linux-kernel - Re: [PATCH 1/3] mm, fs: get rid of PAGE_CACHE_* and page_cache

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Date:	Mon, 21 Mar 2016 13:45:22 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 1/3] mm, fs: get rid of PAGE_CACHE_* and
 page_cache_{get,release} macros

On Mon 21-03-16 15:06:36, Kirill A. Shutemov wrote:
> PAGE_CACHE_{SIZE,SHIFT,MASK,ALIGN} macros were introduced *long* time ago
> with promise that one day it will be possible to implement page cache with
> bigger chunks than PAGE_SIZE.
> 
> This promise never materialized. And unlikely will.
> 
> We have many places where PAGE_CACHE_SIZE assumed to be equal to
> PAGE_SIZE. And it's constant source of confusion on whether PAGE_CACHE_*
> or PAGE_* constant should be used in a particular case, especially on the
> border between fs and mm.
> 
> Global switching to PAGE_CACHE_SIZE != PAGE_SIZE would cause to much
> breakage to be doable.
> 
> Let's stop pretending that pages in page cache are special. They are not.
> 
> The changes are pretty straight-forward:
> 
>  - <foo> << (PAGE_CACHE_SHIFT - PAGE_SHIFT) -> <foo>;
> 
>  - <foo> >> (PAGE_CACHE_SHIFT - PAGE_SHIFT) -> <foo>;
> 
>  - PAGE_CACHE_{SIZE,SHIFT,MASK,ALIGN} -> PAGE_{SIZE,SHIFT,MASK,ALIGN};
> 
>  - page_cache_get() -> get_page();
> 
>  - page_cache_release() -> put_page();
> 
> This patch contains automated changes generated with coccinelle using
> script below. For some reason, coccinelle doesn't patch header files.
> I've called spatch for them manually.
> 
> The only adjustment after coccinelle is revert of changes to
> PAGE_CAHCE_ALIGN definition: we are going to drop it later.
> 
> There are few places in the code where coccinelle didn't reach.
> I'll fix them manually in a separate patch. Comments and documentation
> also will be addressed with the separate patch.
> 
> virtual patch
> 
> @@
> expression E;
> @@
> - E << (PAGE_CACHE_SHIFT - PAGE_SHIFT)
> + E
> 
> @@
> expression E;
> @@
> - E >> (PAGE_CACHE_SHIFT - PAGE_SHIFT)
> + E
> 
> @@
> @@
> - PAGE_CACHE_SHIFT
> + PAGE_SHIFT
> 
> @@
> @@
> - PAGE_CACHE_SIZE
> + PAGE_SIZE
> 
> @@
> @@
> - PAGE_CACHE_MASK
> + PAGE_MASK
> 
> @@
> expression E;
> @@
> - PAGE_CACHE_ALIGN(E)
> + PAGE_ALIGN(E)
> 
> @@
> expression E;
> @@
> - page_cache_get(E)
> + get_page(E)
> 
> @@
> expression E;
> @@
> - page_cache_release(E)
> + put_page(E)
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

It is great to see PAGE_CACHE_* going away finally.
It is even better that a large part of the removal could be automated.
The semantic patch makes sense to me.

Acked-by: Michal Hocko <mhocko@...e.com>

Thanks!

[...]
-- 
Michal Hocko
SUSE Labs