Re: [PATCH v2] mm: compaction: optimize compact_memory to comply with the admin-guide

[Mel Gorman]

On Tue, Apr 25, 2023 at 11:52:35PM +0800, wenyang.linux@xxxxxxxxxxx wrote:
> From: Wen Yang <wenyang.linux@xxxxxxxxxxx>
> 
> For the /proc/sys/vm/compact_memory file, the admin-guide states:
> When 1 is written to the file, all zones are compacted such that free
> memory is available in contiguous blocks where possible. This can be
> important for example in the allocation of huge pages although processes
> will also directly compact memory as required
> 
> But it was not strictly followed, writing any value would cause all
> zones to be compacted.
> It has been slightly optimized to comply with the admin-guide.
> Enforce the 1 on the unlikely chance that the sysctl handler is ever
> extended to do something different.
> 
> Commit ef4984384172 ("mm/compaction: remove unused variable sysctl_compact_memory")
> has also been optimized a bit here, as the declaration in the external header
> file has been eliminated, and sysctl_compact_memory also needs to be verified.
> 
> Signed-off-by: Wen Yang <wenyang.linux@xxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Cc: Oscar Salvador <osalvador@xxxxxxx>
> Cc: William Lam <william.lam@xxxxxxxxxxxxx>
> Cc: Pintu Kumar <pintu@xxxxxxxxxxxxxx>
> Cc: Fu Wei <wefu@xxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  mm/compaction.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/compaction.c b/mm/compaction.c
> index c8bcdea15f5f..ba57d4178866 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1736,6 +1736,7 @@ static int sysctl_compact_unevictable_allowed __read_mostly = CONFIG_COMPACT_UNE
>   */
>  static unsigned int __read_mostly sysctl_compaction_proactiveness = 20;
>  static int sysctl_extfrag_threshold = 500;
> +static int sysctl_compact_memory;
>  

__read_mostly but that aside, it's only used in
sysctl_compaction_handler so could also be declared as static within
that function. That way if CONFIG_SYSCTL is not set, it should be
guaranteed that the compiler does not save storage for it.

It's minor enough that With or without another version;

Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>

-- 
Mel Gorman
SUSE Labs