Re: Prerequisites for Large Anon Folios

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

Hi All,


I want to get serious about getting large anon folios merged. To do that, there
are a number of outstanding prerequistes. I'm hoping the respective owners may
be able to provide an update on progress?

I appreciate everyone is busy and likely juggling multiple things, so understand
if no progress has been made or likely to be made - it would be good to know
that though, so I can attempt to make alternative plans.

See questions/comments below.

Thanks!



On 20/07/2023 10:41, Ryan Roberts wrote:
> Hi All,
> 
> As discussed at Matthew's call yesterday evening, I've put together a list of
> items that need to be done as prerequisites for merging large anonymous folios
> support.
> 
> It would be great to get some review and confirmation as to whether anything is
> missing or incorrect. Most items have an assignee - in that case it would be
> good to check that my understanding that you are working on the item is correct.
> 
> I think most things are independent, with the exception of "shared vs exclusive
> mappings", which I think becomes a dependency for a couple of things (marked in
> depender description); again would be good to confirm.
> 
> Finally, although I'm concentrating on the prerequisites to clear the path for
> merging an MVP Large Anon Folios implementation, I've included one "enhancement"
> item ("large folios in swap cache"), solely because we explicitly discussed it
> last night. My view is that enhancements can come after the initial large anon
> folios merge. Over time, I plan to add other enhancements (e.g. retain large
> folios over COW, etc).
> 
> I'm posting the table as yaml as that seemed easiest for email. You can convert
> to csv with something like this in Python:
> 
>   import yaml
>   import pandas as pd
>   pd.DataFrame(yaml.safe_load(open('work-items.yml'))).to_csv('work-items.csv')
> 
> Thanks,
> Ryan
> 
> -----
> 
> - item:
>     shared vs exclusive mappings
> 
>   priority:
>     prerequisite
> 
>   description: >-
>     New mechanism to allow us to easily determine precisely whether a given
>     folio is mapped exclusively or shared between multiple processes. Required
>     for (from David H):
> 
>     (1) Detecting shared folios, to not mess with them while they are shared.
>     MADV_PAGEOUT, user-triggered page migration, NUMA hinting, khugepaged ...
>     replace cases where folio_estimated_sharers() == 1 would currently be the
>     best we can do (and in some cases, page_mapcount() == 1).
> 
>     (2) COW improvements for PTE-mapped large anon folios after fork(). Before
>     fork(), PageAnonExclusive would have been reliable, after fork() it's not.
> 
>     For (1), "MADV_PAGEOUT" maps to the "madvise" item captured in this list. I
>     *think* "NUMA hinting" maps to "numa balancing" (but need confirmation!).
>     "user-triggered page migration" and "khugepaged" not yet captured (would
>     appreciate someone fleshing it out). I previously understood migration to be
>     working for large folios - is "user-triggered page migration" some specific
>     aspect that does not work?
> 
>     For (2), this relates to Large Anon Folio enhancements which I plan to
>     tackle after we get the basic series merged.
> 
>   links:
>     - 'email thread: Mapcount games: "exclusive mapped" vs. "mapped shared"'
> 
>   location:
>     - shrink_folio_list()
> 
>   assignee:
>     David Hildenbrand <david@redhat.com>

Any comment on this David? I think the last comment I saw was that you were
planning to start an implementation a couple of weeks back? Did that get anywhere?

> 
> 
> 
> - item:
>     compaction
> 
>   priority:
>     prerequisite
> 
>   description: >-
>     Raised at LSFMM: Compaction skips non-order-0 pages. Already problem for
>     page-cache pages today.
> 
>   links:
>     - https://lore.kernel.org/linux-mm/ZKgPIXSrxqymWrsv@casper.infradead.org/
>     - https://lore.kernel.org/linux-mm/C56EA745-E112-4887-8C22-B74FCB6A14EB@nvidia.com/
> 
>   location:
>     - compaction_alloc()
> 
>   assignee:
>     Zi Yan <ziy@nvidia.com>
> 
> 

Are you still planning to work on this, Zi? The last email I have is [1] where
you agreed to take a look.

