On Mon, Jan 21, 2019 at 03:57:01PM +0800, Peter Xu wrote: > The idea comes from a discussion between Linus and Andrea [1]. > > Before this patch we only allow a page fault to retry once. We achieved > this by clearing the FAULT_FLAG_ALLOW_RETRY flag when doing > handle_mm_fault() the second time. This was majorly used to avoid > unexpected starvation of the system by looping over forever to handle > the page fault on a single page. However that should hardly happen, and > after all for each code path to return a VM_FAULT_RETRY we'll first wait > for a condition (during which time we should possibly yield the cpu) to > happen before VM_FAULT_RETRY is really returned. > > This patch removes the restriction by keeping the FAULT_FLAG_ALLOW_RETRY > flag when we receive VM_FAULT_RETRY. It means that the page fault > handler now can retry the page fault for multiple times if necessary > without the need to generate another page fault event. Meanwhile we > still keep the FAULT_FLAG_TRIED flag so page fault handler can still > identify whether a page fault is the first attempt or not. So there is nothing protecting starvation after this patch ? AFAICT. Do we sufficient proof that we never have a scenario where one process might starve fault another ? For instance some page locking could starve one process. > > GUP code is not touched yet and will be covered in follow up patch. > > This will be a nice enhancement for current code at the same time a > supporting material for the future userfaultfd-writeprotect work since > in that work there will always be an explicit userfault writeprotect > retry for protected pages, and if that cannot resolve the page > fault (e.g., when userfaultfd-writeprotect is used in conjunction with > shared memory) then we'll possibly need a 3rd retry of the page fault. > It might also benefit other potential users who will have similar > requirement like userfault write-protection. > > Please read the thread below for more information. > > [1] https://lkml.org/lkml/2017/11/2/833 > > Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > Suggested-by: Andrea Arcangeli <aarcange@xxxxxxxxxx> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx> > ---
