Re: Prerequisites for Large Anon Folios

["Yin, Fengwei" <fengwei.yin@intel.com>]

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@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?
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@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?
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@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.
> 
>>
>> -----
>