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Date:   Sat, 16 Oct 2021 04:28:23 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Kent Overstreet <kent.overstreet@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        "Darrick J. Wong" <djwong@kernel.org>,
        Christoph Hellwig <hch@infradead.org>,
        David Howells <dhowells@redhat.com>
Subject: Re: Folios for 5.15 request - Was: re: Folio discussion recap -
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On Wed, Sep 22, 2021 at 11:08:58AM -0400, Johannes Weiner wrote:
>       mm/memcg: Add folio_memcg() and related functions
>       mm/memcg: Convert commit_charge() to take a folio
>       mm/memcg: Convert mem_cgroup_charge() to take a folio
>       mm/memcg: Convert uncharge_page() to uncharge_folio()
>       mm/memcg: Convert mem_cgroup_uncharge() to take a folio
>       mm/memcg: Convert mem_cgroup_migrate() to take folios
>       mm/memcg: Convert mem_cgroup_track_foreign_dirty_slowpath() to folio
>       mm/memcg: Add folio_memcg_lock() and folio_memcg_unlock()
>       mm/memcg: Convert mem_cgroup_move_account() to use a folio
>       mm/memcg: Add folio_lruvec()
>       mm/memcg: Add folio_lruvec_lock() and similar functions
>       mm/memcg: Add folio_lruvec_relock_irq() and folio_lruvec_relock_irqsave()
>       mm/workingset: Convert workingset_activation to take a folio	
> 
> 		This is all anon+file stuff, not needed for filesystem
> 		folios.

No, that's not true.  A number of these functions are called from
filesystem code.  mem_cgroup_track_foreign_dirty() is only
called from filesystem code.  We at the very least need wrappers
like folio_cgroup_charge(), and folio_memcg_lock().

> 		As per the other email, no conceptual entry point for
> 		tail pages into either subsystem, so no ambiguity
> 		around the necessity of any compound_head() calls,
> 		directly or indirectly. It's easy to rule out
> 		wholesale, so there is no justification for
> 		incrementally annotating every single use of the page.

The justification is that we can remove all those hidden calls to
compound_head().  Hundreds of bytes of text spread throughout this file.

>       mm: Add folio_young and folio_idle
>       mm/swap: Add folio_activate()
>       mm/swap: Add folio_mark_accessed()
> 
> 		This is anon+file aging stuff, not needed.

Again, very much needed.  Take a look at pagecache_get_page().  In Linus'
tree today, it calls if (page_is_idle(page)) clear_page_idle(page);
So either we need wrappers (which are needlessly complicated thanks to
how page_is_idle() is defined) or we just convert it.

>       mm/rmap: Add folio_mkclean()
> 
>       mm/migrate: Add folio_migrate_mapping()
>       mm/migrate: Add folio_migrate_flags()
>       mm/migrate: Add folio_migrate_copy()
> 
> 		More anon+file conversion, not needed.

As far as I can tell, anon never calls any of these three functions.
anon calls migrate_page(), which calls migrate_page_move_mapping(),
but several filesystems do call these individual functions.

>       mm/lru: Add folio_add_lru()
> 
> 		LRU code, not needed.

Again, we need folio_add_lru() for filemap.  This one's more
tractable as a wrapper function.