Re: [PATCH] arch: arm: remove redundant clear_page when CONFIG_INIT_ON_ALLOC_DEFAULT_ON is on

[Zhaoyang Huang]

On Tue, Sep 12, 2023 at 8:18 PM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Tue, Sep 12, 2023 at 06:33:34PM +0800, zhaoyang.huang wrote:
> > From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> >
> > Double times of clear_page observed in an arm SOC(A55) when
> > CONFIG_INIT_ON_ALLOC_DEFAULT_ON is on, which introduced by
> > vma_alloc_zeroed_movable_folio within do_anonymous_pages.
> > Since there is no D-cache operation within v6's clear_user_highpage,
> > I would like to suggest to remove the redundant clear_page.
> >
> > struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
> >                                  unsigned long vaddr)
> > {
> >       struct folio *folio;
> >
> > //first clear_page invoked by vma_alloc_folio==>alloc_page==>post_alloc_hook
> >       folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma, vaddr, false);
> >       if (folio)
> > //second clear_page which is meaningless since it do nothing to D-cache in armv6
> >               clear_user_highpage(&folio->page, vaddr);
>
> This is, of course, not the only place which calls clear_user_highpage().
> Please explain why this patch is safe for all the _other_ places which
> call clear_user_highpage().
Here are all positions called clear_user_highpage which are paired
with alloc_pages. IMO,  it is safe to skip the second clear_page under
armv6.

drivers/media/v4l2-core/videobuf-dma-sg.c:441:
clear_user_highpage(page, vmf->address);
fs/dax.c:1612: clear_user_highpage(vmf->cow_page, vmf->address);
include/linux/highmem.h:231: clear_user_highpage(&folio->page, vaddr);
mm/memory.c:5974: clear_user_highpage(p, addr + i * PAGE_SIZE);
mm/memory.c:5982: clear_user_highpage(page + idx, addr);
mm/shmem.c:2621: clear_user_highpage(&folio->page, dst_addr);
mm/khugepaged.c:796: clear_user_highpage(page, _address);

>
> >       return folio;
> > }
> >
> > Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> > ---
> >  arch/arm/mm/copypage-v6.c | 8 ++++++++
> >  1 file changed, 8 insertions(+)
> >
> > diff --git a/arch/arm/mm/copypage-v6.c b/arch/arm/mm/copypage-v6.c
> > index a1a71f36d850..6f8bee1b3203 100644
> > --- a/arch/arm/mm/copypage-v6.c
> > +++ b/arch/arm/mm/copypage-v6.c
> > @@ -9,6 +9,7 @@
> >  #include <linux/mm.h>
> >  #include <linux/highmem.h>
> >  #include <linux/pagemap.h>
> > +#include <linux/gfp.h>
> >
> >  #include <asm/shmparam.h>
> >  #include <asm/tlbflush.h>
> > @@ -45,6 +46,13 @@ static void v6_copy_user_highpage_nonaliasing(struct page *to,
> >   */
> >  static void v6_clear_user_highpage_nonaliasing(struct page *page, unsigned long vaddr)
> >  {
> > +     /*
> > +      * This criteria only help bailing out when CONFIG_INIT_ON_ALLOC_DEFAULT_ON
> > +      * is on. The page has been memset to zero when it allocated and the
> > +      * bellowing clear_page will do it again.
> > +      */
> > +     if (want_init_on_alloc(GFP_HIGHUSER_MOVABLE))
> > +             return;
> >       void *kaddr = kmap_atomic(page);
> >       clear_page(kaddr);
> >       kunmap_atomic(kaddr);
> > --
> > 2.25.1
> >