linux-kernel - Re: [PATCH v3 2/2] mm: Make alloc_contig

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Date:   Fri, 26 Feb 2021 11:27:38 +0100
From:   Oscar Salvador <osalvador@...e.de>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        Muchun Song <songmuchun@...edance.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] mm: Make alloc_contig_range handle in-use hugetlb
 pages

On Fri, Feb 26, 2021 at 11:24:29AM +0100, Oscar Salvador wrote:
> On Fri, Feb 26, 2021 at 09:46:57AM +0100, Michal Hocko wrote:
> > On Mon 22-02-21 14:51:37, Oscar Salvador wrote:
> > [...]
> > > @@ -2394,9 +2397,19 @@ bool isolate_or_dissolve_huge_page(struct page *page)
> > >  	 */
> > >  	if (hstate_is_gigantic(h))
> > >  		return ret;
> > > -
> > > -	if (!page_count(head) && alloc_and_dissolve_huge_page(h, head))
> > > +retry:
> > > +	if (page_count(head) && isolate_huge_page(head, list)) {
> > >  		ret = true;
> > > +	} else if (!page_count(head)) {
> > 
> > This is rather head spinning. Do we need to test page_count in the else
> > branch? Do you want to optimize for a case where the page cannot be
> > isolated because of page_huge_active?
> 
> Well, I wanted to explictly call out both cases.
> We either 1) have an in-use page and we try to issolate it or 2) we have a free
> page (count == 0).
> 
> If the page could not be dissolved due to page_huge_active, this would either
> mean that page is about to be freed, or that someone has already issolated the
> page.
> Being the former case, one could say that falling-through alloc_and_dissolve is
> ok.
> 
> But no, I did not really want to optimize anything here, just wanted to be explicit
> about what we are checking and why.

Maybe I could add a comment to make it more explicit.
 

-- 
Oscar Salvador
SUSE L3