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From: Michal Hocko <mhocko@suse.com>
To: Oscar Salvador <osalvador@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	David Hildenbrand <david@redhat.com>,
	Muchun Song <songmuchun@bytedance.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v3 1/2] mm: Make alloc_contig_range handle free hugetlb pages
Date: Fri, 26 Feb 2021 09:38:35 +0100	[thread overview]
Message-ID: <YDizi3/TKYnSi5Zo@dhcp22.suse.cz> (raw)
In-Reply-To: <YDiyvQ2SCXxCjPJ2@dhcp22.suse.cz>

On Fri 26-02-21 09:35:10, Michal Hocko wrote:
> On Mon 22-02-21 14:51:36, Oscar Salvador wrote:
> > alloc_contig_range will fail if it ever sees a HugeTLB page within the
> > range we are trying to allocate, even when that page is free and can be
> > easily reallocated.
> > This has proved to be problematic for some users of alloc_contic_range,
> > e.g: CMA and virtio-mem, where those would fail the call even when those
> > pages lay in ZONE_MOVABLE and are free.
> > 
> > We can do better by trying to replace such page.
> > 
> > Free hugepages are tricky to handle so as to no userspace application
> > notices disruption, we need to replace the current free hugepage with
> > a new one.
> > 
> > In order to do that, a new function called alloc_and_dissolve_huge_page
> > is introduced.
> > This function will first try to get a new fresh hugepage, and if it
> > succeeds, it will replace the old one in the free hugepage pool.
> > 
> > All operations are being handled under hugetlb_lock, so no races are
> > possible. The only exception is when page's refcount is 0, but it still
> > has not been flagged as PageHugeFreed.
> 
> I think it would be helpful to call out that specific case explicitly
> here. I can see only one scenario (are there more?)
> __free_huge_page()		isolate_or_dissolve_huge_page
> 				  PageHuge() == T
> 				  alloc_and_dissolve_huge_page
> 				    alloc_fresh_huge_page()
> 				    spin_lock(hugetlb_lock)
> 				    // PageHuge() && !PageHugeFreed &&
> 				    // !PageCount()
> 				    spin_unlock(hugetlb_lock)
>   spin_lock(hugetlb_lock)
>   1) update_and_free_page
>        PageHuge() == F
>        __free_pages()
>   2) enqueue_huge_page
>        SetPageHugeFreed()
>   spin_unlock(&hugetlb_lock)			    
> 
> > In this case we retry as the window race is quite small and we have high
> > chances to succeed next time.
> > 
> > With regard to the allocation, we restrict it to the node the page belongs
> > to with __GFP_THISNODE, meaning we do not fallback on other node's zones.
> > 
> > Note that gigantic hugetlb pages are fenced off since there is a cyclic
> > dependency between them and alloc_contig_range.
> > 
> > Signed-off-by: Oscar Salvador <osalvador@suse.de>
> 
> Thanks this looks much better than the initial version. One nit below.
> Acked-by: Michal Hocko <mhocko@suse.com>

Btw. if David has some numbers it would be great to add them to the
changelog.
-- 
Michal Hocko
SUSE Labs


  reply	other threads:[~2021-02-26  8:38 UTC|newest]

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-22 13:51 [PATCH v3 0/2] Make alloc_contig_range handle Hugetlb pages Oscar Salvador
2021-02-22 13:51 ` [PATCH v3 1/2] mm: Make alloc_contig_range handle free hugetlb pages Oscar Salvador
2021-02-25 20:03   ` Mike Kravetz
2021-02-26  9:48     ` Oscar Salvador
2021-02-26  8:35   ` Michal Hocko
2021-02-26  8:38     ` Michal Hocko [this message]
2021-02-26  9:25       ` David Hildenbrand
2021-02-26  9:47         ` Oscar Salvador
2021-02-26  9:45     ` Oscar Salvador
2021-02-26  9:51       ` Michal Hocko
2021-03-01 14:09   ` David Hildenbrand
2021-03-04 10:19     ` Oscar Salvador
2021-03-04 10:32       ` David Hildenbrand
2021-03-04 10:41         ` Oscar Salvador
2021-02-22 13:51 ` [PATCH v3 2/2] mm: Make alloc_contig_range handle in-use " Oscar Salvador
2021-02-25 23:05   ` Mike Kravetz
2021-02-26  8:46   ` Michal Hocko
2021-02-26 10:24     ` Oscar Salvador
2021-02-26 10:27       ` Oscar Salvador
2021-02-26 12:46       ` Michal Hocko
2021-02-28 13:43         ` Oscar Salvador
2021-03-05 17:30   ` David Hildenbrand
2021-03-01 12:43 ` [PATCH v3 0/2] Make alloc_contig_range handle Hugetlb pages David Hildenbrand
2021-03-01 12:57   ` Oscar Salvador
2021-03-01 12:59     ` David Hildenbrand

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