Re: [External] Re: [PATCH v2 2/6] mm: hugetlbfs: fix cannot migrate the fallocated HugeTLB page

[Muchun Song <songmuchun@bytedance.com>]

On Thu, Jan 7, 2021 at 12:35 AM Michal Hocko <mhocko@suse.com> wrote:
>
> On Wed 06-01-21 16:47:35, Muchun Song wrote:
> > Because we only can isolate a active page via isolate_huge_page()
> > and hugetlbfs_fallocate() forget to mark it as active, we cannot
> > isolate and migrate those pages.
>
> I've little bit hard time to understand this initially and had to dive
> into the code to make sense of it. I would consider the following
> wording easier to grasp. Feel free to reuse if you like.
> "
> If a new hugetlb page is allocated during fallocate it will not be
> marked as active (set_page_huge_active) which will result in a later
> isolate_huge_page failure when the page migration code would like to
> move that page. Such a failure would be unexpected and wrong.
> "

Thanks for your suggestion. I will reuse it.

>
> Now to the fix. I believe that this patch shows that the
> set_page_huge_active is just too subtle. Is there any reason why we
> cannot make all freshly allocated huge pages active by default?
>
> > Only export set_page_huge_active, just leave clear_page_huge_active
> > as static. Because there are no external users.
> >
> > Fixes: 70c3547e36f5 (hugetlbfs: add hugetlbfs_fallocate())
> > Signed-off-by: Muchun Song <songmuchun@bytedance.com>
> > ---
> >  fs/hugetlbfs/inode.c    | 3 ++-
> >  include/linux/hugetlb.h | 2 ++
> >  mm/hugetlb.c            | 2 +-
> >  3 files changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
> > index b5c109703daa..21c20fd5f9ee 100644
> > --- a/fs/hugetlbfs/inode.c
> > +++ b/fs/hugetlbfs/inode.c
> > @@ -735,9 +735,10 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
> >
> >               mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> >
> > +             set_page_huge_active(page);
> >               /*
> >                * unlock_page because locked by add_to_page_cache()
> > -              * page_put due to reference from alloc_huge_page()
> > +              * put_page() due to reference from alloc_huge_page()
> >                */
> >               unlock_page(page);
> >               put_page(page);
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index ebca2ef02212..b5807f23caf8 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -770,6 +770,8 @@ static inline void huge_ptep_modify_prot_commit(struct vm_area_struct *vma,
> >  }
> >  #endif
> >
> > +void set_page_huge_active(struct page *page);
> > +
> >  #else        /* CONFIG_HUGETLB_PAGE */
> >  struct hstate {};
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 1f3bf1710b66..4741d60f8955 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -1348,7 +1348,7 @@ bool page_huge_active(struct page *page)
> >  }
> >
> >  /* never called for tail page */
> > -static void set_page_huge_active(struct page *page)
> > +void set_page_huge_active(struct page *page)
> >  {
> >       VM_BUG_ON_PAGE(!PageHeadHuge(page), page);
> >       SetPagePrivate(&page[1]);
> > --
> > 2.11.0
>
> --
> Michal Hocko
> SUSE Labs