On 04/12/2023 19:30, Andrew Morton wrote: > On Mon, 4 Dec 2023 10:20:17 +0000 Ryan Roberts <ryan.roberts@xxxxxxx> 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 > >
