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

From: Michal Hocko
Date: Fri Jun 23 2017 - 07:48:16 EST


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@xxxxxxxx>
> >> ---
> >> 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