Re: [PATCH v8 00/10] Multi-size THP for anonymous memory

Ryan Roberts
2023-12-05T09:34:56Z

On 04/12/2023 19:30, Andrew Morton wrote:
> On Mon, 4 Dec 2023 10:20:17 +0000 Ryan Roberts <ryan.roberts@arm.com> wrote:
>
>> Hi All,
>>
>>
>> Prerequisites
>> =============
>>
>> Some work items identified as being prerequisites are listed on page 3 at [9].
>> The summary is:
>>
>> | item | status |
>> |:------------------------------|:------------------------|
>> | mlock | In mainline (v6.7) |
>> | madvise | In mainline (v6.6) |
>> | compaction | v1 posted [10] |
>> | numa balancing | Investigated: see below |
>> | user-triggered page migration | In mainline (v6.7) |
>> | khugepaged collapse | In mainline (NOP) |
>
> What does "prerequisites" mean here? Won't compile without? Kernel
> crashes without? Nice-to-have-after? Please expand on this.

Short answer: It's supposed to mean things that either need to be done to prevent the mm from regressing (both correctness and performance) when multi-size THP is present but disabled, or things that need to be done to make the mm robust (but not neccessarily optimially performant) when multi-size THP is enabled. But in reality, all of the things on the list could really be reclassified as "nice-to-have-after", IMHO; their absence will neither cause compilation nor runtime errors.

Longer answer: When I first started looking at this, I was advised that there were likely a number of corners which made assumptions about large folios always being PMD-sized, and if not found and fixed, could lead to stability issues. At the time I was also pursuing a strategy of multi-size THP being a compile-time feature with no runtime control, so I decided it was important for multi-size THP to not effectively disable other features (e.g. various madvise ops used to ignore PTE-mapped large folios). This list represents all the things that I could find based on code review, as well as things suggested by others, and in the end, they all fall into that last category of "PTE-mapped large folios efectively disable existing features". But given we now have runtime controls to opt-in to multi-size THP, I'm not sure we need to classify these as prerequisites. But I didn't want to unilaterally make that decision, given this list has previously been discussed and agreed by others.

It's also worth noting that in the case of compaction, that's already a problem for large folios in the page cache; large folios will be skipped.

>
> I looked at [9], but access is denied.

Sorry about that; its owned by David Rientjes so I can't fix that for you. It's a PDF of a slide with the following table:

+-------------------------------+------------------------------------------------------------------------+--------------+--------------------+
| Item | Description | Assignee | Status |
+-------------------------------+------------------------------------------------------------------------+--------------+--------------------+
| mlock | Large, pte-mapped folios are ignored when mlock is requested. | Yin, Fengwei | In mainline (v6.7) |
| | 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." | | |
| madvise | MADV_COLD, MADV_PAGEOUT, MADV_FREE: For large folios, code assumes | Yin, Fengwei | In mainline (v6.6) |
| | 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. | | |
| compaction | Raised at LSFMM: Compaction skips non-order-0 pages. | Zi Yan | v1 posted |
| | Already problem for page-cache pages today. | | |
| numa balancing | Large, pte-mapped folios are ignored by numa-balancing code. Commit | John Hubbard | Investigated: |
| | comment (e81c480): "We're going to have THP mapped with PTEs. It | | Not prerequisite |
| | will confuse numabalancing. Let's skip them for now." | | |
| user-triggered page migration | mm/migrate.c (migrate_pages syscall) We don't want to migrate folio | Kefeng Wang | In mainline (v6.7) |
| | that is shared. | | |
| khugepaged collapse | collapse small-sized THP to PMD-sized THP in khugepaged/MADV_COLLAPSE. | Ryan Roberts | In mainline (NOP) |
| | Kirill thinks khugepage should already be able to collapse | | |
| | small large folios to PMD-sized THP; verification required. | | |
+-------------------------------+------------------------------------------------------------------------+--------------+--------------------+

Thanks,
Ryan

>
>> [9] https://drive.google.com/file/d/1GnfYFpr7_c1kA41liRUW5YtCb8Cj18Ud/view?usp=sharing&resourcekey=0-U1Mj3-RhLD1JV6EThpyPyA