linux-kernel - Re: [PATCH v2 1/9] mm/hugetlb: remove checking hstate_is_gigantic() in return_unused_surplus

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Date:   Fri, 24 Jun 2022 16:03:42 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     Naoya Horiguchi <nao.horiguchi@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Liu Shixin <liushixin2@...wei.com>,
        Yang Shi <shy828301@...il.com>,
        Oscar Salvador <osalvador@...e.de>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        linux-kernel@...r.kernel.org, Linux-MM <linux-mm@...ck.org>
Subject: Re: [PATCH v2 1/9] mm/hugetlb: remove checking hstate_is_gigantic()
 in return_unused_surplus_pages()

On Fri, Jun 24, 2022 at 10:25:48AM +0800, Miaohe Lin wrote:
> On 2022/6/24 7:51, Naoya Horiguchi wrote:
> > From: Naoya Horiguchi <naoya.horiguchi@....com>
> > 
> > I found a weird state of 1GB hugepage pool, caused by the following
> > procedure:
> > 
> >   - run a process reserving all free 1GB hugepages,
> >   - shrink free 1GB hugepage pool to zero (i.e. writing 0 to
> >     /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages), then
> >   - kill the reserving process.
> > 
> > , then all the hugepages are free *and* surplus at the same time.
> > 
> >   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
> >   3
> >   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/free_hugepages
> >   3
> >   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/resv_hugepages
> >   0
> >   $ cat /sys/kernel/mm/hugepages/hugepages-1048576kB/surplus_hugepages
> >   3
> > 
> > This state is resolved by reserving and allocating the pages then
> > freeing them again, so this seems not to result in serious problem.
> > But it's a little surprizing (shrinking pool suddenly fails).
> > 
> > This behavior is caused by hstate_is_gigantic() check in
> > return_unused_surplus_pages(). This was introduced so long ago in 2008
> > by commit aa888a74977a ("hugetlb: support larger than MAX_ORDER"), and
> > it seems to me that this check is no longer unnecessary. Let's remove it.
> 
> s/unnecessary/necessary/
> 
> > 
> > Signed-off-by: Naoya Horiguchi <naoya.horiguchi@....com>
> > ---
> >  mm/hugetlb.c | 4 ----
> >  1 file changed, 4 deletions(-)
> > 
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index a57e1be41401..c538278170a2 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -2432,10 +2432,6 @@ static void return_unused_surplus_pages(struct hstate *h,
> >  	/* Uncommit the reservation */
> >  	h->resv_huge_pages -= unused_resv_pages;
> >  
> > -	/* Cannot return gigantic pages currently */
> > -	if (hstate_is_gigantic(h))
> > -		goto out;
> > -
> 
> IIUC it might be better to do the below check:
> 	/*
> 	 * Cannot return gigantic pages currently if runtime gigantic page
> 	 * allocation is not supported.
> 	 */
> 	if (hstate_is_gigantic(h) && !gigantic_page_runtime_supported())
> 		goto out;
>

The change looks good to me. However, the comments above is unnecessary
since gigantic_page_runtime_supported() is straightforward.

Thanks.
 
> But I might be miss something.
> 
> Thanks.
> 
> >  	/*
> >  	 * Part (or even all) of the reservation could have been backed
> >  	 * by pre-allocated pages. Only free surplus pages.
> > 
> 
>