Re: [dm-devel] [PATCH] Fix over-zealous flush_disk when changing device size.

[NeilBrown <neilb@suse.de>]

(linux-fsdevel added - seems relevant)

On Mon, 07 Mar 2011 10:46:58 -0600 James Bottomley <James.Bottomley@suse.de>
wrote:

> On Sun, 2011-03-06 at 21:22 -0700, Andrew Patterson wrote:
> > On Sun, 2011-03-06 at 17:47 +1100, NeilBrown wrote:
> > >  Would you be uncomfortable if I asked Linus to revert both my fix and your
> > >  original patch??
> > 
> > James Bottomley wanted me to put this functionality in. I have no
> > problem with reverting it myself, especially if it is causing other
> > problems.  I would have to say that you need to ask him (or rather, I am
> > not qualified to render an opinion here).
> 
> So it seems we have a couple of problems: the first being that
> flush_disk() doesn't actually do what it says (flush the disk).  If it's
> just discarding all data, dirty or clean, then its use in the
> grow/shrink interface is definitely wrong.
> 
> The idea is that before we complete the grow/shrink, we make sure that
> the device doesn't have any errors, so we want to try to write out all
> dirty buffers to make sure they still have a home.  If flush_disk()
> doesn't do that, then we need to use a different interface ... what's
> the interface to flush a disk?
> 

Hi James,

I *always* want to make sure that my device doesn't have any errors, not just
when it changes size... but I'm not sure that regularly flushing out data is
the right way to do it.
But maybe I still misunderstand what the real point of this is.

As for the correct interface to flush a disk - there isn't one.
One doesn't flush a storage device, one flushes a cache - to a storage
device.  And there is not a 1-to-1 mapping.

A block device *is* associated with one cache - one which is used for
caching accesses through /dev/XXX and also by some filesystems to cache some
metadata.  You can flush this cache with sync_blockdev().  This just
flushes out dirty pages in that cache, it doesn't discard clean pages.
invalidate_bdev() can discard clean pages.  Call both, and get both outcomes.

If a filesystem is mounted directly on a given block_device, then it
should have a valid s_bdev pointer and it is possible to find that filesystem
from the block_device using get_active_super().  You could then call
sync_filesystem() to flush out dirty data.  There isn't really a good
interface to discard clean data. shrink_dcache_sb() does some of it, other
bits of code do other bits.

Note that a block_device also can have a pointer to a super_block (bd_super).
This does not seem to be widely used .... ext3 and ext4 use it so that memory
pressure felt by the block-device cache can transmitted to the fs, so the
fs can associate private data with the block device's cache I guess.
I don't think bd_super is sufficiently persistent reference to be usable
for sync_filesystems (it could race with unmount).


But if the block device is within a dm or md device, or is an external
journal for e.g. ext3, or is in some more complex relationship with a
filesystem, then there is no way to request that any particular cache get
flushed to the block device.  And if some user-space app has cached data
for the device .... there is no way to get at that either.

If we wanted a "proper" solution for this we probably need to leverage
the 'bd_claim' concept.  When a device is claimed, allow an 'operations'
structure to be provided with operations like:
   flush_caches  (writes out data)
   purge_caches  (discards clean data)
   discard_caches  (discards all data)
   prepare_resize (is allowed to fail)
   commit_resize
   freeze_io
   thaw_io

But flush_disk is definitely a wrong interface.  It has a meaningless name
(as discussed above), and purges some caches while discarding others.

What do we *really* lose if we just revert that original patch?

Thanks,
NeilBrown