Re: [RFC PATCH v2 1/1] mm/vmscan: move the written-back folios to the tail of LRU after shrinking

[chenridong <chenridong@huawei.com>]

On 2024/11/18 12:14, Barry Song wrote:
> On Mon, Nov 18, 2024 at 5:03 PM Matthew Wilcox <willy@infradead.org> wrote:
>>
>> On Sat, Nov 16, 2024 at 09:16:58AM +0000, Chen Ridong wrote:
>>> 2. In shrink_page_list function, if folioN is THP(2M), it may be splited
>>>    and added to swap cache folio by folio. After adding to swap cache,
>>>    it will submit io to writeback folio to swap, which is asynchronous.
>>>    When shrink_page_list is finished, the isolated folios list will be
>>>    moved back to the head of inactive lru. The inactive lru may just look
>>>    like this, with 512 filioes have been move to the head of inactive lru.
>>
>> I was hoping that we'd be able to stop splitting the folio when adding
>> to the swap cache.  Ideally. we'd add the whole 2MB and write it back
>> as a single unit.
> 
> This is already the case: adding to the swapcache doesn’t require splitting
> THPs, but failing to allocate 2MB of contiguous swap slots will.
> 
>>
>> This is going to become much more important with memdescs.  We'd have to
>> allocate 512 struct folios to do this, which would be about 10 4kB pages,
>> and if we're trying to swap out memory, we're probably low on memory.
>>
>> So I don't like this solution you have at all because it doesn't help us
>> get to the solution we're going to need in about a year's time.
>>
> 
> Ridong might need to clarify why this splitting is occurring. If it’s due to the
> failure to allocate swap slots, we still need a solution to address it.
> 
> Thanks
> Barry

shrink_folio_list
  add_to_swap
    folio_alloc_swap
      get_swap_pages
      	scan_swap_map_slots
    	/*
    	* Swapfile is not block device or not using clusters so unable
    	* to allocate large entries.
    	*/
      	if (!(si->flags & SWP_BLKDEV) || !si->cluster_info)
          return 0;

In my test, I use a file as swap, which is not 'SWP_BLKDEV'. So it
failed to get get_swap_pages.

I think this is a race issue between 'shrink_folio_list' executing and
writing back asynchronously. In my test, 512 folios(THP split) were
added to swap, only about 60 folios had not been written back when
'move_folios_to_lru' was invoked after 'shrink_folio_list'. What if
writing back faster? Maybe this will happen even 32 folios(without THP)
are in the 'folio_list' of shrink_folio_list's inputs.

Best regards,
Ridong