Re: [RFC] Merge PG_private_2 and PG_mappedtodisk

[David Hildenbrand <david@redhat.com>]

On 14.08.24 19:36, Matthew Wilcox wrote:
> I believe these two flags have entirely disjoint uses and there will
> be no confusion in amalgamating them.
> 
> Anonymous memory (re)uses mappedtodisk for anon_exclusive.
> Anonymous memory does not use PG_private_2.

Also not when they are in the swapcache, right?

> 
> $ git grep -El '(folio.*mappedtodisk)|PageMappedToDisk'
> fs/buffer.c
> fs/ext4/readpage.c
> fs/f2fs/data.c
> fs/f2fs/file.c
> fs/fuse/dev.c
> fs/mpage.c
> fs/nilfs2/file.c
> fs/nilfs2/page.c
> include/trace/events/pagemap.h
> mm/memory.c
> mm/migrate.c
> mm/truncate.c
> 
> $ git grep -El '(folio.*private_2)|PagePrivate2'
> fs/btrfs/ctree.h
> fs/ceph/addr.c
> fs/netfs/buffered_read.c
> fs/netfs/fscache_io.c
> fs/netfs/io.c
> fs/nfs/file.c
> fs/nfs/fscache.h
> fs/nfs/write.c
> include/linux/netfs.h
> include/linux/pagemap.h
> mm/filemap.c
> mm/migrate.c
> 
> The one thing that's going to stand in the way of this is that various
> parts of the VFS treat private_2 as a "wait for this bit to be clear",
> due to its use in fscache (which is going away).
> 
> So my approach here is going to be:
> 
>   - Rename mappedtodisk to be PG_owner_priv_2 (add appropriate aliases)
>   - Switch btrfs to use owner_priv_2 instead of private_2
>   - Wait for the fscache use of private2 to finish its deprecation cycle
>   - Remove private_2 entirely
> 
> Sound good?

Yes, one step into the right direction.

-- 
Cheers,

David / dhildenb