On Thu, 2017-03-09 at 12:02 +0100, Jan Kara wrote: > On Thu 09-03-17 05:47:51, Jeff Layton wrote: > > On Thu, 2017-03-09 at 10:04 +0100, Jan Kara wrote: > > > On Wed 08-03-17 21:57:25, Ted Tso wrote: > > > > On Tue, Mar 07, 2017 at 11:26:22AM +0100, Jan Kara wrote: > > > > > On a more general note (DAX is actually fine here), I find the current > > > > > practice of clearing page dirty bits on error and reporting it just once > > > > > problematic. It keeps the system running but data is lost and possibly > > > > > without getting the error anywhere where it is useful. We get away with > > > > > this because it is a rare event but it seems like a problematic behavior. > > > > > But this is more for the discussion at LSF. > > > > > > > > I'm actually running into this in the last day or two because some MM > > > > folks at $WORK have been trying to push hard for GFP_NOFS removal in > > > > ext4 (at least when we are holding some mutex/semaphore like > > > > i_data_sem) because otherwise it's possible for the OOM killer to be > > > > unable to kill processes because they are holding on to locks that > > > > ext4 is holding. > > > > > > > > I've done some initial investigation, and while it's not that hard to > > > > remove GFP_NOFS from certain parts of the writepages() codepath (which > > > > is where we had been are running into problems), a really, REALLY big > > > > problem is if any_filesystem->writepages() returns ENOMEM, it causes > > > > silent data loss, because the pages are marked clean, and so data > > > > written using buffered writeback goes *poof*. > > > > > > > > I confirmed this by creating a test kernel with a simple patch such > > > > that if the ext4 file system is mounted with -o debug, there was a 1 > > > > in 16 chance that ext4_writepages will immediately return with ENOMEM > > > > (and printk the inode number, so I knew which inodes had gotten the > > > > ENOMEM treatment). The result was **NOT** pretty. > > > > > > > > What I think we should strongly consider is at the very least, special > > > > case ENOMEM being returned by writepages() during background > > > > writeback, and *not* mark the pages clean, and make sure the inode > > > > stays on the dirty inode list, so we can retry the write later. This > > > > is especially important since the process that issued the write may > > > > have gone away, so there might not even be a userspace process to > > > > complain to. By converting certain page allocations (most notably in > > > > ext4_mb_load_buddy) from GFP_NOFS to GFP_KMALLOC, this allows us to > > > > release the i_data_sem lock and return an error. This should allow > > > > allow the OOM killer to do its dirty deed, and hopefully we can retry > > > > the writepages() for that inode later. > > > > > > Yeah, so if we can hope the error is transient, keeping pages dirty and > > > retrying the write is definitely better option. For start we can say that > > > ENOMEM, EINTR, EAGAIN, ENOSPC errors are transient, anything else means > > > there's no hope of getting data to disk and so we just discard them. It > > > will be somewhat rough distinction but probably better than what we have > > > now. > > > > > > Honza > > > > I'm not sure about ENOSPC there. That's a return code that is > > specifically expected to be returned by fsync. It seems like that ought > > not be considered a transient error? > > Yeah, for start we should probably keep ENOSPC as is to prevent surprises. > Long term, we may need to make at least some ENOSPC situations behave as > transient to make thin provisioned storage not loose data in case admin > does not supply additional space fast enough (i.e., before ENOSPC is > actually hit). > Maybe we need a systemwide (or fs-level) tunable that makes ENOSPC a transient error? Just have it hang until we get enough space when that tunable is enabled? > EIO is actually in a similar bucket although probably more on the "hard > failure" side - I can imagine there can by types of storage and situations > where the loss of connectivity to the storage is only transient. But for > start I would not bother with this. > > Honza I don't see what we can reasonably do with -EIO other than return a hard error. If we want to deal with loss of connectivity to storage as a transient failure, I think that we'd need to ensure that the lower layers return more distinct error codes in those cases (ENODEV or ENXIO maybe? Or declare a new kernel-internal code -- EDEVGONE?). In any case, I think that the basic idea of marking certain writepage/writepages/launder_page errors as transient might be a reasonable approach to handling this sanely. The problem with all of this though is that we have a pile of existing code that will likely need to be reworked for the new error handling. I expect that we'll have to walk all of the writepage/writepages/launder_page implementations and fix them up one by one once we sort out the rules for this. -- Jeff Layton <jlayton@xxxxxxxxxx>
