Re: [PATCH 3/4] Swapping

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

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 :)


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