Re: [PATCH v1 1/5] mm/shmem: support deterministic charging of tmpfs

[Matthew Wilcox <willy-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

On Tue, Nov 09, 2021 at 09:10:47AM +1100, Dave Chinner wrote:
> > +	rcu_read_lock();
> > +	memcg = rcu_dereference(mapping->host->i_sb->s_memcg_to_charge);
> 
> Anything doing pointer chasing to obtain static, unchanging
> superblock state is poorly implemented. The s_memcg_to_charge value never
> changes, so this code should associate the memcg to charge directly
> on the mapping when the mapping is first initialised by the
> filesystem. We already do this with things like attaching address
> space ops and mapping specific gfp masks (i.e
> mapping_set_gfp_mask()), so this association should be set up that
> way, too (e.g. mapping_set_memcg_to_charge()).

I'm not a fan of enlarging struct address_space with another pointer
unless it's going to be used by all/most filesystems.  If this is
destined to be a shmem-only feature, then it should be in the
shmem_inode instead of the mapping.

If we are to have this for all filesystems, then let's do that properly
and make it generic functionality from its introduction.