Messages in this thread | | | Date | Thu, 24 Nov 2022 09:50:43 +0100 | Subject | Re: [PATCH] mm: migrate: Fix THP's mapcount on isolation | From | David Hildenbrand <> |
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On 23.11.22 17:07, Matthew Wilcox wrote: > On Wed, Nov 23, 2022 at 09:56:38AM +0100, David Hildenbrand wrote: >> But we do have an even better helper in place already: >> mm/huge_memory.c:can_split_folio() >> >> Which cares about >> >> a) Swapcache for THP: each subpage could be in the swapcache >> b) Requires the caller to hold one reference to be safe >> >> But I am a bit confused about the "extra_pins" for !anon. Where do the >> folio_nr_pages() references come from? > > When we add a folio to the page cache, we increment its refcount by > folio_nr_pages() instead of by 1. I suspect this is no longer needed > (if it was ever needed) and it could be changed. See > __filemap_add_folio(): > > long nr = 1; > if (!huge) { > nr = folio_nr_pages(folio); > folio_ref_add(folio, nr); > >> So *maybe* it makes sense to factor out can_split_folio() and call it >> something like: "folio_maybe_additionally_referenced" [to clearly >> distinguish it from "folio_maybe_dma_pinned" that cares about actual page >> pinning (read/write page content)]. >> >> Such a function could return false positives/negatives due to races and the >> caller would have to hold one reference and be able to deal with the >> semantics. > > I don't like the 'pextra_pins' parameter to a generic function ...
Right, that part should remain khugepaged specific. The assumption would be, that the caller of the generic function holds exactly one additional reference.
-- Thanks,
David / dhildenb
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