Re: [PATCH v3 4/4] mm: swap: Swap-out small-sized THP without splitting

[Ryan Roberts <ryan.roberts@arm.com>]

On 23/02/2024 09:46, Barry Song wrote:
> On Thu, Feb 22, 2024 at 11:09 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 22.02.24 08:05, Barry Song wrote:
>>> Hi Ryan,
>>>
>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>>>> index 2cc0cb41fb32..ea19710aa4cd 100644
>>>> --- a/mm/vmscan.c
>>>> +++ b/mm/vmscan.c
>>>> @@ -1212,11 +1212,13 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>>>>                                      if (!can_split_folio(folio, NULL))
>>>>                                              goto activate_locked;
>>>>                                      /*
>>>> -                                     * Split folios without a PMD map right
>>>> -                                     * away. Chances are some or all of the
>>>> -                                     * tail pages can be freed without IO.
>>>> +                                     * Split PMD-mappable folios without a
>>>> +                                     * PMD map right away. Chances are some
>>>> +                                     * or all of the tail pages can be freed
>>>> +                                     * without IO.
>>>>                                       */
>>>> -                                    if (!folio_entire_mapcount(folio) &&
>>>> +                                    if (folio_test_pmd_mappable(folio) &&
>>>> +                                        !folio_entire_mapcount(folio) &&
>>>>                                          split_folio_to_list(folio,
>>>>                                                              folio_list))
>>>>                                              goto activate_locked;
>>>
>>> I ran a test to investigate what would happen while reclaiming a partially
>>> unmapped large folio. for example, for 64KiB large folios, MADV_DONTNEED
>>> 4KB~64KB, and keep the first subpage 0~4KiB.
>>
>> IOW, something that already happens with ordinary THP already IIRC.
>>
>>>
>>> My test wants to address my three concerns,
>>> a. whether we will have leak on swap slots
>>> b. whether we will have redundant I/O
>>> c. whether we will cause races on swapcache
>>>
>>> what i have done is printing folio->_nr_pages_mapped and dumping 16 swap_map[]
>>> at some specific stage
>>> 1. just after add_to_swap   (swap slots are allocated)
>>> 2. before and after try_to_unmap   (ptes are set to swap_entry)
>>> 3. before and after pageout (also add printk in zram driver to dump all I/O write)
>>> 4. before and after remove_mapping
>>>
>>> The below is the dumped info for a particular large folio,
>>>
>>> 1. after add_to_swap
>>> [   27.267357] vmscan: After add_to_swap shrink_folio_list 1947 mapnr:1
>>> [   27.267650] vmscan: offset:101b0 swp_map 40-40-40-40-40-40-40-40-40-40-40-40-40-40-40-40
>>>
>>> as you can see,
>>> _nr_pages_mapped is 1 and all 16 swap_map are SWAP_HAS_CACHE (0x40)
>>>
>>>
>>> 2. before and after try_to_unmap
>>> [   27.268067] vmscan: before try to unmap shrink_folio_list 1991 mapnr:1
>>> [   27.268372] try_to_unmap_one address:ffff731f0000 pte:e8000103cd0b43 pte_p:ffff0000c36a8f80
>>> [   27.268854] vmscan: after try to unmap shrink_folio_list 1997 mapnr:0
>>> [   27.269180] vmscan: offset:101b0 swp_map 41-40-40-40-40-40-40-40-40-40-40-40-40-40-40-40
>>>
>>> as you can see, one pte is set to swp_entry, and _nr_pages_mapped becomes
>>> 0 from 1. The 1st swp_map becomes 0x41, SWAP_HAS_CACHE + 1
>>>
>>> 3. before and after pageout
>>> [   27.269602] vmscan: before pageout shrink_folio_list 2065 mapnr:0
>>> [   27.269880] vmscan: offset:101b0 swp_map 41-40-40-40-40-40-40-40-40-40-40-40-40-40-40-40
>>> [   27.270691] zram: zram_write_page page:fffffc00030f3400 index:101b0
>>> [   27.271061] zram: zram_write_page page:fffffc00030f3440 index:101b1
>>> [   27.271416] zram: zram_write_page page:fffffc00030f3480 index:101b2
>>> [   27.271751] zram: zram_write_page page:fffffc00030f34c0 index:101b3
>>> [   27.272046] zram: zram_write_page page:fffffc00030f3500 index:101b4
>>> [   27.272384] zram: zram_write_page page:fffffc00030f3540 index:101b5
>>> [   27.272746] zram: zram_write_page page:fffffc00030f3580 index:101b6
>>> [   27.273042] zram: zram_write_page page:fffffc00030f35c0 index:101b7
>>> [   27.273339] zram: zram_write_page page:fffffc00030f3600 index:101b8
>>> [   27.273676] zram: zram_write_page page:fffffc00030f3640 index:101b9
>>> [   27.274044] zram: zram_write_page page:fffffc00030f3680 index:101ba
>>> [   27.274554] zram: zram_write_page page:fffffc00030f36c0 index:101bb
>>> [   27.274870] zram: zram_write_page page:fffffc00030f3700 index:101bc
>>> [   27.275166] zram: zram_write_page page:fffffc00030f3740 index:101bd
>>> [   27.275463] zram: zram_write_page page:fffffc00030f3780 index:101be
>>> [   27.275760] zram: zram_write_page page:fffffc00030f37c0 index:101bf
>>> [   27.276102] vmscan: after pageout and before needs_release shrink_folio_list 2124 mapnr:0
>>>
>>> as you can see, obviously, we have done redundant I/O - 16 zram_write_page though
>>> 4~64KiB has been zap_pte_range before, we still write them to zRAM.
>>>
>>> 4. before and after remove_mapping
>>> [   27.276428] vmscan: offset:101b0 swp_map 41-40-40-40-40-40-40-40-40-40-40-40-40-40-40-40
>>> [   27.277485] vmscan: after remove_mapping shrink_folio_list 2169 mapnr:0 offset:0
>>> [   27.277802] vmscan: offset:101b0 01-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
>>>
>>> as you can see, swp_map 1-15 becomes 0 and only the first swp_map is 1.
>>> all SWAP_HAS_CACHE has been removed. This is perfect and there is no swap
>>> slot leak at all!
>>>
>>> Thus, only two concerns are left for me,
>>> 1. as we don't split anyway, we have done 15 unnecessary I/O if a large folio
>>> is partially unmapped.

