On Wed, 2018-09-05 at 16:24 +0800, 焦晓冬 wrote: > On Wed, Sep 5, 2018 at 4:18 AM Jeff Layton <jlayton@xxxxxxxxxx> wrote: > > > > On Tue, 2018-09-04 at 14:54 -0400, J. Bruce Fields wrote: > > > On Tue, Sep 04, 2018 at 06:23:48PM +0200, Rogier Wolff wrote: > > > > On Tue, Sep 04, 2018 at 12:12:03PM -0400, J. Bruce Fields wrote: > > > > > Well, I think the point was that in the above examples you'd prefer that > > > > > the read just fail--no need to keep the data. A bit marking the file > > > > > (or even the entire filesystem) unreadable would satisfy posix, I guess. > > > > > Whether that's practical, I don't know. > > > > > > > > When you would do it like that (mark the whole filesystem as "in > > > > error") things go from bad to worse even faster. The Linux kernel > > > > tries to keep the system up even in the face of errors. > > > > > > > > With that suggestion, having one application run into a writeback > > > > error would effectively crash the whole system because the filesystem > > > > may be the root filesystem and stuff like "sshd" that you need to > > > > diagnose the problem needs to be read from the disk.... > > > > > > Well, the absolutist position on posix compliance here would be that a > > > crash is still preferable to returning the wrong data. And for the > > > cases 焦晓冬 gives, that sounds right? Maybe it's the wrong balance in > > > general, I don't know. And we do already have filesystems with > > > panic-on-error options, so if they aren't used maybe then maybe users > > > have already voted against that level of strictness. > > > > > > > Yeah, idk. The problem here is that this is squarely in the domain of > > implementation defined behavior. I do think that the current "policy" > > (if you call it that) of what to do after a wb error is weird and wrong. > > What we probably ought to do is start considering how we'd like it to > > behave. > > > > How about something like this? > > > > Mark the pages as "uncleanable" after a writeback error. We'll satisfy > > reads from the cached data until someone calls fsync, at which point > > we'd return the error and invalidate the uncleanable pages. > > Totally agree with you. > > > > > If no one calls fsync and scrapes the error, we'll hold on to it for as > > long as we can (or up to some predefined limit) and then after that > > we'll invalidate the uncleanable pages and start returning errors on > > reads. If someone eventually calls fsync afterward, we can return to > > normal operation. > > Agree with you except that using fsync() as `clear_error_mark()` seems > weird and counter-intuitive. > That is essentially how fsync (and the errseq_t infrastructure) works. Once the kernel has hit a wb error, it reports that error to fsync exactly once per fd. In practice, the errors are not "cleared", but it appears that way to the fsync caller. > > > > As always though...what about mmap? Would we need to SIGBUS at the point > > where we'd start returning errors on read()? > > I think SIGBUS to mmap() is the same thing as EIO to read(). > > > > > Would that approximate the current behavior enough and make sense? > > Implementing it all sounds non-trivial though... > > No. > No problem is reported because nowadays we are relying on the > underlying disk drives. They transparently redirect bad sectors and > use S.M.A.R.T to waning us long before a real EIO could be seen. > As to network filesystems, if I'm not wrong, close() op calls fsync() > inside the implementation. So there is also no problem. There is no requirement for a filesystem to flush data on close(). In fact, most local filesystems do not. NFS does, but that's because it has to in order to provide close-to-open cache consistency semantics. -- Jeff Layton <jlayton@xxxxxxxxxx>
