Re: [PATCH] mm: gfp_to_alloc_flags()

Andrew Morton
2009-02-23T23:01:19Z

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?