[1]
https://lore.kernel.org/linux-mm/4DD00BE6-4141-4887-B5E5-0B7E8D1E2086@nvidia.com/


> 
> - item:
>     mlock
> 
>   priority:
>     prerequisite
> 
>   description: >-
>     Large, pte-mapped folios are ignored when mlock is requested. Code comment
>     for mlock_vma_folio() says "...filter out pte mappings of THPs, which cannot
>     be consistently counted: a pte mapping of the THP head cannot be
>     distinguished by the page alone."
> 
>   location:
>     - mlock_pte_range()
>     - mlock_vma_folio()
> 
>   links:
>     - https://lore.kernel.org/linux-mm/20230712060144.3006358-1-fengwei.yin@intel.com/
> 
>   assignee:
>     Yin, Fengwei <fengwei.yin@intel.com>
> 
> 

series on list at [2]. Does this series cover everything?

[2] https://lore.kernel.org/linux-mm/20230809061105.3369958-1-fengwei.yin@intel.com/


> 
> - item:
>     madvise
> 
>   priority:
>     prerequisite
> 
>   description: >-
>     MADV_COLD, MADV_PAGEOUT, MADV_FREE: For large folios, code assumes exclusive
>     only if mapcount==1, else skips remainder of operation. For large,
>     pte-mapped folios, exclusive folios can have mapcount upto nr_pages and
>     still be exclusive. Even better; don't split the folio if it fits entirely
>     within the range. Likely depends on "shared vs exclusive mappings".
> 
>   links:
>     - https://lore.kernel.org/linux-mm/20230713150558.200545-1-fengwei.yin@intel.com/
> 
>   location:
>     - madvise_cold_or_pageout_pte_range()
>     - madvise_free_pte_range()
> 
>   assignee:
>     Yin, Fengwei <fengwei.yin@intel.com>

As I understand it: initial solution based on folio_estimated_sharers() has gone
into v6.5. Have a dependecy on David's precise shared vs exclusive work for an
improved solution. And I think you mentioned you are planning to do a change
that avoids splitting a large folio if it is entirely covered by the range?


> 
> 
> 
> - item:
>     deferred_split_folio
> 
>   priority:
>     prerequisite
> 
>   description: >-
>     zap_pte_range() will remove each page of a large folio from the rmap, one at
>     a time, causing the rmap code to see the folio as partially mapped and call
>     deferred_split_folio() for it. Then it subsquently becmes fully unmapped and
>     it is removed from the queue. This can cause some lock contention. Proposed
>     fix is to modify to zap_pte_range() to "batch zap" a whole pte range that
>     corresponds to a folio to avoid the unneccessary deferred_split_folio()
>     call.
> 
>   links:
>     - https://lore.kernel.org/linux-mm/20230719135450.545227-1-ryan.roberts@arm.com/
> 
>   location:
>     - zap_pte_range()
> 
>   assignee:
>     Ryan Roberts <ryan.roberts@arm.com>

I have a series at [3] to solve this (different approach than described above).
Although Yu has suggested this is not a prerequisite after all [4].

[3] https://lore.kernel.org/linux-mm/20230830095011.1228673-1-ryan.roberts@arm.com/
[4]
https://lore.kernel.org/linux-mm/CAOUHufZr8ym0kzoa99=k3Gquc4AdoYXMaj-kv99u5FPv1KkezA@mail.gmail.com/


> 
> 
> 
> - item:
>     numa balancing
> 
>   priority:
>     prerequisite
> 
>   description: >-
>     Large, pte-mapped folios are ignored by numa-balancing code. Commit comment
>     (e81c480): "We're going to have THP mapped with PTEs. It will confuse
>     numabalancing. Let's skip them for now." Likely depends on "shared vs
>     exclusive mappings".
> 
>   links: []
> 
>   location:
>     - do_numa_page()
> 
>   assignee:
>     <none>
> 

Vaguely sounded like David might be planning to tackle this as part of his work
on "shared vs exclusive mappings" ("NUMA hinting"??). David?

> 
> 
> - item:
>     large folios in swap cache
> 
>   priority:
>     enhancement
> 
>   description: >-
>     shrink_folio_list() currently splits large folios to single pages before
>     adding them to the swap cache. It would be preferred to add the large folio
>     as an atomic unit to the swap cache. It is still expected that each page
>     would use a separate swap entry when swapped out. This represents an
>     efficiency improvement. There is risk that this change will expose bad
>     assumptions in the swap cache that assume any large folio is pmd-mappable.
> 
>   links:
>     - https://lore.kernel.org/linux-mm/CAOUHufbC76OdP16mRsY3i920qB7khcu8FM+nUOG0kx5BMRdKXw@mail.gmail.com/
> 
>   location:
>     - shrink_folio_list()
> 
>   assignee:
>     <none>

Not a prerequisite so not worrying about it for now.

> 
> -----