linux-kernel - Re: [PATCH 4/6] x86, mm: make split_mem

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Date:   Thu, 28 Mar 2019 15:20:27 +0800
From:   Wei Yang <richardw.yang@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Wei Yang <richardw.yang@...ux.intel.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, dave.hansen@...ux.intel.com,
        luto@...nel.org, peterz@...radead.or
Subject: Re: [PATCH 4/6] x86, mm: make split_mem_range() more easy to read

On Sun, Mar 24, 2019 at 03:29:04PM +0100, Thomas Gleixner wrote:
> 
>+static int __meminit split_mem_range(struct map_range *mr, unsigned long start,
>+				     unsigned long end)
>+{
>+	static const struct mapinfo mapinfos[] = {
> #ifdef CONFIG_X86_64
>+		{ .mask = 1U << PG_LEVEL_1G, .size = PUD_SIZE },
> #endif
>+		{ .mask = 1U << PG_LEVEL_2M, .size = PMD_SIZE },
>+		{ .mask = 0, .size = PAGE_SIZE },
>+	};
>+	const struct mapinfo *mi;
>+	struct map_range *curmr;
>+	unsigned long addr;
>+	int idx;
>+
>+	for (idx = 0, addr = start, curmr = mr; addr < end; idx++, curmr++) {
>+		BUG_ON(idx == NR_RANGE_MR);
>+		mr_setup(curmr, addr, end);
> 
>+		/* Try map sizes top down. PAGE_SIZE will always succeed. */
>+		for (mi = mapinfos; !mr_try_map(curmr, mi); mi++);
>
>+		/* Get the start address for the next range */
>+		addr = curmr->end;
> 	}

I re-arrange the code to make split_mem_range() here easy to read.

My question is to the for loop.

For example, we have a range

       +--+---------+-----------------------+
       ^ 128M       1G                      2G
   128M - 4K

If my understanding is correct, the original behavior will split this into
three ranges:

   4K size: [128M - 4K, 128M]
   2M size: [128M, 1G]
   1G size: [1G, 2G]

While after your change, it will split this into two ranges:

   ?? size: [128M - 4K, 1G]
   2M size: [1G, 2G]

The question mark here is because you leave the page_size_mask unchanged in
this case.

Is my understanding correct? Or I missed something?

-- 
Wei Yang
Help you, Help me