linux-kernel - Re: [RFC PATCH 06/12] mm/cma: define and act on CMA

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Message-ID: <190cd3842bc0776611c73a439e49ba57c86849d1.camel@surriel.com>
Date: Tue, 16 Sep 2025 23:30:20 -0400
From: Rik van Riel <riel@...riel.com>
To: Frank van der Linden <fvdl@...gle.com>, akpm@...ux-foundation.org, 
	muchun.song@...ux.dev, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc: hannes@...xchg.org, david@...hat.com, roman.gushchin@...ux.dev
Subject: Re: [RFC PATCH 06/12] mm/cma: define and act on CMA_BALANCE flag

On Mon, 2025-09-15 at 19:51 +0000, Frank van der Linden wrote:
> 
> @@ -851,7 +862,10 @@ int __init
> cma_declare_contiguous_multi(phys_addr_t total_size,
>  	list_for_each_safe(mp, next, &ranges) {
>  		mlp = list_entry(mp, struct cma_init_memrange,
> list);
>  		list_del(mp);
> -		list_insert_sorted(&final_ranges, mlp, basecmp);
> +		if (flags & CMA_BALANCE)
> +			list_insert_sorted(&final_ranges, mlp,
> revbasecmp);
> +		else
> +			list_insert_sorted(&final_ranges, mlp,
> basecmp);
>  		sizesum += mlp->size;
>  		if (sizesum >= total_size)
>  			break;

Wait, what is this supposed to do when presented
with a mix of CMA_BALANCE and !CMA_BALANCE ranges?

I can't convince myself this would always keep
the CMA_BALANCE range near the top of the list,
and the others near the bottom.

Also, does the regular compaction code pay any
attention to the order of this list?

I have the feeling I am overlooking something big
in how this patch is supposed to work, but I am
not sure what that would be...

> @@ -866,7 +880,12 @@ int __init
> cma_declare_contiguous_multi(phys_addr_t total_size,
>  	list_for_each(mp, &final_ranges) {
>  		mlp = list_entry(mp, struct cma_init_memrange,
> list);
>  		size = min(sizeleft, mlp->size);
> -		if (memblock_reserve(mlp->base, size)) {
> +		if (flags & CMA_BALANCE)
> +			start = (mlp->base + mlp->size - size);
> +		else
> +			start = mlp->base;
> +
> +		if (memblock_reserve(start, size)) {
>  			/*
>  			 * Unexpected error. Could go on to
>  			 * the next one, but just abort to
> 

-- 
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