Re: [PATCH] mm: gfp_to_alloc_flags()

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 23 Feb 2009 12:55:03 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> +static int gfp_to_alloc_flags(gfp_t gfp_mask)
> +{
> +	struct task_struct *p = current;
> +	int alloc_flags = ALLOC_WMARK_MIN | ALLOC_CPUSET;
> +	const gfp_t wait = gfp_mask & __GFP_WAIT;
> +
> +	/*
> +	 * The caller may dip into page reserves a bit more if the caller
> +	 * cannot run direct reclaim, or if the caller has realtime scheduling
> +	 * policy or is asking for __GFP_HIGH memory.  GFP_ATOMIC requests will
> +	 * set both ALLOC_HARDER (!wait) and ALLOC_HIGH (__GFP_HIGH).
> +	 */
> +	if (gfp_mask & __GFP_HIGH)
> +		alloc_flags |= ALLOC_HIGH;

This could be sped up by making ALLOC_HIGH==__GFP_HIGH (hack)

> +	if (!wait) {
> +		alloc_flags |= ALLOC_HARDER;
> +		/*
> +		 * Ignore cpuset if GFP_ATOMIC (!wait) rather than fail alloc.
> +		 * See also cpuset_zone_allowed() comment in kernel/cpuset.c.
> +		 */
> +		alloc_flags &= ~ALLOC_CPUSET;
> +	} else if (unlikely(rt_task(p)) && !in_interrupt())
> +		alloc_flags |= ALLOC_HARDER;
> +
> +	if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
> +		if (!in_interrupt() &&
> +		    ((p->flags & PF_MEMALLOC) ||
> +		     unlikely(test_thread_flag(TIF_MEMDIE))))
> +			alloc_flags |= ALLOC_NO_WATERMARKS;
> +	}
> +	return alloc_flags;
> +}


But really, the whole function can be elided on the fastpath.  Try the
allocation with the current flags (and __GFP_NOWARN) and only if it
failed will we try altering the flags to try harder?