Re: Why __alloc_contig_migrate_range calls migrate_prep() at first?

[Minchan Kim <minchan@kernel.org>]

On Wed, Jun 01, 2016 at 11:42:29AM +0800, Wang Sheng-Hui wrote:
> Dear,
> 
> Sorry to trouble you.
> 
> I noticed cma_alloc would turn to  __alloc_contig_migrate_range for allocating pages.
> But  __alloc_contig_migrate_range calls  migrate_prep() at first, even if the requested page
> is single and free, lru_add_drain_all still run (called by  migrate_prep())?
> 
> Image a large chunk of free contig pages for CMA, various drivers may request a single page from
> the CMA area, we'll get  lru_add_drain_all run for each page.
> 
> Should we detect if the required pages are free before migrate_prep(), or detect at least for single 
> page allocation?

That makes sense to me.

How about calling migrate_prep once migrate_pages fails in the first trial?

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 9d666df5ef95..c504c1a623d2 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6623,8 +6623,6 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
 	unsigned int tries = 0;
 	int ret = 0;
 
-	migrate_prep();
-
 	while (pfn < end || !list_empty(&cc->migratepages)) {
 		if (fatal_signal_pending(current)) {
 			ret = -EINTR;
@@ -6650,6 +6648,8 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
 
 		ret = migrate_pages(&cc->migratepages, alloc_migrate_target,
 				    NULL, 0, cc->mode, MR_CMA);
+		if (ret)
+			migrate_prep();
 	}
 	if (ret < 0) {
 		putback_movable_pages(&cc->migratepages);


> 
> ------------------
> Regards,
> Wang Sheng-HuiN‹§²æìr¸›zǧu©ž²Æ {\b­†éì¹»\x1c®&Þ–)îÆi¢žØ^n‡r¶‰šŽŠÝ¢j$½§$¢¸\x05¢¹¨­è§~Š'.)îÄÃ,yèm¶ŸÿÃ\f%Š{±šj+ƒðèž×¦j)Z†·Ÿþf¢–Ú\x1d¢{d½§$¢¸\x1e™¨¥’öœ’Šà

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