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

Barry Song
2024-02-28T01:23:37Z

On Wed, Feb 28, 2024 at 1:06 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>
> 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.

yes. it should still be safe. just more I/O and more swap spaces. but they will
be removed while remove_mapping happens if try_to_unmap_one makes
the folio unmapped.

but with the potential possibility even mapped PTEs can be skipped by
try_to_unmap_one (reported intermediate PTEs issue - PTL is held till
a valid PTE, some PTEs might be skipped by try_to_unmap without being
set to swap entries), we could have the possibility folio_mapped() is still true
after try_to_unmap_one. so we can't get to __remove_mapping() for a long
time. but it still doesn't cause a crash.

>
> [...]
> >>>
> >>> 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?

I agree, this is just one question to David, not my proposal. if
deferred_list is sufficient,
I prefer we use deferred_list.

I actually don't quite understand why David dislikes _nr_pages_mapped being used
though I do think _nr_pages_mapped cannot precisely reflect how a
folio is mapped
by multi-processes. but _nr_pages_mapped < nr_pages seems be safe to
tell the folio
is partially unmapped :-)

>
> >
> >> 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 agree. btw, do we need a spinlock ds_queue->split_queue_lock for checking
the list? deferred_split_folio(), for itself, has no spinlock while checking
if (!list_empty(&folio->_deferred_list)), but why? the read and write
need to be exclusive.....

void deferred_split_folio(struct folio *folio)
{
...

if (!list_empty(&folio->_deferred_list))
return;

spin_lock_irqsave(&ds_queue->split_queue_lock, flags);
if (list_empty(&folio->_deferred_list)) {
count_vm_event(THP_DEFERRED_SPLIT_PAGE);
list_add_tail(&folio->_deferred_list, &ds_queue->split_queue);
ds_queue->split_queue_len++;
#ifdef CONFIG_MEMCG
if (memcg)
set_shrinker_bit(memcg, folio_nid(folio),
deferred_split_shrinker->id);
#endif
}
spin_unlock_irqrestore(&ds_queue->split_queue_lock, flags);
}

>
> >
> > 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 don't think there are fundamental differences for pmd and pte. i was
testing pte-mapped folio at that time, so kept the behavior of pmd as is.

>
> I'll incorporate this into my next version.

Great!

>
> > + &&
> > 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