linux-kernel - Re: [PATCH] mm: Refactor conversion of pages to bytes macro definitions

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Date:   Fri, 23 Jun 2017 13:48:07 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Nikolay Borisov <nborisov@...e.com>
Cc:     linux-mm@...ck.org, mgorman@...hsingularity.net,
        cmetcalf@...lanox.com, minchan@...nel.org, vbabka@...e.cz,
        kirill.shutemov@...ux.intel.com, tj@...nel.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Refactor conversion of pages to bytes macro
 definitions

On Thu 22-06-17 09:58:17, Nikolay Borisov wrote:
> 
> 
> On 22.06.2017 09:44, Michal Hocko wrote:
> > On Tue 20-06-17 18:14:28, Nikolay Borisov wrote:
> >> Currently there are a multiple files with the following code:
> >>  #define K(x) ((x) << (PAGE_SHIFT - 10))
> >>  ... some code..
> >>  #undef K
> >>
> >> This is mainly used to print out some memory-related statistics, where X is
> >> given in pages and the macro just converts it to kilobytes. In the future
> >> there is going to be more macros since there are intention to introduce
> >> byte-based memory counters [1]. This could lead to proliferation of
> >> multiple duplicated definition of various macros used to convert a quantity
> >> from one unit to another. Let's try and consolidate such definition in the
> >> mm.h header since currently it's being included in all files which exhibit
> >> this pattern. Also let's rename it to something a bit more verbose.
> >>
> >> This patch doesn't introduce any functional changes
> >>
> >> [1] https://patchwork.kernel.org/patch/9395205/
> >>
> >> Signed-off-by: Nikolay Borisov <nborisov@...e.com>
> >> ---
> >>  arch/tile/mm/pgtable.c      |  2 --
> >>  drivers/base/node.c         | 66 ++++++++++++++++++-------------------
> >>  include/linux/mm.h          |  2 ++
> >>  kernel/debug/kdb/kdb_main.c |  3 +-
> >>  mm/backing-dev.c            | 22 +++++--------
> >>  mm/memcontrol.c             | 17 +++++-----
> >>  mm/oom_kill.c               | 19 +++++------
> >>  mm/page_alloc.c             | 80 ++++++++++++++++++++++-----------------------
> >>  8 files changed, 100 insertions(+), 111 deletions(-)
> > 
> > Those macros are quite trivial and we do not really save much code while
> > this touches a lot of code potentially causing some conflicts. So do we
> > really need this? I am usually very keen on removing duplication but
> > this doesn't seem to be worth all the troubles IMHO.
> > 
> 
> There are 2 problems I see: 
> 
> 1. K is in fact used for other macros than converting pages to kbytes. 
> Simple grep before my patch is applied yields the following: 
> 
> arch/tile/mm/pgtable.c:#define K(x) ((x) << (PAGE_SHIFT-10))
> arch/x86/crypto/serpent-sse2-i586-asm_32.S:#define K(x0, x1, x2, x3, x4, i) \
> crypto/serpent_generic.c:#define K(x0, x1, x2, x3, i) ({                                \
> drivers/base/node.c:#define K(x) ((x) << (PAGE_SHIFT - 10))
> drivers/net/hamradio/scc.c:#define K(x) kiss->x
> include/uapi/linux/keyboard.h:#define K(t,v)            (((t)<<8)|(v))
> kernel/debug/kdb/kdb_main.c:#define K(x) ((x) << (PAGE_SHIFT - 10))
> mm/backing-dev.c:#define K(x) ((x) << (PAGE_SHIFT - 10))
> mm/backing-dev.c:#define K(pages) ((pages) << (PAGE_SHIFT - 10))
> mm/memcontrol.c:#define K(x) ((x) << (PAGE_SHIFT-10))
> mm/oom_kill.c:#define K(x) ((x) << (PAGE_SHIFT-10))
> mm/page_alloc.c:#define K(x) ((x) << (PAGE_SHIFT-10))
> 
> 
> Furthermore, I intend on sending another patchset which introduces 2 more macros:
> drivers/base/node.c:#define BtoK(x) ((x) >> 10)
> drivers/video/fbdev/intelfb/intelfb.h:#define BtoKB(x)          ((x) / 1024)
> mm/backing-dev.c:#define BtoK(x) ((x) >> 10)
> mm/page_alloc.c:#define BtoK(x) ((x) >> 10)
> 
> fs/fs-writeback.c:#define BtoP(x) ((x) >> PAGE_SHIFT)
> include/trace/events/writeback.h:#define BtoP(x) ((x) >> PAGE_SHIFT)
> mm/page_alloc.c:#define BtoP(x) ((x) >> PAGE_SHIFT)
> 
> As you can see this ends up in spreading those macros. Ideally 
> they should be in a header which is shared among all affected 
> files. This was inspired by the feedback that Tejun has given 
> here: https://patchwork.kernel.org/patch/9395205/ and I believe
> he is right. 

Fair enough, if this is part of a larger work then I would incline to do
all of them in a single series.

-- 
Michal Hocko
SUSE Labs