Re: [PATCH 3/4] Swapping

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

Izik Eidus wrote:
> Anthony Liguori wrote:
>> 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.
> if we can do that, yes swap will work on older userspace.

I think it's just a matter of calling do_mmap() with the appropriate 
parameters.  It looks likes there's some drivers call do_mmap() directly.

Regards,

Anthony Liguori

>> Regards,
>>
>> Anthony Liguori
>>
>>
>
>


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