linux-kernel - Re: [PATCH v2 3/3] vsprintf: dump full information of page flags in pGp

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Date:   Mon, 01 Feb 2021 05:05:32 -0800
From:   Joe Perches <joe@...ches.com>
To:     Yafang Shao <laoar.shao@...il.com>,
        andriy.shevchenko@...ux.intel.com, david@...hat.com,
        vbabka@...e.cz, linmiaohe@...wei.com, cl@...ux.com,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, pmladek@...e.com, rostedt@...dmis.org,
        sergey.senozhatsky@...il.com, linux@...musvillemoes.dk
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] vsprintf: dump full information of page flags in
 pGp

On Mon, 2021-02-01 at 19:56 +0800, Yafang Shao wrote:
> Currently the pGp only shows the names of page flags, rather than
> the full information including section, node, zone, last cpupid and
> kasan tag. While it is not easy to parse these information manually
> because there're so many flavors. Let's interpret them in pGp as well.
> 
> - Before the patch,
> [ 6343.396602] Slab 0x000000004382e02b objects=33 used=3 fp=0x000000009ae06ffc flags=0x17ffffc0010200(slab|head)
> 
> - After the patch,
> [ 6871.296131] Slab 0x00000000c0e19a37 objects=33 used=3 fp=0x00000000c4902159 flags=0x17ffffc0010200(Node 0,Zone 2,Lastcpupid 0x1fffff,slab|head)

While debugfs is not an ABI, this format is exported in debugfs to
userspace via mm/page_owner.c read_page_owner/print_page_owner.

Does changing the output format matter to anyone?

> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
[] 
> +static
> +char *format_page_flags(char *buf, char *end, unsigned long page_flags)
> +{
> +	unsigned long flags = page_flags & ((1UL << NR_PAGEFLAGS) - 1);
> +	int size = ARRAY_SIZE(pfl);

There's no real value in used-once variables.

> +	bool separator = false;
> +	int i;
> +
> +	for (i = 0; i < size; i++) {

Use ARRAY_SIZE here instead

	for (i = 0; i < ARRAY_SIZE(pfl); i++) {

> +		if (pfl[i].width == 0)
> +			continue;
> +
> +		if (separator) {
> +			if (buf < end)
> +				*buf = ',';
> +			buf++;
> +		}
> +
> +
> +		buf = string(buf, end, pfl[i].name, *pfl[i].spec);
> +
> +		buf = number(buf, end, (page_flags >> pfl[i].shift) & pfl[i].mask,
> +			     *pfl[i].spec);
> +		separator = true;
> +	}

Style question:
Might this array be more intelligible with pointers instead of indexes?
Something like:

	struct page_flags_layout *p;

	for (p = pfl; p < pfl + ARRAY_SIZE(pfl); p++) {
		if (p->width == 0)
			continue;

		if (p > pfl) {
			if (buf < end)
				*buf = ',';
			buf++;
		}

		buf = string(buf, end, p->name, *p->spec);
		buf = number(buf, end, (page_flags >> p->shift) & p->mask, *p->spec);
	}

> +
> +	if (flags) {

Maybe:

	if (page_flags & (BIT(NR_PAGEFLAGS) - 1)) {

> +		if (buf < end)
> +			*buf = ',';
> +		buf++;
> +	}
> +
> +	return buf;
> +}
> +