Re: [PATCH 01/18] xfs: pass an on-disk extent to xfs_bmbt_validate_extent

[Christoph Hellwig <hch@lst.de>]

On Thu, Nov 02, 2017 at 12:54:19PM -0400, Brian Foster wrote:
> > <shrug> Same feeling here.  TBH I wonder about how costly those
> > unaligned be64 accesses are on things like sparc such that we should
> > minimize the number of calls and just check the (probably aligned)
> > incore version instead...
> > 
> 
> I'm not totally sure.. I guess it's potentially a function call where an
> immediate memory copy would suffice..? Anyways, that's pretty much what
> caused the function to stand out to me as starting to look a little too
> busy/ugly for its own good. We now have a validation helper that has to
> handle unaligned accesses because its caller may or may not refer to
> unaligned memory. It's not even totally clear to me when we can expect
> the memory to be unaligned.. when we're referring to an on-disk inode
> fork?

We are iterating over each possibly unaligned on-disk extent, so we
already are taking the hit.  In all modern CPUs unaligned access
don't cause a function call.  They either require a slightly different
load instruction or multiple small (e.g. byte) loads.  I'd really like
to see anyone who can demonstrate a difference vs reading the inode or
bmap btree from disk.

> So rather than start to propagate that logic, much more clean to me is
> to do the unaligned accesses only where necessary and fix up the error
> checks to examine the read values. I don't see any logical difference
> between checking the bit vs. the extent state since the unwritten bit
> basically maps directly to XFS_EXT_UNWRITTEN/XFS_EXT_NORM in the incore
> record.

With the new incore extent list I wanted to keep the incore extent
format private to the xfs_iext_tree.c file, and so far succeeded.

With the final version of this tree I could switch to checking
the xfs_bmbt_irec structure, which makes a whole lot sense, but I
can't think of an easy way to stage this.  Let me thing if I can come
up with something, else I'll leave this patch as-is and will change it
again in the end.