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SubjectRe: [PATCH] mm/page_alloc: don't reserve ZONE_HIGHMEM for ZONE_MOVABLE request
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On 08/24/2017 07:45 AM, js1304@gmail.com wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@lge.com>
>
> Freepage on ZONE_HIGHMEM doesn't work for kernel memory so it's not that
> important to reserve. When ZONE_MOVABLE is used, this problem would
> theorectically cause to decrease usable memory for GFP_HIGHUSER_MOVABLE
> allocation request which is mainly used for page cache and anon page
> allocation. So, fix it.
>
> And, defining sysctl_lowmem_reserve_ratio array by MAX_NR_ZONES - 1 size
> makes code complex. For example, if there is highmem system, following
> reserve ratio is activated for *NORMAL ZONE* which would be easyily
> misleading people.
>
> #ifdef CONFIG_HIGHMEM
> 32
> #endif
>
> This patch also fix this situation by defining sysctl_lowmem_reserve_ratio
> array by MAX_NR_ZONES and place "#ifdef" to right place.
>
> Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>

Looks like I did that almost year ago, so definitely had to refresh my
memory now :)

Anyway now I looked more thoroughly and noticed that this change leaks
into the reported sysctl. On a 64bit system with ZONE_MOVABLE:

before the patch:
vm.lowmem_reserve_ratio = 256 256 32

after the patch:
vm.lowmem_reserve_ratio = 256 256 32 2147483647

So if we indeed remove HIGHMEM from protection (c.f. Michal's mail), we
should do that differently than with the INT_MAX trick, IMHO.

> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> ---
> include/linux/mmzone.h | 2 +-
> mm/page_alloc.c | 11 ++++++-----
> 2 files changed, 7 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index e7e92c8..e5f134b 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -882,7 +882,7 @@ int min_free_kbytes_sysctl_handler(struct ctl_table *, int,
> void __user *, size_t *, loff_t *);
> int watermark_scale_factor_sysctl_handler(struct ctl_table *, int,
> void __user *, size_t *, loff_t *);
> -extern int sysctl_lowmem_reserve_ratio[MAX_NR_ZONES-1];
> +extern int sysctl_lowmem_reserve_ratio[MAX_NR_ZONES];
> int lowmem_reserve_ratio_sysctl_handler(struct ctl_table *, int,
> void __user *, size_t *, loff_t *);
> int percpu_pagelist_fraction_sysctl_handler(struct ctl_table *, int,
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 90b1996..6faa53d 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -202,17 +202,18 @@ static void __free_pages_ok(struct page *page, unsigned int order);
> * TBD: should special case ZONE_DMA32 machines here - in those we normally
> * don't need any ZONE_NORMAL reservation
> */
> -int sysctl_lowmem_reserve_ratio[MAX_NR_ZONES-1] = {
> +int sysctl_lowmem_reserve_ratio[MAX_NR_ZONES] = {
> #ifdef CONFIG_ZONE_DMA
> - 256,
> + [ZONE_DMA] = 256,
> #endif
> #ifdef CONFIG_ZONE_DMA32
> - 256,
> + [ZONE_DMA32] = 256,
> #endif
> + [ZONE_NORMAL] = 32,
> #ifdef CONFIG_HIGHMEM
> - 32,
> + [ZONE_HIGHMEM] = INT_MAX,
> #endif
> - 32,
> + [ZONE_MOVABLE] = INT_MAX,
> };
>
> EXPORT_SYMBOL(totalram_pages);
>

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