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

[Vivek Goyal <vgoyal@redhat.com>]

On Mon, Feb 24, 2020 at 12:52:13PM -0800, Dan Williams wrote:
> On Mon, Feb 24, 2020 at 12:13 PM Vivek Goyal <vgoyal@redhat.com> wrote:
> >
> > On Mon, Feb 24, 2020 at 10:03:30AM +1100, Dave Chinner 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.
> > >
> > >
> > > > 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.
> >
> > Ok, how about if I add one more patch to the series which will check
> > if unwritten portion of the page has known poison. If it has, then
> > -EIO is returned.
> >
> >
> > Subject: pmem: zero page range return error if poisoned memory in unwritten area
> >
> > Filesystems call into pmem_dax_zero_page_range() to zero partial page upon
> > truncate. If partial page is being zeroed, then at the end of operation
> > file systems expect that there is no poison in the whole page (atleast
> > known poison).
> >
> > So make sure part of the partial page which is not being written, does not
> > have poison. If it does, return error. If there is poison in area of page
> > being written, it will be cleared.
> 
> No, I don't like that the zero operation is special cased compared to
> the write case. I'd say let's make them identical for now. I.e. fail
> the I/O at dax_direct_access() time.

So basically __dax_zero_page_range() will only write zeros (and not
try to clear any poison). Right?

> I think the error clearing
> interface should be an explicit / separate op rather than a
> side-effect. What about an explicit interface for initializing newly
> allocated blocks, and the only reliable way to destroy poison through
> the filesystem is to free the block?

Effectively pmem_make_request() is already that interface filesystems
use to initialize blocks and clear poison. So we don't really have to
introduce a new interface?

Or you are suggesting separate dax_zero_page_range() interface which will
always call into firmware to clear poison. And that will make sure latent
poison is cleared as well and filesystem should use that for block
initialization instead? I do like the idea of not having to differentiate
between known poison and latent poison. Once a block has been initialized
all poison should be cleared (known/latent). I am worried though that
on large devices this might slowdown filesystem initialization a lot
if they are zeroing large range of blocks.

If yes, this sounds like two different patch series. First patch series
takes care of removing blkdev_issue_zeroout() from
__dax_zero_page_range() and couple of iomap related cleans christoph
wanted.

And second patch series for adding new dax operation to zero a range
and always call info firmware to clear poison and modify filesystems
accordingly.

Thanks
Vivek
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