So the cost of this is increased IO and swap storage, correct? Is this a big
problem in practice? i.e. do you see a lot of partially mapped large folios in
your workload? (I agree the proposed fix below is simple, so I think we should
do it anyway - I'm just interested in the scale of the problem).

>>> 2. large folio is added as a whole as a swapcache covering the range whose
>>> part has been zapped. I am not quite sure if this will cause some problems
>>> while some concurrent do_anon_page, swapin and swapout occurs between 3 and
>>> 4 on zapped subpage1~subpage15. still struggling.. my brain is exploding...

Yes mine too. I would only expect the ptes that map the folio will get replaced
with swap entries? So I would expect it to be safe. Although I understand the
concern with the extra swap consumption.

[...]
>>>
>>> To me, it seems safer to split or do some other similar optimization if we find a
>>> large folio has partial map and unmap.
>>
>> I'm hoping that we can avoid any new direct users of _nr_pages_mapped if
>> possible.
>>
> 
> Is _nr_pages_mapped < nr_pages a reasonable case to split as we
> have known the folio has at least some subpages zapped?

I'm not sure we need this - the folio's presence on the split list will tell us
everything we need to know I think?

> 
>> If we find that the folio is on the deferred split list, we might as
>> well just split it right away, before swapping it out. That might be a
>> reasonable optimization for the case you describe.

Yes, agreed. I think there is still chance of a race though; Some other thread
could be munmapping in parallel. But in that case, I think we just end up with
the increased IO and swap storage? That's not the end of the world if its a
corner case.

> 
> i tried to change Ryan's code as below
> 
> @@ -1905,11 +1922,12 @@ static unsigned int shrink_folio_list(struct
> list_head *folio_list,
>                                          * PMD map right away. Chances are some
>                                          * or all of the tail pages can be freed
>                                          * without IO.
> +                                        * Similarly, split PTE-mapped folios if
> +                                        * they have been already
> deferred_split.
>                                          */
> -                                       if (folio_test_pmd_mappable(folio) &&
> -                                           !folio_entire_mapcount(folio) &&
> -                                           split_folio_to_list(folio,
> -                                                               folio_list))
> +                                       if
> (((folio_test_pmd_mappable(folio) && !folio_entire_mapcount(folio)) ||
> +
> (!folio_test_pmd_mappable(folio) &&
> !list_empty(&folio->_deferred_list)))

I'm not sure we need the different tests for pmd_mappable vs !pmd_mappable. I
think presence on the deferred list is a sufficient indicator that there are
unmapped subpages?

I'll incorporate this into my next version.

> +                                           &&
> split_folio_to_list(folio, folio_list))
>                                                 goto activate_locked;
>                                 }
>                                 if (!add_to_swap(folio)) {
> 
> It seems to work as expected. only one I/O is left for a large folio
> with 16 PTEs
> but 15 of them have been zapped before.
> 
>>
>> --
>> Cheers,
>>
>> David / dhildenb
>>
> 
> Thanks
> Barry