Re: [PATCH v3 0/5] btrfs: remove buffer heads form superblock handling

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jan 30, 2020 at 03:53:37PM +0000, Johannes Thumshirn wrote:
> On 30/01/2020 14:39, Christoph Hellwig wrote:
> > On Thu, Jan 30, 2020 at 01:15:30PM +0100, David Sterba wrote:
> >>> Sure but with hch's proposed change to using read_cache_page_gfp() this
> >>> doesn't make too much sense anymore at least for the read path.
> >>>
> >>> Maybe "use page cache for superblock reading"?
> >>
> >> That works too. We might need a new iteration that summarizes up all the
> >> feedback so far, so we have same code to refer to.
> > 
> > Per my question on the second patch:  why even use the page cache at
> > all.  btrfs already caches the value outside the pagecache, so why
> > even bother with the page cache overhead?
> > 
> This is what my first version did, alloc_page() and submit_bio() 
> directly [1]. But reviewers told me to go the route via page cache.

I only see your patch at the url, not any reply.  What is the issue
of not using the page cache?  Also you really shoudn't need a separate
alloc_page - you should be able to use the already cached superblock
as the destination and source of I/O, assuming they are properly aligned
(and if not that could be fixed easily).