linux-kernel - Re: [PATCH] rss_stat: Add support to detect RSS updates of external mm

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Message-ID: <20191106085959.ae2dgvmny3njnk7n@pathway.suse.cz>
Date:   Wed, 6 Nov 2019 09:59:59 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, Ioannis Ilkos <ilkos@...gle.com>,
        minchan@...gle.com, primiano@...gle.com, fmayer@...gle.com,
        hjd@...gle.com, joaodias@...gle.com, joelaf@...gle.com,
        lalitm@...gle.com, rslawik@...gle.com, sspatil@...gle.com,
        timmurray@...gle.com, Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Changbin Du <changbin.du@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Joe Perches <joe@...ches.com>,
        Kees Cook <keescook@...omium.org>, linux-mm@...ck.org,
        Michal Hocko <mhocko@...e.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] rss_stat: Add support to detect RSS updates of external
 mm

On Tue 2019-11-05 21:44:51, Joel Fernandes (Google) wrote:
> Also vsprintf.c is refactored a bit to allow reuse of hashing code.

I agree with Sergey that it would make sense to move this outside
vsprintf.c.

> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index dee8fc467fcf..401baaac1813 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -761,11 +761,34 @@ static int __init initialize_ptr_random(void)
>  early_initcall(initialize_ptr_random);
>  
>  /* Maps a pointer to a 32 bit unique identifier. */
> +int ptr_to_hashval(const void *ptr, unsigned long *hashval_out)
> +{
> +	const char *str = sizeof(ptr) == 8 ? "(____ptrval____)" : "(ptrval)";

str is unused.

> +	unsigned long hashval;

IMHO, this local variable unnecessarily complicates the code.
I would use the pointer directly and let compiler to optimize.

> +	if (static_branch_unlikely(&not_filled_random_ptr_key))
> +		return -EAGAIN;
> +
> +#ifdef CONFIG_64BIT
> +	hashval = (unsigned long)siphash_1u64((u64)ptr, &ptr_key);
> +	/*
> +	 * Mask off the first 32 bits, this makes explicit that we have
> +	 * modified the address (and 32 bits is plenty for a unique ID).
> +	 */
> +	hashval = hashval & 0xffffffff;
> +#else
> +	hashval = (unsigned long)siphash_1u32((u32)ptr, &ptr_key);
> +#endif
> +	*hashval_out = hashval;
> +	return 0;
> +}

Best Regards,
Petr