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

[Vlastimil Babka <vbabka@suse.cz>]

On 05/10/2016 01:28 PM, Tetsuo Handa wrote:
> Vlastimil Babka wrote:
>> 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.
> 
> Not true. gfp_to_alloc_flags() will include ALLOC_NO_WATERMARKS if current
> thread got TIF_MEMDIE after gfp_to_alloc_flags() was called for the first

Oh, right. Stupid global state.

> time. Do you want to make TIF_MEMDIE threads fail their allocations without
> using memory reserves?

No, thanks for catching this. How about the following version? I think
that's even nicer cleanup, if correct. Note it causes a conflict in
patch 03/13 but it's simple to resolve.

Thanks

----8<----
>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().

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