linux-kernel - Re: [PATCH v2 1/2] mm: cma: allocate cma areas bottom-up

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Date:   Sun, 20 Dec 2020 08:48:48 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Rik van Riel <riel@...riel.com>,
        Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH v2 1/2] mm: cma: allocate cma areas bottom-up

On Thu, Dec 17, 2020 at 12:12:13PM -0800, Roman Gushchin wrote:
> Currently cma areas without a fixed base are allocated close to the
> end of the node. This placement is sub-optimal because of compaction:
> it brings pages into the cma area. In particular, it can bring in hot
> executable pages, even if there is a plenty of free memory on the
> machine. This results in cma allocation failures.
> 
> Instead let's place cma areas close to the beginning of a node.
> In this case the compaction will help to free cma areas, resulting
> in better cma allocation success rates.
> 
> If there is enough memory let's try to allocate bottom-up starting
> with 4GB to exclude any possible interference with DMA32. On smaller
> machines or in a case of a failure, stick with the old behavior.
> 
> 16GB vm, 2GB cma area:
> With this patch:
> [    0.000000] Command line: root=/dev/vda3 rootflags=subvol=/root systemd.unified_cgroup_hierarchy=1 enforcing=0 console=ttyS0,115200 hugetlb_cma=2G
> [    0.002928] hugetlb_cma: reserve 2048 MiB, up to 2048 MiB per node
> [    0.002930] cma: Reserved 2048 MiB at 0x0000000100000000
> [    0.002931] hugetlb_cma: reserved 2048 MiB on node 0
> 
> Without this patch:
> [    0.000000] Command line: root=/dev/vda3 rootflags=subvol=/root systemd.unified_cgroup_hierarchy=1 enforcing=0 console=ttyS0,115200 hugetlb_cma=2G
> [    0.002930] hugetlb_cma: reserve 2048 MiB, up to 2048 MiB per node
> [    0.002933] cma: Reserved 2048 MiB at 0x00000003c0000000
> [    0.002934] hugetlb_cma: reserved 2048 MiB on node 0
> 
> v2:
>   - switched to memblock_set_bottom_up(true), by Mike
>   - start with 4GB, by Mike
> 
> Signed-off-by: Roman Gushchin <guro@...com>

With one nit below 

Reviewed-by: Mike Rapoport <rppt@...ux.ibm.com>

> ---
>  mm/cma.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
> 
> diff --git a/mm/cma.c b/mm/cma.c
> index 7f415d7cda9f..21fd40c092f0 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -337,6 +337,22 @@ int __init cma_declare_contiguous_nid(phys_addr_t base,
>  			limit = highmem_start;
>  		}
>  
> +		/*
> +		 * If there is enough memory, try a bottom-up allocation first.
> +		 * It will place the new cma area close to the start of the node
> +		 * and guarantee that the compaction is moving pages out of the
> +		 * cma area and not into it.
> +		 * Avoid using first 4GB to not interfere with constrained zones
> +		 * like DMA/DMA32.
> +		 */
> +		if (!memblock_bottom_up() &&
> +		    memblock_end >= SZ_4G + size) {

This seems short enough to fit a single line

> +			memblock_set_bottom_up(true);
> +			addr = memblock_alloc_range_nid(size, alignment, SZ_4G,
> +							limit, nid, true);
> +			memblock_set_bottom_up(false);
> +		}
> +
>  		if (!addr) {
>  			addr = memblock_alloc_range_nid(size, alignment, base,
>  					limit, nid, true);
> -- 
> 2.26.2
> 

-- 
Sincerely yours,
Mike.