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

[Jeff Moyer <jmoyer@redhat.com>]

Dan Williams <dan.j.williams@intel.com> writes:

> On Sun, Feb 23, 2020 at 3:03 PM Dave Chinner <david@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@redhat.com> writes:
>> > >
>> > > > On Thu, Feb 20, 2020 at 04:35:17PM -0500, Jeff Moyer wrote:
>> > > >> Vivek Goyal <vgoyal@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.

I believe we're discussing the case where we know there is a bad block.
Obviously we can't know about latent errors.

>> > 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.

Let's just focus on reporting errors when we know we have them.

-Jeff
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