Re: [PATCH 2/3] xfs: use folios in the buffer cache

[Christoph Hellwig <hch@infradead.org>]

On Mon, Jan 22, 2024 at 11:05:20PM +1100, Dave Chinner wrote:
> I haven't looked at what using vmalloc means for packing the buffer
> into a bio - we currently use bio_add_page(), so does that mean we
> have to use some variant of virt_to_page() to break the vmalloc
> region up into it's backing pages to feed them to the bio? Or is
> there some helper that I'm unaware of that does it all for us
> magically?

We have a kmem_to_page helper for chuking any kind of kernel virtual
address space into pages.  xfs_rw_bdev in fs/xfs/xfs_bio_io.c uses
that for a bio, we should probably hav an async version of that
and maybe move it to the block layer instead of duplicating the
logic in various places.

> Yeah, that's kind of where I'm going with this. Large folios already
> turn off unmapped buffers, and I'd really like to get rid of that
> page straddling mess that unmapped buffers require in the buffer
> item dirty region tracking. That means we have to get rid of
> unmapped buffers....

I actually have an old series to kill unmapped buffers and use
vmalloc, but decided I'd need use folios for the fast path instead
of paying the vmalloc overhead.  I can dust it off and you can decide
if you want to pick up parts of it.