Re: [PATCH RFC v3 02/35] mm: page_alloc: Add an arch hook early in free_pages_prepare()

[Alexandru Elisei]

Hi,

On Mon, Jan 29, 2024 at 01:49:44PM +0530, Anshuman Khandual wrote:
> 
> 
> On 1/25/24 22:12, Alexandru Elisei wrote:
> > The arm64 MTE code uses the PG_arch_2 page flag, which it renames to
> > PG_mte_tagged, to track if a page has been mapped with tagging enabled.
> > That flag is cleared by free_pages_prepare() by doing:
> > 
> > 	page->flags &= ~PAGE_FLAGS_CHECK_AT_PREP;
> > 
> > When tag storage management is added, tag storage will be reserved for a
> > page if and only if the page is mapped as tagged (the page flag
> > PG_mte_tagged is set). When a page is freed, likewise, the code will have
> > to look at the the page flags to determine if the page has tag storage
> > reserved, which should also be freed.
> > 
> > For this purpose, add an arch_free_pages_prepare() hook that is called
> > before that page flags are cleared. The function arch_free_page() has also
> > been considered for this purpose, but it is called after the flags are
> > cleared.
> 
> arch_free_pages_prepare() makes sense as a prologue to arch_free_page().  

Thanks!

> 
> s/arch_free_pages_prepare/arch_free_page_prepare to match similar functions.

The function free_pages_prepare() calls the function arch_free_pages_prepare().
I find that consistent, and it makes it easy to identify from where
arch_free_pages_prepare() is called.

Thanks,
Alex

> 
> > 
> > Signed-off-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
> > ---
> > 
> > Changes since rfc v2:
> > 
> > * Expanded commit message (David Hildenbrand).
> > 
> >  include/linux/pgtable.h | 4 ++++
> >  mm/page_alloc.c         | 1 +
> >  2 files changed, 5 insertions(+)
> > 
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index f6d0e3513948..6d98d5fdd697 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -901,6 +901,10 @@ static inline void arch_do_swap_page(struct mm_struct *mm,
> >  }
> >  #endif
> >  
> > +#ifndef __HAVE_ARCH_FREE_PAGES_PREPARE
> 
> I guess new __HAVE_ARCH_ constructs are not being added lately. Instead
> something like '#ifndef arch_free_pages_prepare' might be better suited.
> 
> > +static inline void arch_free_pages_prepare(struct page *page, int order) { }
> > +#endif
> > +
> >  #ifndef __HAVE_ARCH_UNMAP_ONE
> >  /*
> >   * Some architectures support metadata associated with a page. When a
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 2c140abe5ee6..27282a1c82fe 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1092,6 +1092,7 @@ static __always_inline bool free_pages_prepare(struct page *page,
> >  
> >  	trace_mm_page_free(page, order);
> >  	kmsan_free_page(page, order);
> > +	arch_free_pages_prepare(page, order);
> >  
> >  	if (memcg_kmem_online() && PageMemcgKmem(page))
> >  		__memcg_kmem_uncharge_page(page, order);