On Tue, 09 Nov 2010 16:15:52 -0500 Rik van Riel <riel@xxxxxxxxxx> wrote: > On 11/09/2010 04:07 PM, Ted Ts'o wrote: > > On Tue, Nov 09, 2010 at 02:33:29PM -0500, Rik van Riel wrote: > >> > >> There are essentially two possibilities: > >> 1) the VM can potentially be filled up with uncleanable dirty pages, or > >> 2) pages that hit an IO error are left in a clean state, so they can > >> be reclaimed under memory pressure > >> > >> Alternative 1 could cause the entire system to deadlock, while > >> option 2 puts the onus on userland apps to rewrite the data > >> from a failed msync/fsync. > >> > >> Currently the VM has behaviour #2 which is preserved with my > >> patch. > >> > >> The only difference with my patch is, we won't keep returning > >> -EIO on subsequent, error free, msync or fsync calls to files > >> that had an IO error at some previous point in the past. > > > > Do we guarantee that the application will get EIO at least once? I > > thought there were issues where the error bit could get lost if the > > page writeback was triggered by sync() run by a third-party > > application. > > There is no such guarantee in the current kernel, either > with or without my patch. > > A third application calling fsync or msync can get the > EIO cleared, so the application that did the write does > not see it. yup. It's a userspace bug, really. Although that bug might be expressed as "userspace didn't know about linux-specific EIO behaviour". > The VM could also reclaim the PageError page due to > memory pressure, so the application calling fsync or > msync does not see it. That would be a kernel bug, methinks. The page's end_io handler should set the address_space's AS_EIO flag (see mpage_end_io_write()), to be later returned to (and cleared by) the fsync/msync caller. It wouldn't surprise me if lots of end_io handlers got that wrong. > I see no good way in which we could guarantee that > every process calling msync or fsync on a file that > had an IO error in the past gets EIO once - at least, > not without every one of them always getting EIO on > the file even after the IO path is good again... Yes, there's no obviously good design here. And a lot of these problems also apply to ENOSPC, and an ENOSPC condition most certainly does magically fix itself up in real time... -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
