Messages in this thread | | | Date | Tue, 19 Sep 2006 16:59:06 -0700 | From | Andrew Morton <> | Subject | Re: [RFC] page fault retry with NOPAGE_RETRY |
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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.
> 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.
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