Re: [PATCH 3/4] Swapping

[Izik Eidus <izike-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

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.
> Regards,
>
> Anthony Liguori
>
>


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