From: George Dunlap <george.dunlap@citrix.com>
To: Jan Beulich <jbeulich@suse.com>,
Stewart Hildebrand <stewart.hildebrand@dornerworks.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
Julien Grall <julien@xen.org>, Wei Liu <wl@xen.org>,
Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
George Dunlap <George.Dunlap@eu.citrix.com>,
Andrew Cooper <andrew.cooper3@citrix.com>,
Ian Jackson <ian.jackson@eu.citrix.com>,
Jeff Kubascik <jeff.kubascik@dornerworks.com>,
xen-devel@lists.xenproject.org,
David Woodhouse <dwmw2@infradead.org>
Subject: Re: [Xen-devel] [XEN PATCH v2 1/2] Check zone before merging adjacent blocks in heap
Date: Tue, 4 Feb 2020 15:37:04 +0000 [thread overview]
Message-ID: <428eb589-ec3d-1527-02ce-92e2d4367a1a@citrix.com> (raw)
In-Reply-To: <89f9cdc5-bd93-7c15-7832-740db273aca6@suse.com>
On 2/4/20 3:22 PM, Jan Beulich wrote:
> On 04.02.2020 16:14, Stewart Hildebrand wrote:
>> From: Jeff Kubascik <jeff.kubascik@dornerworks.com>
>>
>> The Xen heap is split up into nodes and zones. Each node + zone is
>> managed as a separate pool of memory.
>>
>> When returning pages to the heap, free_heap_pages will check adjacent
>> blocks to see if they can be combined into a larger block. However, the
>> zone of the adjacent block is not checked. This results in blocks that
>> migrate from one zone to another.
>>
>> When a block migrates to the adjacent zone, the avail counters for the
>> old and new node + zone is not updated accordingly. The avail counter
>> is used when allocating pages to determine whether to skip over a zone.
>> With this behavior, it is possible for free pages to collect in a zone
>> with the avail counter smaller than the actual page count, resulting
>> in free pages that are not allocable.
>
> "When a block migrates" - fine. But is this situation possible to
> occur, without "xen/page_alloc: Keep away MFN 0 from the buddy
> allocator" reverted? If not, there's no bug, no need for a change,
> and even less so ...
>
>> --- a/xen/common/page_alloc.c
>> +++ b/xen/common/page_alloc.c
>> @@ -1462,6 +1462,7 @@ static void free_heap_pages(
>> if ( !mfn_valid(page_to_mfn(predecessor)) ||
>> !page_state_is(predecessor, free) ||
>> (PFN_ORDER(predecessor) != order) ||
>> + (page_to_zone(predecessor) != zone) ||
>> (phys_to_nid(page_to_maddr(predecessor)) != node) )
>> break;
>>
>> @@ -1485,6 +1486,7 @@ static void free_heap_pages(
>> if ( !mfn_valid(page_to_mfn(successor)) ||
>> !page_state_is(successor, free) ||
>> (PFN_ORDER(successor) != order) ||
>> + (page_to_zone(successor) != zone) ||
>> (phys_to_nid(page_to_maddr(successor)) != node) )
>> break;
>
> ... for one that slows down many free operations, even if just
> slightly. IOW afaict either the change is not needed, or its
> description needs updating.
At very least it's more robust this way; the algorithm is also less
"magic". We just had a long discussion this morning trying to re-create
the logic for why "Remove MFN 0" was sufficient to prevent this issue,
and even then David wasn't sure it was correct at first.
-George
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next prev parent reply other threads:[~2020-02-04 15:37 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-04 15:14 [Xen-devel] [XEN PATCH v2 1/2] Check zone before merging adjacent blocks in heap Stewart Hildebrand
2020-02-04 15:14 ` [Xen-devel] [DO NOT APPLY XEN PATCH v2 2/2] Test case for buddy allocator merging issue Stewart Hildebrand
2020-02-04 15:22 ` [Xen-devel] [XEN PATCH v2 1/2] Check zone before merging adjacent blocks in heap Jan Beulich
2020-02-04 15:37 ` George Dunlap [this message]
2020-02-05 9:50 ` David Woodhouse
2020-02-05 10:02 ` Jan Beulich
2020-02-05 10:24 ` David Woodhouse
2020-02-05 10:49 ` Jan Beulich
2020-02-05 11:23 ` David Woodhouse
2020-02-05 13:37 ` Jan Beulich
2020-02-05 14:12 ` David Woodhouse
2020-02-07 15:49 ` David Woodhouse
2020-02-07 15:57 ` [Xen-devel] [PATCH 1/2] xen/mm: fold PGC_broken into PGC_state bits David Woodhouse
2020-02-07 20:27 ` Julien Grall
2020-02-09 13:22 ` David Woodhouse
2020-02-09 17:59 ` Julien Grall
2020-03-17 21:39 ` David Woodhouse
2020-02-20 11:10 ` Jan Beulich
2020-03-17 21:52 ` David Woodhouse
2020-03-18 9:56 ` Jan Beulich
2020-03-18 12:31 ` Julien Grall
2020-03-18 13:23 ` Jan Beulich
2020-03-18 17:13 ` David Woodhouse
2020-03-19 8:49 ` Jan Beulich
2020-03-19 10:26 ` David Woodhouse
2020-03-19 11:59 ` Jan Beulich
2020-03-19 13:54 ` David Woodhouse
2020-03-19 14:46 ` Jan Beulich
2020-02-07 15:57 ` [Xen-devel] [PATCH 2/2] xen/mm: Introduce PG_state_uninitialised David Woodhouse
2020-02-07 16:30 ` Xia, Hongyan
2020-02-07 16:32 ` David Woodhouse
2020-02-07 16:40 ` Xia, Hongyan
2020-02-07 17:06 ` David Woodhouse
2020-02-07 18:04 ` David Woodhouse
2020-02-20 11:59 ` Jan Beulich
2020-02-20 13:27 ` Julien Grall
2020-03-17 22:15 ` David Woodhouse
2020-03-18 8:53 ` Paul Durrant
2020-03-18 10:10 ` Jan Beulich
2020-03-18 10:41 ` Paul Durrant
2020-03-18 11:12 ` Jan Beulich
2020-03-18 10:03 ` Jan Beulich
2020-03-18 12:11 ` David Woodhouse
2020-03-18 13:27 ` Jan Beulich
2020-02-05 10:22 ` [Xen-devel] [XEN PATCH v2 1/2] Check zone before merging adjacent blocks in heap Julien Grall
2020-02-05 10:32 ` David Woodhouse
2020-02-05 11:36 ` David Woodhouse
2020-02-04 15:37 ` Stewart Hildebrand
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