linux-kernel - Re: [PATCH 1/8] mm, page_alloc: Enable/disable IRQs once when freeing a list of pages

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Message-ID: <20171018101547.mjycw7zreb66jzpa@techsingularity.net>
Date:   Wed, 18 Oct 2017 11:15:47 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, Jan Kara <jack@...e.cz>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Dave Chinner <david@...morbit.com>
Subject: Re: [PATCH 1/8] mm, page_alloc: Enable/disable IRQs once when
 freeing a list of pages

On Wed, Oct 18, 2017 at 11:02:18AM +0200, Vlastimil Babka wrote:
> On 10/18/2017 09:59 AM, Mel Gorman wrote:
> > Freeing a list of pages current enables/disables IRQs for each page freed.
> > This patch splits freeing a list of pages into two operations -- preparing
> > the pages for freeing and the actual freeing. This is a tradeoff - we're
> > taking two passes of the list to free in exchange for avoiding multiple
> > enable/disable of IRQs.
> 
> There's also some overhead of storing pfn in page->private, but all that
> seems negligible compared to irq disable/enable...
> 

Exactly and it's cheaper than doing a second page to pfn lookup.

> <SNIP>
> Looks good.
> 
> > Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> 
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
> 

Thanks.

> A nit below.
> 
> > @@ -2647,11 +2663,25 @@ void free_hot_cold_page(struct page *page, bool cold)
> >  void free_hot_cold_page_list(struct list_head *list, bool cold)
> >  {
> >  	struct page *page, *next;
> > +	unsigned long flags, pfn;
> > +
> > +	/* Prepare pages for freeing */
> > +	list_for_each_entry_safe(page, next, list, lru) {
> > +		pfn = page_to_pfn(page);
> > +		if (!free_hot_cold_page_prepare(page, pfn))
> > +			list_del(&page->lru);
> > +		page->private = pfn;
> 
> We have (set_)page_private() helpers so better to use them (makes it a
> bit easier to check for all places where page->private is used to e.g.
> avoid a clash)?
> 

Agreed and it's trivial to do so

---8<---
mm, page_alloc: Enable/disable IRQs once when freeing a list of page -fix

Use page_private and set_page_private helpers.

Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
---
 mm/page_alloc.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 167e163cf733..092973014c1e 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2670,14 +2670,14 @@ void free_hot_cold_page_list(struct list_head *list, bool cold)
 		pfn = page_to_pfn(page);
 		if (!free_hot_cold_page_prepare(page, pfn))
 			list_del(&page->lru);
-		page->private = pfn;
+		set_page_private(page, pfn);
 	}
 
 	local_irq_save(flags);
 	list_for_each_entry_safe(page, next, list, lru) {
-		unsigned long pfn = page->private;
+		unsigned long pfn = page_private(page);
 
-		page->private = 0;
+		set_page_private(page, 0);
 		trace_mm_page_free_batched(page, cold);
 		free_hot_cold_page_commit(page, pfn, cold);
 	}