[dm-devel] [PATCH v5 2/8] drivers/pmem: Allow pmem_clear_poison() to accept arbitrary offset and len

[Dan Williams]

On Sun, Feb 23, 2020 at 3:03 PM Dave Chinner <david at fromorbit.com> wrote:
>
> On Fri, Feb 21, 2020 at 03:17:59PM -0500, Vivek Goyal wrote:
> > On Fri, Feb 21, 2020 at 01:32:48PM -0500, Jeff Moyer wrote:
> > > Vivek Goyal <vgoyal at redhat.com> writes:
> > >
> > > > On Thu, Feb 20, 2020 at 04:35:17PM -0500, Jeff Moyer wrote:
> > > >> Vivek Goyal <vgoyal at redhat.com> writes:
> > > >>
> > > >> > Currently pmem_clear_poison() expects offset and len to be sector aligned.
> > > >> > Atleast that seems to be the assumption with which code has been written.
> > > >> > It is called only from pmem_do_bvec() which is called only from pmem_rw_page()
> > > >> > and pmem_make_request() which will only passe sector aligned offset and len.
> > > >> >
> > > >> > Soon we want use this function from dax_zero_page_range() code path which
> > > >> > can try to zero arbitrary range of memory with-in a page. So update this
> > > >> > function to assume that offset and length can be arbitrary and do the
> > > >> > necessary alignments as needed.
> > > >>
> > > >> What caller will try to zero a range that is smaller than a sector?
> > > >
> > > > Hi Jeff,
> > > >
> > > > New dax zeroing interface (dax_zero_page_range()) can technically pass
> > > > a range which is less than a sector. Or which is bigger than a sector
> > > > but start and end are not aligned on sector boundaries.
> > >
> > > Sure, but who will call it with misaligned ranges?
> >
> > create a file foo.txt of size 4K and then truncate it.
> >
> > "truncate -s 23 foo.txt". Filesystems try to zero the bytes from 24 to
> > 4095.
>
> This should fail with EIO. Only full page writes should clear the
> bad page state, and partial writes should therefore fail because
> they do not guarantee the data in the filesystem block is all good.
>
> If this zeroing was a buffered write to an address with a bad
> sector, then the writeback will fail and the user will (eventually)
> get an EIO on the file.
>
> DAX should do the same thing, except because the zeroing is
> synchronous (i.e. done directly by the truncate syscall) we can -
> and should - return EIO immediately.
>
> Indeed, with your code, if we then extend the file by truncating up
> back to 4k, then the range between 23 and 512 is still bad, even
> though we've successfully zeroed it and the user knows it. An
> attempt to read anywhere in this range (e.g. 10 bytes at offset 100)
> will fail with EIO, but reading 10 bytes at offset 2000 will
> succeed.
>
> That's *awful* behaviour to expose to userspace, especially when
> they look at the fs config and see that it's using both 4kB block
> and sector sizes...
>
> The only thing that makes sense from a filesystem perspective is
> clearing bad page state when entire filesystem blocks are
> overwritten. The data in a filesystem block is either good or bad,
> and it doesn't matter how many internal (kernel or device) sectors
> it has.
>
> > > And what happens to the rest?  The caller is left to trip over the
> > > errors?  That sounds pretty terrible.  I really think there needs to be
> > > an explicit contract here.
> >
> > Ok, I think is is the contentious bit. Current interface
> > (__dax_zero_page_range()) either clears the poison (if I/O is aligned to
> > sector) or expects page to be free of poison.
> >
> > So in above example, of "truncate -s 23 foo.txt", currently I get an error
> > because range being zeroed is not sector aligned. So
> > __dax_zero_page_range() falls back to calling direct_access(). Which
> > fails because there are poisoned sectors in the page.
> >
> > With my patches, dax_zero_page_range(), clears the poison from sector 1 to
> > 7 but leaves sector 0 untouched and just writes zeroes from byte 0 to 511
> > and returns success.
>
> Ok, kernel sectors are not the unit of granularity bad page state
> should be managed at. They don't match page state granularity, and
> they don't match filesystem block granularity, and the whacky
> "partial writes silently succeed, reads fail unpredictably"
> assymetry it leads to will just cause problems for users.
>
> > So question is, is this better behavior or worse behavior. If sector 0
> > was poisoned, it will continue to remain poisoned and caller will come
> > to know about it on next read and then it should try to truncate file
> > to length 0 or unlink file or restore that file to get rid of poison.
>
> Worse, because the filesystem can't track what sub-parts of the
> block are bad and that leads to inconsistent data integrity status
> being exposed to userspace.

The driver can't track it either. Latent poison isn't know until it is
consumed, and writes to latent poison are not guaranteed to clear it.

>
>
> > IOW, if a partial block is being zeroed and if it is poisoned, caller
> > will not be return an error and poison will not be cleared and memory
> > will be zeroed. What do we expect in such cases.
> >
> > Do we expect an interface where if there are any bad blocks in the range
> > being zeroed, then they all should be cleared (and hence all I/O should
> > be aligned) otherwise error is returned. If yes, I could make that
> > change.
> >
> > Downside of current interface is that it will clear as many blocks as
> > possible in the given range and leave starting and end blocks poisoned
> > (if it is unaligned) and not return error. That means a reader will
> > get error on these blocks again and they will have to try to clear it
> > again.
>
> Which is solved by having partial page writes always EIO on poisoned
> memory.

The problem with the above is that partial page writes can not be
guaranteed to return EIO. Poison is only detected on consumed reads,
or a periodic scrub, not writes. IFF poison detection was always
synchronous with poison creation then the above makes sense. However,
with asynchronous signaling, it's fundamentally a false security
blanket to assume even full block writes will clear poison unless a
callback to firmware is made for every block.