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

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Date:   Wed, 10 Feb 2021 13:17:33 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Oscar Salvador <osalvador@...e.de>,
        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>
Subject: Re: [PATCH] mm: remove lru_add_drain_all in alloc_contig_range

On 2/9/21 8:03 PM, Oscar Salvador wrote:
> On Tue, Feb 09, 2021 at 07:17:59PM +0100, David Hildenbrand wrote:
>> 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.

AFAICS, at the time page migration used putback_lru_page() to release the
migration source page. This would put the page on lru pvec even if it was in
fact not mapped anywhere anymore, and only the drain would actually free it.
Seems Minchan optimized this in 2016 by c6c919eb90e0 ("mm: use put_page() to
free page instead of putback_lru_page()")

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