Messages in this thread |  | | | Subject | Re: [RFC] page fault retry with NOPAGE_RETRY | | From | Benjamin Herrenschmidt <> | | Date | Wed, 20 Sep 2006 10:06:46 +1000 |
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On Tue, 2006-09-19 at 16:59 -0700, Andrew Morton wrote: > On Wed, 20 Sep 2006 09:50:35 +1000 > Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote: > > > On Tue, 2006-09-19 at 16:35 -0700, Mike Waychison wrote: > > > Patch attached. > > > > > > As Andrew points out, the logic is a bit hacky and using a flag in > > > current->flags to determine whether we have done the retry or not already. > > > > > > I too think the right approach to being able to handle these kinds of > > > retries in a more general fashion is to introduce a struct > > > pagefault_args along the page faulting path. Within it, we could > > > introduce a reason for the retry so the higher levels would be able to > > > better understand what to do. > > > > .../... > > > > I need to re-read your mail and Andrew as at this point, I don't quite > > see why we need that args and/or that current->flags bit instead of > > always returning all the way to userland and let the faulting > > instruction happen again (which means you don't block in the kernel, can > > take signals etc... > > That would amount to a busy wait, waiting for the disk IO to complete. > > So we need to go to sleep somewhere (in D state, because we _are_ waiting > for disk IO). Returning all the way to userspace and immediately retaking > the fault is unneeded extra work.
No, I'm not saying immediately... you do the wait thing in filemap.c. Anyway, see my other message.
> > thus do you actually need to prevent multiple > > retries ?) > > I expect there are livelock scenarios. For example, process A could spin > on posix_fadvise(some libc text page, POSIX_FADV_DONTNEED), perhaps causing > other applications to get permanently stuck in the kernel.
Unless you add a way to handle signals.. see my other mail.
Ben.
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