PM +0100, Lorenzo Stoakes wrote: > So I don't think this route is plausible unless you were thinking of > somehow offloading to a thread? ah, fair enough > In any case, if we institute the FOLL_ALLOW_BROKEN_FILE_MAPPINGS flag we > can just drop FOLL_ANON altogether right, as this will be implied and > hugetlb should work here too? Well.. no, as I said read-only access to the pages works fine, so GUP should not block that. It is only write that has issues > Separately, I find the semantics of access_remote_vm() kind of weird, and > with a possible mmap_lock-free future it does make me wonder whether > something better could be done there. Yes, it is very weird, kthread_use_mm is much nicer. > (Section where I sound like I might be going mad) Perhaps having some means > of context switching into the kernel portion of the remote process as if > were a system call or soft interrupt handler and having that actually do > the uaccess operation could be useful here? This is the kthread_use_mm() approach, that is basically what it does. You are suggesting to extend it to kthreads that already have a process attached... access_remote_vm is basically copy_to/from_user built using kmap and GUP. even a simple step of localizing FOLL_ANON to __access_remote_vm, since it must have the VMA nyhow, would be an improvement. Jason
