Re: LSF/MM/BPF 2023 IOMAP conversion status update

[Matthew Wilcox <willy@infradead.org>]

On Sat, Jan 28, 2023 at 08:46:45PM -0800, Luis Chamberlain wrote:
> I'm hoping this *might* be useful to some, but I fear it may leave quite
> a bit of folks with more questions than answers as it did for me. And
> hence I figured that *this aspect of this topic* perhaps might be a good
> topic for LSF.  The end goal would hopefully then be finally enabling us
> to document IOMAP API properly and helping with the whole conversion
> effort.

+1 from me.

I've made a couple of abortive efforts to try and convert a "trivial"
filesystem like ext2/ufs/sysv/jfs to iomap, and I always get hung up on
what the semantics are for get_block_t and iomap_begin().

> Perhaps fs/buffers.c could be converted to folios only, and be done
> with it. But would we be loosing out on something? What would that be?

buffer_heads are inefficient for multi-page folios because some of the
algorthims are O(n^2) for n being the number of buffers in a folio.
It's fine for 8x 512b buffers in a 4k page, but for 512x 4kb buffers in
a 2MB folio, it's pretty sticky.  Things like "Read I/O has completed on
this buffer, can I mark the folio as Uptodate now?"  For iomap, that's a
scan of a 64 byte bitmap up to 512 times; for BHs, it's a loop over 512
allocations, looking at one bit in each BH before moving on to the next.
Similarly for writeback, iirc.

So +1 from me for a "How do we convert 35-ish block based filesystems
from BHs to iomap for their buffered & direct IO paths".  There's maybe a
separate discussion to be had for "What should the API be for filesystems
to access metadata on the block device" because I don't believe the
page-cache based APIs are easy for fs authors to use.

Maybe some related topics are
"What testing should we require for some of these ancient filesystems?"
"Whose job is it to convert these 35 filesystems anyway, can we just
delete some of them?"
"Is there a lower-performance but easier-to-implement API than iomap
for old filesystems that only exist for compatibiity reasons?"