Re: [Xen-devel] [PATCH 1/2] PV hugepages - Xen patch

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 8/10/08 18:05, "Dave McCracken" <dcm@xxxxxxxx> wrote:

> On Friday 03 October 2008, Keir Fraser wrote:
>> Some issues:
>>  * You need to check return value of get_page_from_pagenr() on every page
>> of the superpage. Any one of them can fail, causing you to undo your work
>> so far and then fail.
>>  * You need to get_page_type(PGT_writable) on every page if the superpage
>> mapping asserts _PAGE_RW. Otherwise the guest is getting write access
>> without that being asserted in the reference counts.
>>  * Look at get_page_from_l1e() for an example of how this is done for a
>> single page. You need to do similar work for every page of the super-page.
> 
> Ok, here's a version of the patch with all these issues addressed.
> 
>>  * This surely breaks save/restore, since the restore code is not
>> superpage-aware.
> 
> I don't have this one solved yet.  I'm working on it.

Actually this is an interesting one. For a PV guest it may be in general
unsolvable, since the target machine may not have allocatable 2MB extents.
It may also screw live migration since 2MB is a very coarse granularity to
do dirty-page tracking. One option: perhaps the PV kernel could shatter and
then reconstruct (as best it can) superpage mappings across save/restore?
I'm actually not sure what's for the best here. Perhaps just make 2MB
mappings and save/restore mutually exclusive for now?

 -- Keir



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