On Tue, Jan 22, 2019 at 04:22:38PM +0800, Peter Xu wrote: > On Mon, Jan 21, 2019 at 10:55:36AM -0500, Jerome Glisse wrote: > > 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. > > Hi, Jerome, > > Do you mean lock_page()? > > AFAIU lock_page() will only yield the process itself until the lock is > released, so IMHO it's not really starving the process but a natural > behavior. After all the process may not continue without handling the > page fault correctly. > > Or when you say "starvation" do you mean that we might return > VM_FAULT_RETRY from handle_mm_fault() continuously so we'll looping > over and over inside the page fault handler? That one ie every time we retry someone else is holding the lock and thus lock_page_or_retry() will continuously retry. Some process just get unlucky ;) With existing code because we remove the retry flag then on the second try we end up waiting for the page lock while holding the mmap_sem so we know that we are in line for the page lock and we will get it once it is our turn. Cheers, Jérôme
