Re: [Xen-devel] [PATCH 1/2] swap: change swap_info singly-linked list to list_head

[Bob Liu <bob.liu@xxxxxxxxxx>]

On 05/03/2014 04:00 AM, Dan Streetman wrote:
> On Fri, Apr 25, 2014 at 12:15 AM, Weijie Yang <weijie.yang.kh@xxxxxxxxx> 
> wrote:
>> On Fri, Apr 25, 2014 at 2:48 AM, Dan Streetman <ddstreet@xxxxxxxx> wrote:
>>> On Wed, Apr 23, 2014 at 6:34 AM, Mel Gorman <mgorman@xxxxxxx> wrote:
>>>> On Sat, Apr 12, 2014 at 05:00:53PM -0400, Dan Streetman wrote:
> <SNIP>
>>>>> diff --git a/mm/frontswap.c b/mm/frontswap.c
>>>>> index 1b24bdc..fae1160 100644
>>>>> --- a/mm/frontswap.c
>>>>> +++ b/mm/frontswap.c
>>>>> @@ -327,15 +327,12 @@ EXPORT_SYMBOL(__frontswap_invalidate_area);
>>>>>
>>>>>  static unsigned long __frontswap_curr_pages(void)
>>>>>  {
>>>>> -     int type;
>>>>>       unsigned long totalpages = 0;
>>>>>       struct swap_info_struct *si = NULL;
>>>>>
>>>>>       assert_spin_locked(&swap_lock);
>>>>> -     for (type = swap_list.head; type >= 0; type = si->next) {
>>>>> -             si = swap_info[type];
>>>>> +     list_for_each_entry(si, &swap_list_head, list)
>>>>>               totalpages += atomic_read(&si->frontswap_pages);
>>>>> -     }
>>>>>       return totalpages;
>>>>>  }
>>>>>
>>>>> @@ -347,11 +344,9 @@ static int __frontswap_unuse_pages(unsigned long 
>>>>> total, unsigned long *unused,
>>>>>       int si_frontswap_pages;
>>>>>       unsigned long total_pages_to_unuse = total;
>>>>>       unsigned long pages = 0, pages_to_unuse = 0;
>>>>> -     int type;
>>>>>
>>>>>       assert_spin_locked(&swap_lock);
>>>>> -     for (type = swap_list.head; type >= 0; type = si->next) {
>>>>> -             si = swap_info[type];
>>>>> +     list_for_each_entry(si, &swap_list_head, list) {
>>>>>               si_frontswap_pages = atomic_read(&si->frontswap_pages);
>>>>>               if (total_pages_to_unuse < si_frontswap_pages) {
>>>>>                       pages = pages_to_unuse = total_pages_to_unuse;
>>>>
>>>> The frontswap shrink code looks suspicious. If the target is smaller than
>>>> the total number of frontswap pages then it does nothing. The callers

__frontswap_unuse_pages() is called only to get the correct value of
pages_to_unuse which will pass to try_to_unuse(), perhaps we should
rename it to __frontswap_unuse_pages_nr()..

------
ret = __frontswap_shrink(target_pages, &pages_to_unuse, &type);
        ->  __frontswap_unuse_pages(total_pages_to_unuse, pages_to_unuse, type);

try_to_unuse(type, true, pages_to_unuse);
------

>>>> appear to get this right at least. Similarly, if the first swapfile has
>>>> fewer frontswap pages than the target then it does not unuse the target
>>>> number of pages because it only handles one swap file. It's outside the
>>>> scope of your patch to address this or wonder if xen balloon driver is
>>>> really using it the way it's expected.
>>>
>>> I didn't look into the frontswap shrinking code, but I agree the
>>> existing logic there doesn't look right.  I'll review frontswap in
>>> more detail to see if it needs changing here, unless anyone else gets
>>> it to first :-)
>>>
>>
>> FYI, I drop the frontswap_shrink code in a patch
>> see: https://lkml.org/lkml/2014/1/27/98
> 
> frontswap_shrink() is actually used (only) by drivers/xen/xen-selfballoon.c.
> 
> However, I completely agree with you that the backend should be doing
> the shrinking, not from a frontswap api.  Forcing frontswap to shrink
> is backwards - xen-selfballoon appears to be assuming that xem/tmem is
> the only possible frontswap backend.  It certainly doensn't make any
> sense for xen-selfballoon to force zswap to shrink itself, does it?
> 
> If xen-selfballoon wants to shrink its frontswap backend tmem, it
> should do that by telling tmem directly to shrink itself (which it
> looks like tmem would have to implement, just like zswap sends its LRU
> pages back to swapcache when it becomes full).
> 

Yes, it's possible in theory, but tmem is located in xen(host) which
can't put back pages to swap cache(in guest os) directly. Use
frontswap_shrink() can make things simple and easier.

And I think frontswap shrink isn't a blocker of this patch set, so
please keep it.

-- 
Regards,
-Bob

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