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Date: Wed, 20 Sep 2006 10:06:46 +1000 From: Benjamin Herrenschmidt <benh@...nel.crashing.org> To: Andrew Morton <akpm@...l.org> Cc: Mike Waychison <mikew@...gle.com>, linux-mm@...ck.org, Linux Kernel list <linux-kernel@...r.kernel.org>, Linus Torvalds <torvalds@...l.org> Subject: Re: [RFC] page fault retry with NOPAGE_RETRY On Tue, 2006-09-19 at 16:59 -0700, Andrew Morton wrote: > On Wed, 20 Sep 2006 09:50:35 +1000 > Benjamin Herrenschmidt <benh@...nel.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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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