Re: [PATCH] libxl: introduce an option for disabling the non-O_DIRECT workaround

[George Dunlap <george.dunlap@eu.citrix.com>]

On 11/26/2013 05:46 PM, Konrad Rzeszutek Wilk wrote:
> On Tue, Nov 26, 2013 at 11:42:31AM +0000, Ian Jackson wrote:
>> From: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
>>
>> Document and implement a new option that permits disk backends which
>> would otherwise have to avoid O_DIRECT (because of the network memory
>> lifetime bug) to use it anyway.  This is:
>
> I thought this was fixed in the Linux kernel with:
>
> commit cd9151e26d31048b2b5e00fd02e110e07d2200c9
> Author: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
> Date:   Sun Aug 4 15:39:40 2013 +0100
>
>      xen/balloon: set a mapping for ballooned out pages
>
>      Currently ballooned out pages are mapped to 0 and have INVALID_P2M_ENTRY
>      in the p2m. These ballooned out pages are used to map foreign grants
>      by gntdev and blkback (see alloc_xenballooned_pages).
>
>      Allocate a page per cpu and map all the ballooned out pages to the
>      corresponding mfn. Set the p2m accordingly. This way reading from a
>      ballooned out page won't cause a kernel crash (see
>      http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html).
>
>
> ?

Not everyone builds her own kernel from the latest release; until we can 
be relatively sure that this fix has hit distros (including older 
LTS-style ones), we have to deal with the fact that the O_DIRECT bug may 
be present.  The purpose of this flag is to enable you to turn it on 
when you know it's safe -- for instance, if you're running a kernel with 
this changeset.

It is worth asking the question now though: Given that this has been 
checked in, would it make sense to switch the polarity of this -- 
default to O_DIRECT on and have a flag to allow people to switch it off? 
  If not now, maybe for 4.5?  And in that case, maybe we should change 
the argument to be tristate: default, true, or false (i.e., true means I 
know it's safe, false means I know it's unsafe, default means I haven't 
specified either way)?

  -George