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doc: clarify refcount section by referring to folios & pages

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Include the rename of subpages_mapcount to _nr_pages_mapped.

Link: https://lkml.kernel.org/r/20230111142915.1001531-4-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Matthew Wilcox (Oracle) 2023-01-11 14:28:49 +00:00 committed by Andrew Morton
parent eec20426d4
commit 6eee1a0062
1 changed files with 9 additions and 9 deletions
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18
Documentation/mm/transhuge.rst
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@ -112,20 +112,20 @@ Refcounts and transparent huge pages
Refcounting on THP is mostly consistent with refcounting on other compound
pages:
- get_page()/put_page() and GUP operate on head page's ->_refcount.
- get_page()/put_page() and GUP operate on the folio->_refcount.
- ->_refcount in tail pages is always zero: get_page_unless_zero() never
succeeds on tail pages.
- map/unmap of PMD entry for the whole compound page increment/decrement
->compound_mapcount, stored in the first tail page of the compound page;
and also increment/decrement ->subpages_mapcount (also in the first tail)
by COMPOUND_MAPPED when compound_mapcount goes from -1 to 0 or 0 to -1.
- map/unmap of a PMD entry for the whole THP increment/decrement
folio->_entire_mapcount and also increment/decrement
folio->_nr_pages_mapped by COMPOUND_MAPPED when _entire_mapcount
goes from -1 to 0 or 0 to -1.
- map/unmap of sub-pages with PTE entry increment/decrement ->_mapcount
on relevant sub-page of the compound page, and also increment/decrement
->subpages_mapcount, stored in first tail page of the compound page, when
_mapcount goes from -1 to 0 or 0 to -1: counting sub-pages mapped by PTE.
- map/unmap of individual pages with PTE entry increment/decrement
page->_mapcount and also increment/decrement folio->_nr_pages_mapped
when page->_mapcount goes from -1 to 0 or 0 to -1 as this counts
the number of pages mapped by PTE.
split_huge_page internally has to distribute the refcounts in the head
page to the tail pages before clearing all PG_head/tail bits from the page