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Re: [Xen-devel] [PATCH 2/3] x86/mem_sharing: replace use of page_lock/unlock with our own lock
- To: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 25 Apr 2019 21:01:19 +0100
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- Cc: George Dunlap <george.dunlap@xxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Thu, 25 Apr 2019 20:01:26 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 25/04/2019 20:57, Tamas K Lengyel wrote:
> On Thu, Apr 25, 2019 at 12:58 PM Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 25/04/2019 16:32, Tamas K Lengyel wrote:
>>> diff --git a/xen/include/asm-x86/mm.h b/xen/include/asm-x86/mm.h
>>> index 6faa563167..594de6148f 100644
>>> --- a/xen/include/asm-x86/mm.h
>>> +++ b/xen/include/asm-x86/mm.h
>>> @@ -133,7 +133,10 @@ struct page_info
>>> * of sharing share the version they expect to.
>>> * This list is allocated and freed when a page is shared/unshared.
>>> */
>>> - struct page_sharing_info *sharing;
>>> + struct {
>>> + struct page_sharing_info *info;
>>> + rwlock_t lock;
>>> + } sharing;
>>> };
>>>
>>> /* Reference count and various PGC_xxx flags and fields. */
>> I'm afraid this is a no-go, but for some reasons which are rather more
>> subtle that they appear here.
>>
>> There is one struct page_info per 4k frame, and you've added an extra 16
>> bytes, taking it from 32 bytes to 48 bytes.
>>
>> Other than making it a non-power-of-2 (net diffstat grow/shrink: 256/27
>> up/down: 7750/-5696 (2054) due to less efficient arithmetic[1]), the
>> framtable has a fixed virtual size (128G by default), so you've ended up
>> dropping Xen's memory limit (16TB by default) by 1/3.
> Interesting, I'm not sure I completely follow but does this mean that
> this structure is now not allowed to grow, like ever, or that I just
> need to add padding to fix up its alignment?
Basically, it needs exceedingly good reasons to grow. Even "supporting
more than 16T of RAM" wasn't a good enough reason to grow it from 32 to
64 bytes, which is why CONFIG_BIGMEM exists and is off by default.
At the moment, I don't have a good solution for your problem to suggest.
~Andrew
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