Re: [LSF/MM TOPIC] Badblocks checking/representation in filesystems

["Verma, Vishal L" <vishal.l.verma@intel.com>]

On Tue, 2017-01-17 at 17:58 -0800, Andiry Xu wrote:
> On Tue, Jan 17, 2017 at 3:51 PM, Vishal Verma <vishal.l.verma@intel.co
> m> wrote:
> > On 01/17, Andiry Xu wrote:
> > 
> > <snip>
> > 
> > > > > 
> > > > > The pmem_do_bvec() read logic is like this:
> > > > > 
> > > > > pmem_do_bvec()
> > > > >     if (is_bad_pmem())
> > > > >         return -EIO;
> > > > >     else
> > > > >         memcpy_from_pmem();
> > > > > 
> > > > > Note memcpy_from_pmem() is calling memcpy_mcsafe(). Does this
> > > > > imply
> > > > > that even if a block is not in the badblock list, it still can
> > > > > be bad
> > > > > and causes MCE? Does the badblock list get changed during file
> > > > > system
> > > > > running? If that is the case, should the file system get a
> > > > > notification when it gets changed? If a block is good when I
> > > > > first
> > > > > read it, can I still trust it to be good for the second
> > > > > access?
> > > > 
> > > > Yes, if a block is not in the badblocks list, it can still cause
> > > > an
> > > > MCE. This is the latent error case I described above. For a
> > > > simple read()
> > > > via the pmem driver, this will get handled by memcpy_mcsafe. For
> > > > mmap,
> > > > an MCE is inevitable.
> > > > 
> > > > Yes the badblocks list may change while a filesystem is running.
> > > > The RFC
> > > > patches[1] I linked to add a notification for the filesystem
> > > > when this
> > > > happens.
> > > > 
> > > 
> > > This is really bad and it makes file system implementation much
> > > more
> > > complicated. And badblock notification does not help very much,
> > > because any block can be bad potentially, no matter it is in
> > > badblock
> > > list or not. And file system has to perform checking for every
> > > read,
> > > using memcpy_mcsafe. This is disaster for file system like NOVA,
> > > which
> > > uses pointer de-reference to access data structures on pmem. Now
> > > if I
> > > want to read a field in an inode on pmem, I have to copy it to
> > > DRAM
> > > first and make sure memcpy_mcsafe() does not report anything
> > > wrong.
> > 
> > You have a good point, and I don't know if I have an answer for
> > this..
> > Assuming a system with MCE recovery, maybe NOVA can add a mce
> > handler
> > similar to nfit_handle_mce(), and handle errors as they happen, but
> > I'm
> > being very hand-wavey here and don't know how much/how well that
> > might
> > work..
> > 
> > > 
> > > > No, if the media, for some reason, 'dvelops' a bad cell, a
> > > > second
> > > > consecutive read does have a chance of being bad. Once a
> > > > location has
> > > > been marked as bad, it will stay bad till the ACPI clear error
> > > > 'DSM' has
> > > > been called to mark it as clean.
> > > > 
> > > 
> > > I wonder what happens to write in this case? If a block is bad but
> > > not
> > > reported in badblock list. Now I write to it without reading
> > > first. Do
> > > I clear the poison with the write? Or still require a ACPI DSM?
> > 
> > With writes, my understanding is there is still a possibility that
> > an
> > internal read-modify-write can happen, and cause a MCE (this is the
> > same
> > as writing to a bad DRAM cell, which can also cause an MCE). You
> > can't
> > really use the ACPI DSM preemptively because you don't know whether
> > the
> > location was bad. The error flow will be something like write causes
> > the
> > MCE, a badblock gets added (either through the mce handler or after
> > the
> > next reboot), and the recovery path is now the same as a regular
> > badblock.
> > 
> 
> This is different from my understanding. Right now write_pmem() in
> pmem_do_bvec() does not use memcpy_mcsafe(). If the block is bad it
> clears poison and writes to pmem again. Seems to me writing to bad
> blocks does not cause MCE. Do we need memcpy_mcsafe for pmem stores?

You are right, writes don't use memcpy_mcsafe, and will not directly
cause an MCE. However a write can cause an asynchronous 'CMCI' -
corrected machine check interrupt, but this is not critical, and wont be
a memory error as the core didn't consume poison. memcpy_mcsafe cannot
protect against this because the write is 'posted' and the CMCI is not
synchronous. Note that this is only in the latent error or memmap-store
case.

> 
> Thanks,
> Andiry
> 
> > > 
> > > > [1]: http://www.linux.sgi.com/archives/xfs/2016-06/msg00299.html
> > > > 
> > > 
> > > Thank you for the patchset. I will look into it.
> > >