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
