On 8/30/2023 6:44 PM, Ryan Roberts wrote:
> 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@xxxxxxxxxx>
>
> 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@xxxxxxxxxxxxxxxxxxxx/
>> - https://lore.kernel.org/linux-mm/C56EA745-E112-4887-8C22-B74FCB6A14EB@xxxxxxxxxx/
>>
>> location:
>> - compaction_alloc()
>>
>> assignee:
>> Zi Yan <ziy@xxxxxxxxxx>
>>
>>
>
> 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@xxxxxxxxxx/
>
>
>>
>> - 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@xxxxxxxxx/
>>
>> assignee:
>> Yin, Fengwei <fengwei.yin@xxxxxxxxx>
>>
>>
>
> series on list at [2]. Does this series cover everything?
Yes. I suppose so. I already collected comment from you. And I am waiting for review comment
from Yu who is on vacation now. Then, I will work on v3.
>
> [2] https://lore.kernel.org/linux-mm/20230809061105.3369958-1-fengwei.yin@xxxxxxxxx/
>
>
>>
>> - 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@xxxxxxxxx/
>>
>> location:
>> - madvise_cold_or_pageout_pte_range()
>> - madvise_free_pte_range()
>>
>> assignee:
>> Yin, Fengwei <fengwei.yin@xxxxxxxxx>
>
> 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?
The changes based on folio_estimated_sharers() is in. Once David's solution is
ready, will switch to new solution.
For avoids splitting large folio, it was in the patchset I posted (before split
folio_estimated_sharers() part out).
Regards
Yin, Fengwei
>
>
>>
>>
>>
>> - 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@xxxxxxx/
>>
>> location:
>> - zap_pte_range()
>>
>> assignee:
>> Ryan Roberts <ryan.roberts@xxxxxxx>
>
> 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@xxxxxxx/
> [4]
> https://lore.kernel.org/linux-mm/CAOUHufZr8ym0kzoa99=k3Gquc4AdoYXMaj-kv99u5FPv1KkezA@xxxxxxxxxxxxxx/
>
>
>>
>>
>>
>> - 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@xxxxxxxxxxxxxx/
>>
>> location:
>> - shrink_folio_list()
>>
>> assignee:
>> <none>
>
> Not a prerequisite so not worrying about it for now.
>
>>
>> -----
>
