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?
[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?
>
>
>
> - 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.
>
> -----
