Re: [PATCH] mm: gup: remove FOLL_SPLIT

[Yang Shi <shy828301@gmail.com>]

On Tue, Mar 30, 2021 at 12:08 AM John Hubbard <jhubbard@nvidia.com> wrote:
>
> On 3/29/21 12:38 PM, Yang Shi wrote:
> > Since commit 5a52c9df62b4 ("uprobe: use FOLL_SPLIT_PMD instead of FOLL_SPLIT")
> > and commit ba925fa35057 ("s390/gmap: improve THP splitting") FOLL_SPLIT
> > has not been used anymore.  Remove the dead code.
> >
> > Signed-off-by: Yang Shi <shy828301@gmail.com>
> > ---
> >   include/linux/mm.h |  1 -
> >   mm/gup.c           | 28 ++--------------------------
> >   2 files changed, 2 insertions(+), 27 deletions(-)
> >
>
> Looks nice.
>
> As long as I'm running git grep here, there is one more search hit that should also
> be fixed up, as part of a "remove FOLL_SPLIT" patch:
>
> git grep -nw FOLL_SPLIT
> Documentation/vm/transhuge.rst:57:follow_page, the FOLL_SPLIT bit can be specified as a parameter to
>
> Reviewed-by: John Hubbard <jhubbard@nvidia.com>

Thanks. Removed the reference to FOLL_SPLIT in documentation for v2.

>
> thanks,
> --
> John Hubbard
> NVIDIA
>
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index 8ba434287387..3568836841f9 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -2780,7 +2780,6 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
> >   #define FOLL_NOWAIT 0x20    /* if a disk transfer is needed, start the IO
> >                                * and return without waiting upon it */
> >   #define FOLL_POPULATE       0x40    /* fault in page */
> > -#define FOLL_SPLIT   0x80    /* don't return transhuge pages, split them */
> >   #define FOLL_HWPOISON       0x100   /* check page is hwpoisoned */
> >   #define FOLL_NUMA   0x200   /* force NUMA hinting page fault */
> >   #define FOLL_MIGRATION      0x400   /* wait for page to replace migration entry */
> > diff --git a/mm/gup.c b/mm/gup.c
> > index e40579624f10..f3d45a8f18ae 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -435,18 +435,6 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
> >               }
> >       }
> >
> > -     if (flags & FOLL_SPLIT && PageTransCompound(page)) {
> > -             get_page(page);
> > -             pte_unmap_unlock(ptep, ptl);
> > -             lock_page(page);
> > -             ret = split_huge_page(page);
> > -             unlock_page(page);
> > -             put_page(page);
> > -             if (ret)
> > -                     return ERR_PTR(ret);
> > -             goto retry;
> > -     }
> > -
> >       /* try_grab_page() does nothing unless FOLL_GET or FOLL_PIN is set. */
> >       if (unlikely(!try_grab_page(page, flags))) {
> >               page = ERR_PTR(-ENOMEM);
> > @@ -591,7 +579,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
> >               spin_unlock(ptl);
> >               return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
> >       }
> > -     if (flags & (FOLL_SPLIT | FOLL_SPLIT_PMD)) {
> > +     if (flags & FOLL_SPLIT_PMD) {
> >               int ret;
> >               page = pmd_page(*pmd);
> >               if (is_huge_zero_page(page)) {
> > @@ -600,19 +588,7 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
> >                       split_huge_pmd(vma, pmd, address);
> >                       if (pmd_trans_unstable(pmd))
> >                               ret = -EBUSY;
> > -             } else if (flags & FOLL_SPLIT) {
> > -                     if (unlikely(!try_get_page(page))) {
> > -                             spin_unlock(ptl);
> > -                             return ERR_PTR(-ENOMEM);
> > -                     }
> > -                     spin_unlock(ptl);
> > -                     lock_page(page);
> > -                     ret = split_huge_page(page);
> > -                     unlock_page(page);
> > -                     put_page(page);
> > -                     if (pmd_none(*pmd))
> > -                             return no_page_table(vma, flags);
> > -             } else {  /* flags & FOLL_SPLIT_PMD */
> > +             } else {
> >                       spin_unlock(ptl);
> >                       split_huge_pmd(vma, pmd, address);
> >                       ret = pte_alloc(mm, pmd) ? -ENOMEM : 0;
> >
>