Re: [PATCH] zonefs: move super block reading from page to folio

[Damien Le Moal <dlemoal@kernel.org>]

On 6/4/24 14:33, Christoph Hellwig wrote:
> On Sat, Jun 01, 2024 at 06:51:45PM +0100, Matthew Wilcox wrote:
>> On Fri, May 31, 2024 at 10:28:50AM +0900, Damien Le Moal wrote:
>>>>> This will stop working at some point.  It'll return NULL once we get
>>>>> to the memdesc future (because the memdesc will be a slab, not a folio).
>>>>
>>>> Hmmm, xfs_buf.c plays a similar trick here for sub-page buffers.  I'm
>>>> assuming that will get ported to ... whatever the memdesc future holds?
>>
>> I don't think it does, exactly?  Are you referring to kmem_to_page()?
>> That will continue to work.  You're not trying to get a folio from a
>> slab allocation; that will start to fail.
> 
> The point is that we doing block I/O on a slab allocation is heavily
> used, and zonefs does this.  If you dislike going through the folio
> we can just keep using pages in zonefs for now.
> 
> Eventually I'll get around lifting the logic to greedily build a bio
> from arbitrary kernel virtual addresses from various places in XFS
> into common code and we can convert to that.
> 
>> I think you should use read_mapping_folio() for now instead of
>> complicating zonefs.  Once there's a grand new buffer cache, switch to
>> that, but I don't think you're introducing a significant vulnerability
>> by using the block device's page cache.
> 
> Please don't add pointless uses of the bdev page cache for users that
> don't need caching.  This just creates more headaches later on to
> untangle it again.

OK. Will drop this patch then.

-- 
Damien Le Moal
Western Digital Research