Re: Freeing page flags

[Mike Rapoport <rppt@kernel.org>]

On Thu, May 12, 2022 at 09:54:59PM +0100, Matthew Wilcox wrote:
> The LWN writeup [1] on merging the MGLRU reminded me that I need to send
> out a plan for removing page flags that we can do without.
> 
> 4. I think I can also consolidate PG_slab and PG_reserved into a "single
> bit" (not really, but change the encoding so that effectively they only
> take a single bit).

PG_reserved could be a PageType, AFAIR no reserved pages are ever mapped to
userspace
 
> That gives us 4 bits back, which should relieve the pressure on page flag
> bits for a while.  I have Thoughts on PG_private_2 and PG_owner_priv_1,
> as well as a suspicion that not all combinations of referenced, lru,
> active, workingset, reclaim and unevictable are possible, and there
> might be scope for a better encoding.  But I don't know that we need to
> do that work; gaining back 4 bits is already a Big Deal.
> 
> I'm slowly doing the PG_private transition as part of the folio work.
> For example, eagle eyed reviewers may have spotted that there is no
> folio_has_buffers().  Converted code calls folio_buffers() and checks
> if it's NULL.  Help from filesystem maintainers on removing the uses of
> PG_error gratefully appreciated.
> 
> [1] https://lwn.net/Articles/894859/
> 

-- 
Sincerely yours,
Mike.