On Fri, Mar 22, 2024 at 10:43:38AM +0100, Vlastimil Babka wrote:
> On 3/21/24 15:24, Matthew Wilcox (Oracle) wrote:
> > Return 0 for pages which can't be mapped. This matches how page_mapped()
> > works. It is more convenient for users to not have to filter out
> > these pages.
> >
> > Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
>
> Hm strictly speaking you shouldn't be removing those PageSlab tests until
> it's changed to a PageType in 7/9? If we're paranoid enough about not
> breaking bisection between this and that patch.
I thought about that. Slub currently doesn't use the field while will
become __page_type, so it's left set to -1 by the page allocator. So
this is safe.
Thanks for checking that though ;-)
> Otherwise
>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
>
> > ---
> > fs/proc/page.c | 7 ++-----
> > include/linux/mm.h | 8 +++++---
> > include/linux/page-flags.h | 4 ++--
> > 3 files changed, 9 insertions(+), 10 deletions(-)
> >
> > diff --git a/fs/proc/page.c b/fs/proc/page.c
> > index 195b077c0fac..9223856c934b 100644
> > --- a/fs/proc/page.c
> > +++ b/fs/proc/page.c
> > @@ -67,7 +67,7 @@ static ssize_t kpagecount_read(struct file *file, char __user *buf,
> > */
> > ppage = pfn_to_online_page(pfn);
> >
> > - if (!ppage || PageSlab(ppage) || page_has_type(ppage))
> > + if (!ppage)
> > pcount = 0;
> > else
> > pcount = page_mapcount(ppage);
> > @@ -124,11 +124,8 @@ u64 stable_page_flags(struct page *page)
> >
> > /*
> > * pseudo flags for the well known (anonymous) memory mapped pages
> > - *
> > - * Note that page->_mapcount is overloaded in SLAB, so the
> > - * simple test in page_mapped() is not enough.
> > */
> > - if (!PageSlab(page) && page_mapped(page))
> > + if (page_mapped(page))
> > u |= 1 << KPF_MMAP;
> > if (PageAnon(page))
> > u |= 1 << KPF_ANON;
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 0436b919f1c7..5ff3d687bc6c 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -1223,14 +1223,16 @@ static inline void page_mapcount_reset(struct page *page)
> > * a large folio, it includes the number of times this page is mapped
> > * as part of that folio.
> > *
> > - * The result is undefined for pages which cannot be mapped into userspace.
> > - * For example SLAB or special types of pages. See function page_has_type().
> > - * They use this field in struct page differently.
> > + * Will report 0 for pages which cannot be mapped into userspace, eg
> > + * slab, page tables and similar.
> > */
> > static inline int page_mapcount(struct page *page)
> > {
> > int mapcount = atomic_read(&page->_mapcount) + 1;
> >
> > + /* Handle page_has_type() pages */
> > + if (mapcount < 0)
> > + mapcount = 0;
> > if (unlikely(PageCompound(page)))
> > mapcount += folio_entire_mapcount(page_folio(page));
> >
> > diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> > index 8d0e6ce25ca2..5852f967c640 100644
> > --- a/include/linux/page-flags.h
> > +++ b/include/linux/page-flags.h
> > @@ -971,12 +971,12 @@ static inline bool is_page_hwpoison(struct page *page)
> > * page_type may be used. Because it is initialised to -1, we invert the
> > * sense of the bit, so __SetPageFoo *clears* the bit used for PageFoo, and
> > * __ClearPageFoo *sets* the bit used for PageFoo. We reserve a few high and
> > - * low bits so that an underflow or overflow of page_mapcount() won't be
> > + * low bits so that an underflow or overflow of _mapcount won't be
> > * mistaken for a page type value.
> > */
> >
> > #define PAGE_TYPE_BASE 0xf0000000
> > -/* Reserve 0x0000007f to catch underflows of page_mapcount */
> > +/* Reserve 0x0000007f to catch underflows of _mapcount */
> > #define PAGE_MAPCOUNT_RESERVE -128
> > #define PG_buddy 0x00000080
> > #define PG_offline 0x00000100
>
