linux-kernel - Re: [PATCH] Xen backend support for paged out grant targets V4.

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Message-Id: <5B5132A4-93B2-41D0-B1A6-048810565DB5@gridcentric.ca>
Date:	Mon, 17 Sep 2012 05:29:24 -0400
From:	Andres Lagar-Cavilla <andreslc@...dcentric.ca>
To:	Ian Campbell <Ian.Campbell@...rix.com>
Cc:	Andres Lagar-Cavilla <andres@...arcavilla.org>,
	xen-devel <xen-devel@...ts.xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	"David Miller" <davem@...emloft.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH] Xen backend support for paged out grant targets V4.

On Sep 17, 2012, at 4:17 AM, Ian Campbell wrote:

> (I think I forgot to hit send on this on Friday, sorry. Also
> s/xen.lists.org/lists.xen.org in the CC line…)
I'm on a roll here…

> 
> On Fri, 2012-09-14 at 15:26 +0100, Andres Lagar-Cavilla wrote:
>> Since Xen-4.2, hvm domains may have portions of their memory paged out. When a
>> foreign domain (such as dom0) attempts to map these frames, the map will
>> initially fail. The hypervisor returns a suitable errno, and kicks an
>> asynchronous page-in operation carried out by a helper. The foreign domain is
>> expected to retry the mapping operation until it eventually succeeds. The
>> foreign domain is not put to sleep because itself could be the one running the
>> pager assist (typical scenario for dom0).
>> 
>> This patch adds support for this mechanism for backend drivers using grant
>> mapping and copying operations. Specifically, this covers the blkback and
>> gntdev drivers (which map foregin grants), and the netback driver (which copies
> 
>                           foreign
> 
>> foreign grants).
>> 
>> * Add a retry method for grants that fail with GNTST_eagain (i.e. because the
>>  target foregin frame is paged out).
> 
>          foreign
> 
>> * Insert hooks with appropriate wrappers in the aforementioned drivers.
>> 
>> The retry loop is only invoked if the grant operation status is GNTST_eagain.
>> It guarantees to leave a new status code different from GNTST_eagain. Any other
>> status code results in identical code execution as before.
>> 
>> The retry loop performs 256 attempts with increasing time intervals through a
>> 32 second period. It uses msleep to yield while waiting for the next retry.
> [...]
>> Signed-off-by: Andres Lagar-Cavilla <andres@...arcavilla.org>
> 
> Acked-by: Ian Campbell <ian.campbell@...rix.com>
> 
> Since this is more about grant tables than netback this should probably
> go via Konrad rather than Dave, is that OK with you Dave?

If that is the case hopefully Konrad can deal with the two typos? Otherwise happy to re-spin the patch.
Thanks!
Andres

> 
> (there's one more typo below)
> 
>> ---
>> drivers/net/xen-netback/netback.c  |   11 ++------
>> drivers/xen/grant-table.c          |   53 ++++++++++++++++++++++++++++++++++++
>> drivers/xen/xenbus/xenbus_client.c |    6 ++--
>> include/xen/grant_table.h          |   12 ++++++++
>> 4 files changed, 70 insertions(+), 12 deletions(-)
> [...]
>> diff --git a/include/xen/grant_table.h b/include/xen/grant_table.h
>> index 11e27c3..da9386e 100644
>> --- a/include/xen/grant_table.h
>> +++ b/include/xen/grant_table.h
>> @@ -189,4 +189,16 @@ int gnttab_map_refs(struct gnttab_map_grant_ref *map_ops,
>> int gnttab_unmap_refs(struct gnttab_unmap_grant_ref *unmap_ops,
>> 		      struct page **pages, unsigned int count, bool clear_pte);
>> 
>> +/* Perform a batch of grant map/copy operations. Retry every batch slot
>> + * for which the hypervisor returns GNTST_eagain. This is typically due 
>> + * to paged out target frames.
>> + *
>> + * Will retry for 1, 2, ... 255 ms, i.e. 256 times during 32 seconds.
>> + *
>> + * Return value in each iand every status field of the batch guaranteed
> 
>                          and
> 
>> + * to not be GNTST_eagain. 
>> + */
>> +void gnttab_batch_map(struct gnttab_map_grant_ref *batch, unsigned count);
>> +void gnttab_batch_copy(struct gnttab_copy *batch, unsigned count);
>> +
>> #endif /* __ASM_GNTTAB_H__ */
> 
> 
> 
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