Re: [RFC 02/13] mm, page_alloc: set alloc_flags only once in slowpath

Michal Hocko
2016-05-12T12:41:45Z

On Tue 10-05-16 14:30:11, Vlastimil Babka wrote:
[...]
> >From 68f09f1d4381c7451238b4575557580380d8bf30 Mon Sep 17 00:00:00 2001
> From: Vlastimil Babka <vbabka@suse.cz>
> Date: Fri, 29 Apr 2016 11:51:17 +0200
> Subject: [RFC 02/13] mm, page_alloc: set alloc_flags only once in slowpath
>
> In __alloc_pages_slowpath(), alloc_flags doesn't change after it's initialized,
> so move the initialization above the retry: label. Also make the comment above
> the initialization more descriptive.
>
> The only exception in the alloc_flags being constant is ALLOC_NO_WATERMARKS,
> which may change due to TIF_MEMDIE being set on the allocating thread. We can
> fix this, and make the code simpler and a bit more effective at the same time,
> by moving the part that determines ALLOC_NO_WATERMARKS from
> gfp_to_alloc_flags() to gfp_pfmemalloc_allowed(). This means we don't have to
> mask out ALLOC_NO_WATERMARKS in several places in __alloc_pages_slowpath()
> anymore. The only test for the flag can instead call gfp_pfmemalloc_allowed().

I like this _very_ much! gfp_to_alloc_flags was really ugly doing two
separate things and it is nice to split them up and give the whole thing
more sense. gfp_pfmemalloc_allowed() is the bright example of it.

> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> mm/page_alloc.c | 49 ++++++++++++++++++++++++-------------------------
> 1 file changed, 24 insertions(+), 25 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index a50184ec6ca0..1b58facf4b5e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -3216,8 +3216,7 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
> */
> count_vm_event(COMPACTSTALL);
>
> - page = get_page_from_freelist(gfp_mask, order,
> - alloc_flags & ~ALLOC_NO_WATERMARKS, ac);
> + page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);
>
> if (page) {
> struct zone *zone = page_zone(page);
> @@ -3366,8 +3365,7 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
> return NULL;
>
> retry:
> - page = get_page_from_freelist(gfp_mask, order,
> - alloc_flags & ~ALLOC_NO_WATERMARKS, ac);
> + page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);
>
> /*
> * If an allocation failed after direct reclaim, it could be because
> @@ -3425,16 +3423,6 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
> } else if (unlikely(rt_task(current)) && !in_interrupt())
> alloc_flags |= ALLOC_HARDER;
>
> - if (likely(!(gfp_mask & __GFP_NOMEMALLOC))) {
> - if (gfp_mask & __GFP_MEMALLOC)
> - alloc_flags |= ALLOC_NO_WATERMARKS;
> - else if (in_serving_softirq() && (current->flags & PF_MEMALLOC))
> - alloc_flags |= ALLOC_NO_WATERMARKS;
> - else if (!in_interrupt() &&
> - ((current->flags & PF_MEMALLOC) ||
> - unlikely(test_thread_flag(TIF_MEMDIE))))
> - alloc_flags |= ALLOC_NO_WATERMARKS;
> - }
> #ifdef CONFIG_CMA
> if (gfpflags_to_migratetype(gfp_mask) == MIGRATE_MOVABLE)
> alloc_flags |= ALLOC_CMA;
> @@ -3444,7 +3432,19 @@ gfp_to_alloc_flags(gfp_t gfp_mask)
>
> bool gfp_pfmemalloc_allowed(gfp_t gfp_mask)
> {
> - return !!(gfp_to_alloc_flags(gfp_mask) & ALLOC_NO_WATERMARKS);
> + if (unlikely(gfp_mask & __GFP_NOMEMALLOC))
> + return false;
> +
> + if (gfp_mask & __GFP_MEMALLOC)
> + return true;
> + if (in_serving_softirq() && (current->flags & PF_MEMALLOC))
> + return true;
> + if (!in_interrupt() &&
> + ((current->flags & PF_MEMALLOC) ||
> + unlikely(test_thread_flag(TIF_MEMDIE))))
> + return true;
> +
> + return false;
> }
>
> static inline bool is_thp_gfp_mask(gfp_t gfp_mask)
> @@ -3579,25 +3579,24 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> (__GFP_ATOMIC|__GFP_DIRECT_RECLAIM)))
> gfp_mask &= ~__GFP_ATOMIC;
>
> -retry:
> - if (gfp_mask & __GFP_KSWAPD_RECLAIM)
> - wake_all_kswapds(order, ac);
> -
> /*
> - * OK, we're below the kswapd watermark and have kicked background
> - * reclaim. Now things get more complex, so set up alloc_flags according
> - * to how we want to proceed.
> + * The fast path uses conservative alloc_flags to succeed only until
> + * kswapd needs to be woken up, and to avoid the cost of setting up
> + * alloc_flags precisely. So we do that now.
> */
> alloc_flags = gfp_to_alloc_flags(gfp_mask);
>
> +retry:
> + if (gfp_mask & __GFP_KSWAPD_RECLAIM)
> + wake_all_kswapds(order, ac);
> +
> /* This is the last chance, in general, before the goto nopage. */
> - page = get_page_from_freelist(gfp_mask, order,
> - alloc_flags & ~ALLOC_NO_WATERMARKS, ac);
> + page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);
> if (page)
> goto got_pg;
>
> /* Allocate without watermarks if the context allows */
> - if (alloc_flags & ALLOC_NO_WATERMARKS) {
> + if (gfp_pfmemalloc_allowed(gfp_mask)) {
> /*
> * Ignore mempolicies if ALLOC_NO_WATERMARKS on the grounds
> * the allocation is high priority and these type of
> --
> 2.8.2
>

--
Michal Hocko
SUSE Labs