linux-kernel - Re: [Xen-devel] [PATCH v6] xen/grant-table: Avoid m2p

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Message-ID: <52E24727.6090809@citrix.com>
Date:	Fri, 24 Jan 2014 10:57:43 +0000
From:	David Vrabel <david.vrabel@...rix.com>
To:	Matt Wilson <msw@...ux.com>
CC:	Zoltan Kiss <zoltan.kiss@...rix.com>, <jonathan.davies@...rix.com>,
	<wei.liu2@...rix.com>, <ian.campbell@...rix.com>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Anthony Liguori <aliguori@...zon.com>,
	<xen-devel@...ts.xenproject.org>, Matt Wilson <msw@...zon.com>
Subject: Re: [Xen-devel] [PATCH v6] xen/grant-table: Avoid m2p_override	during
 mapping

On 24/01/14 05:48, Matt Wilson wrote:
> On Thu, Jan 23, 2014 at 09:23:44PM +0000, Zoltan Kiss wrote:
>> The grant mapping API does m2p_override unnecessarily: only gntdev needs it,
>> for blkback and future netback patches it just cause a lock contention, as
>> those pages never go to userspace. Therefore this series does the following:
>> - the original functions were renamed to __gnttab_[un]map_refs, with a new
>>   parameter m2p_override
>> - based on m2p_override either they follow the original behaviour, or just set
>>   the private flag and call set_phys_to_machine
>> - gnttab_[un]map_refs are now a wrapper to call __gnttab_[un]map_refs with
>>   m2p_override false
>> - a new function gnttab_[un]map_refs_userspace provides the old behaviour
>>
>> It also removes a stray space from page.h and change ret to 0 if
>> XENFEAT_auto_translated_physmap, as that is the only possible return value
>> there.
>>
>> v2:
>> - move the storing of the old mfn in page->index to gnttab_map_refs
>> - move the function header update to a separate patch
>>
>> v3:
>> - a new approach to retain old behaviour where it needed
>> - squash the patches into one
>>
>> v4:
>> - move out the common bits from m2p* functions, and pass pfn/mfn as parameter
>> - clear page->private before doing anything with the page, so m2p_find_override
>>   won't race with this
>>
>> v5:
>> - change return value handling in __gnttab_[un]map_refs
>> - remove a stray space in page.h
>> - add detail why ret = 0 now at some places
>>
>> v6:
>> - don't pass pfn to m2p* functions, just get it locally
>>
>> Signed-off-by: Zoltan Kiss <zoltan.kiss@...rix.com>
>> Suggested-by: David Vrabel <david.vrabel@...rix.com>
> 
> Apologies for coming in late on this thread. I'm quite behind on
> xen-devel mail that isn't CC: to me.
> 
> It seems to have been forgotten that Anthony and I proposed a similar
> change last November.
> 
> https://lkml.kernel.org/r/1384307336-5328-1-git-send-email-anthony@codemonkey.ws
> 
> Or am I misunderstanding the change?

Yes, it's equivalent.  There doesn't seem to have been a follow up patch
posted for Anthony's patch so it's not surprising that it's fallen
through the cracks.

David
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