linux-kernel - Re: [PATCH] mm: remove lru_add_drain_all in alloc_contig

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Date:   Tue, 9 Feb 2021 20:03:32 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     David Hildenbrand <david@...hat.com>
Cc:     Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm: remove lru_add_drain_all in alloc_contig_range

On Tue, Feb 09, 2021 at 07:17:59PM +0100, David Hildenbrand wrote:
> On 09.02.21 18:50, Minchan Kim wrote:
> > __alloc_contig_migrate_range already has lru_add_drain_all call
> > via migrate_prep. It's necessary to move LRU taget pages into
> > LRU list to be able to isolated. However, lru_add_drain_all call
> > after __alloc_contig_migrate_range is called is pointless.
> > 
> > This patch removes it.
> > 
> > Signed-off-by: Minchan Kim <minchan@...nel.org>
> > ---
> >   mm/page_alloc.c | 2 --
> >   1 file changed, 2 deletions(-)
> > 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 6446778cbc6b..f8fbee73dd6d 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -8603,8 +8603,6 @@ int alloc_contig_range(unsigned long start, unsigned long end,
> >   	 * isolated thus they won't get removed from buddy.
> >   	 */
> > -	lru_add_drain_all();
> > -
> >   	order = 0;
> >   	outer_start = start;
> >   	while (!PageBuddy(pfn_to_page(outer_start))) {
> > 
> 
> I was expecting some magical reason why this is still required but I am not
> able to find a compelling one. Maybe this is really some historical
> artifact.
> 
> Let's see if other people know why this call here still exists.

I also stumbled upon this while working on adding hugetlb support for
alloc_acontig_range [1].
I have to confess I puzzled me a bit.

I saw it going back to when the function was first introduced by 

commit 041d3a8cdc18dc375a128d90bbb753949a81b1fb
Author: Michal Nazarewicz <mina86@...a86.com>
Date:   Thu Dec 29 13:09:50 2011 +0100

    mm: page_alloc: introduce alloc_contig_range()


It does not make much sense to me. At this point our pages are free, so
we do not care about LRU handling here.
But I might be missing something.

[1] https://lore.kernel.org/linux-mm/20210208103935.GA32103@linux/T/#md651fc6e73c656105179382f92f8b2d6073051d1


-- 
Oscar Salvador
SUSE L3