| Message ID | 20240522210341.1030552-2-david@redhat.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm: page_type, zsmalloc and page_mapcount_reset() | expand |
On Wed, May 22, 2024 at 11:03:36PM +0200, David Hildenbrand wrote: > Let's make it clearer that _mapcount must no longer be used for own > purposes, and how _mapcount and page_type behaves nowadays (also in the > context of hugetlb folios, which are typed folios that will be mapped > to user space). > > Move the documentation regarding "-1" over from page_mapcount_reset(), > which we will remove next. > > Signed-off-by: David Hildenbrand <david@redhat.com> > --- > include/linux/mm.h | 5 ----- > include/linux/mm_types.h | 24 +++++++++++++++--------- > 2 files changed, 15 insertions(+), 14 deletions(-) > > diff --git a/include/linux/mm.h b/include/linux/mm.h > index 9849dfda44d4..018e7c0265ca 100644 > --- a/include/linux/mm.h > +++ b/include/linux/mm.h > @@ -1206,11 +1206,6 @@ static inline int folio_entire_mapcount(const struct folio *folio) > return atomic_read(&folio->_entire_mapcount) + 1; > } > > -/* > - * The atomic page->_mapcount, starts from -1: so that transitions > - * both from it and to it can be tracked, using atomic_inc_and_test > - * and atomic_add_negative(-1). > - */ > static inline void page_mapcount_reset(struct page *page) > { > atomic_set(&(page)->_mapcount, -1); > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 24323c7d0bd4..532a3030405d 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -46,9 +46,7 @@ struct mem_cgroup; > * which is guaranteed to be aligned. If you use the same storage as > * page->mapping, you must restore it to NULL before freeing the page. > * > - * If your page will not be mapped to userspace, you can also use the four > - * bytes in the mapcount union, but you must call page_mapcount_reset() > - * before freeing it. > + * The mapcount field must not be used for own purposes. > * > * If you want to use the refcount field, it must be used in such a way > * that other CPUs temporarily incrementing and then decrementing the > @@ -152,16 +150,24 @@ struct page { > > union { /* This union is 4 bytes in size. */ > /* > - * If the page can be mapped to userspace, encodes the number > - * of times this page is referenced by a page table. > + * For pages part of non-typed folios for which mappings are Maybe For pages that are part ... > + * tracked via the RMAP, encodes the number of times this page > + * is directly referenced by a page table. > + * > + * Note that the mapcount is always initialized to -1, so that > + * transitions both from it and to it can be tracked, using > + * atomic_inc_and_test() and atomic_add_negative(-1). > */ > atomic_t _mapcount; > > /* > - * If the page is neither PageSlab nor mappable to userspace, > - * the value stored here may help determine what this page > - * is used for. See page-flags.h for a list of page types > - * which are currently stored here. > + * For head pages of typed folios, the value stored here > + * allows for determining what this page is used for. The > + * tail pages of typed folios will not store a type > + * (mapcount == -1). > + * > + * See page-flags.h for a list of page types which are currently > + * stored here. > */ > unsigned int page_type; and maybe move page_type before _mapcount, so that it will be a bit clearer what are "typed" pages and folios are. > }; > -- > 2.45.0 >
diff --git a/include/linux/mm.h b/include/linux/mm.h index 9849dfda44d4..018e7c0265ca 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -1206,11 +1206,6 @@ static inline int folio_entire_mapcount(const struct folio *folio) return atomic_read(&folio->_entire_mapcount) + 1; } -/* - * The atomic page->_mapcount, starts from -1: so that transitions - * both from it and to it can be tracked, using atomic_inc_and_test - * and atomic_add_negative(-1). - */ static inline void page_mapcount_reset(struct page *page) { atomic_set(&(page)->_mapcount, -1); diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 24323c7d0bd4..532a3030405d 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -46,9 +46,7 @@ struct mem_cgroup; * which is guaranteed to be aligned. If you use the same storage as * page->mapping, you must restore it to NULL before freeing the page. * - * If your page will not be mapped to userspace, you can also use the four - * bytes in the mapcount union, but you must call page_mapcount_reset() - * before freeing it. + * The mapcount field must not be used for own purposes. * * If you want to use the refcount field, it must be used in such a way * that other CPUs temporarily incrementing and then decrementing the @@ -152,16 +150,24 @@ struct page { union { /* This union is 4 bytes in size. */ /* - * If the page can be mapped to userspace, encodes the number - * of times this page is referenced by a page table. + * For pages part of non-typed folios for which mappings are + * tracked via the RMAP, encodes the number of times this page + * is directly referenced by a page table. + * + * Note that the mapcount is always initialized to -1, so that + * transitions both from it and to it can be tracked, using + * atomic_inc_and_test() and atomic_add_negative(-1). */ atomic_t _mapcount; /* - * If the page is neither PageSlab nor mappable to userspace, - * the value stored here may help determine what this page - * is used for. See page-flags.h for a list of page types - * which are currently stored here. + * For head pages of typed folios, the value stored here + * allows for determining what this page is used for. The + * tail pages of typed folios will not store a type + * (mapcount == -1). + * + * See page-flags.h for a list of page types which are currently + * stored here. */ unsigned int page_type; };
Let's make it clearer that _mapcount must no longer be used for own purposes, and how _mapcount and page_type behaves nowadays (also in the context of hugetlb folios, which are typed folios that will be mapped to user space). Move the documentation regarding "-1" over from page_mapcount_reset(), which we will remove next. Signed-off-by: David Hildenbrand <david@redhat.com> --- include/linux/mm.h | 5 ----- include/linux/mm_types.h | 24 +++++++++++++++--------- 2 files changed, 15 insertions(+), 14 deletions(-)