Re: [PATCH 3/4] Swapping

[Anthony Liguori <anthony-rdkfGonbjUSkNkDKm+mE6A@public.gmane.org>]

Izik Eidus wrote:
> Izik Eidus wrote:
>> Anthony Liguori wrote:
>>> Izik Eidus wrote:
>>>>
>>>> @@ -1058,8 +1038,27 @@ struct page *gfn_to_page(struct kvm *kvm, 
>>>> gfn_t gfn)
>>>>  
>>>>      gfn = unalias_gfn(kvm, gfn);
>>>>      slot = __gfn_to_memslot(kvm, gfn);
>>>> -    if (!slot)
>>>> +    if (!slot) {
>>>> +        get_page(bad_page);
>>>>          return bad_page;
>>>> +    }
>>>> +    if (slot->user_alloc) {
>>>> +        struct page *page[1];
>>>> +        int npages;
>>>> +
>>>> +        down_read(&current->mm->mmap_sem);
>>>> +        npages = get_user_pages(current, current->mm,
>>>> +                    slot->userspace_addr
>>>> +                    + (gfn - slot->base_gfn) * PAGE_SIZE, 1,
>>>> +                    1, 0, page, NULL);
>>>> +        up_read(&current->mm->mmap_sem);
>>>> +        if (npages != 1) {
>>>> +            get_page(bad_page);
>>>> +            return bad_page;
>>>> +        }
>>>> +        return page[0];
>>>>   
>>>
>>> Wouldn't it be necessary to assign page[0] to slot->phys_mem[gfn - 
>>> slot->base_gfn]?
>>
> sorry, it seems like i missunderstand you in the answer i gave you.
> it wouldnt be necessary to assign page[0] to slot->phys_mem[gfn - 
> slot->base_gfn], beacuse phys_mem wont have any memory allocate by 
> this time.
>
> with this patch, we are not holding anymore (when using userspace 
> allocation) array of all the memory at phys_mem.
> beacuse now that the pages are swappable, the physical address pointed 
> by the virtual address all the time change (for example when swapping 
> happn)
> so no one promise us that slot->phys_mem[gfn - slot->base_gfn] will 
> really point to page holding the gfn page.
>
> so what we did, is throw away the phys_mem array (also nice beacuse it 
> waste less ram), and at runtime we are getting the pages by using the 
> virtual address
> beacuse the reference of the page get increased, it promised us that 
> untill we release it point to the gfn (release it by doing put_page)
>
> hope i was more clear this time :)

Yes, that makes sense!

I wonder if there's a more elegant way dealing with older userspaces.  
For instance, is there any reason why we can allocate a userspace memory 
region on behalf of userspace.  That way swap would even work with older 
userspaces.

Regards,

Anthony Liguori



-------------------------------------------------------------------------
This SF.net email is sponsored by: Splunk Inc.
Still grepping through log files to find problems?  Stop.
Now Search log events and configuration files using AJAX and a browser.
Download your FREE copy of Splunk now >> http://get.splunk.com/