Re: [RFC PATCH 17/26] hugetlb: update follow_hugetlb_page to support HGM

[James Houghton <jthoughton@google.com>]

On Tue, Jul 19, 2022 at 3:48 AM manish.mishra <manish.mishra@nutanix.com> wrote:
>
>
> On 24/06/22 11:06 pm, James Houghton wrote:
> > This enables support for GUP, and it is needed for the KVM demand paging
> > self-test to work.
> >
> > One important change here is that, before, we never needed to grab the
> > i_mmap_sem, but now, to prevent someone from collapsing the page tables
> > out from under us, we grab it for reading when doing high-granularity PT
> > walks.
> >
> > Signed-off-by: James Houghton <jthoughton@google.com>
> > ---
> >   mm/hugetlb.c | 70 ++++++++++++++++++++++++++++++++++++++++++----------
> >   1 file changed, 57 insertions(+), 13 deletions(-)
> >
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index f9c7daa6c090..aadfcee947cf 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -6298,14 +6298,18 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> >       unsigned long vaddr = *position;
> >       unsigned long remainder = *nr_pages;
> >       struct hstate *h = hstate_vma(vma);
> > +     struct address_space *mapping = vma->vm_file->f_mapping;
> >       int err = -EFAULT, refs;
> > +     bool has_i_mmap_sem = false;
> >
> >       while (vaddr < vma->vm_end && remainder) {
> >               pte_t *pte;
> >               spinlock_t *ptl = NULL;
> >               bool unshare = false;
> >               int absent;
> > +             unsigned long pages_per_hpte;
> >               struct page *page;
> > +             struct hugetlb_pte hpte;
> >
> >               /*
> >                * If we have a pending SIGKILL, don't keep faulting pages and
> > @@ -6325,9 +6329,23 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> >                */
> >               pte = huge_pte_offset(mm, vaddr & huge_page_mask(h),
> >                                     huge_page_size(h));
> > -             if (pte)
> > -                     ptl = huge_pte_lock(h, mm, pte);
> > -             absent = !pte || huge_pte_none(huge_ptep_get(pte));
> > +             if (pte) {
> > +                     hugetlb_pte_populate(&hpte, pte, huge_page_shift(h));
> > +                     if (hugetlb_hgm_enabled(vma)) {
> > +                             BUG_ON(has_i_mmap_sem);
>
> Just thinking can we do without i_mmap_lock_read in most cases. Like earlier
>
> this function was good without i_mmap_lock_read doing almost everything
>
> which is happening now?

We need something to prevent the page tables from being rearranged
while we're walking them. In this RFC, I used the i_mmap_lock. I'm
going to change it, probably to a per-VMA lock (or maybe a per-hpage
lock. I'm trying to figure out if a system with PTLs/hugetlb_pte_lock
could work too :)).

>
> > +                             i_mmap_lock_read(mapping);
> > +                             /*
> > +                              * Need to hold the mapping semaphore for
> > +                              * reading to do a HGM walk.
> > +                              */
> > +                             has_i_mmap_sem = true;
> > +                             hugetlb_walk_to(mm, &hpte, vaddr, PAGE_SIZE,
> > +                                             /*stop_at_none=*/true);
> > +                     }
> > +                     ptl = hugetlb_pte_lock(mm, &hpte);
> > +             }
> > +
> > +             absent = !pte || hugetlb_pte_none(&hpte);
> >
> >               /*
> >                * When coredumping, it suits get_dump_page if we just return
> > @@ -6338,8 +6356,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> >                */
> >               if (absent && (flags & FOLL_DUMP) &&
> >                   !hugetlbfs_pagecache_present(h, vma, vaddr)) {
> > -                     if (pte)
> > +                     if (pte) {
> > +                             if (has_i_mmap_sem) {
> > +                                     i_mmap_unlock_read(mapping);
> > +                                     has_i_mmap_sem = false;
> > +                             }
> >                               spin_unlock(ptl);
> > +                     }
> >                       remainder = 0;
> >                       break;
> >               }
> > @@ -6359,8 +6382,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> >                       vm_fault_t ret;
> >                       unsigned int fault_flags = 0;
> >
> > -                     if (pte)
> > +                     if (pte) {
> > +                             if (has_i_mmap_sem) {
> > +                                     i_mmap_unlock_read(mapping);
> > +                                     has_i_mmap_sem = false;
> > +                             }
> >                               spin_unlock(ptl);
> > +                     }
> >                       if (flags & FOLL_WRITE)
> >                               fault_flags |= FAULT_FLAG_WRITE;
> >                       else if (unshare)
> > @@ -6403,8 +6431,11 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> >                       continue;
> >               }
> >
> > -             pfn_offset = (vaddr & ~huge_page_mask(h)) >> PAGE_SHIFT;
> > -             page = pte_page(huge_ptep_get(pte));
> > +             pfn_offset = (vaddr & ~hugetlb_pte_mask(&hpte)) >> PAGE_SHIFT;
> > +             page = pte_page(hugetlb_ptep_get(&hpte));
> > +             pages_per_hpte = hugetlb_pte_size(&hpte) / PAGE_SIZE;
> > +             if (hugetlb_hgm_enabled(vma))
> > +                     page = compound_head(page);
> >
> >               VM_BUG_ON_PAGE((flags & FOLL_PIN) && PageAnon(page) &&
> >                              !PageAnonExclusive(page), page);
> > @@ -6414,17 +6445,21 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> >                * and skip the same_page loop below.
> >                */
> >               if (!pages && !vmas && !pfn_offset &&
> > -                 (vaddr + huge_page_size(h) < vma->vm_end) &&
> > -                 (remainder >= pages_per_huge_page(h))) {
> > -                     vaddr += huge_page_size(h);
> > -                     remainder -= pages_per_huge_page(h);
> > -                     i += pages_per_huge_page(h);
> > +                 (vaddr + pages_per_hpte < vma->vm_end) &&
> > +                 (remainder >= pages_per_hpte)) {
> > +                     vaddr += pages_per_hpte;
> > +                     remainder -= pages_per_hpte;
> > +                     i += pages_per_hpte;
> >                       spin_unlock(ptl);
> > +                     if (has_i_mmap_sem) {
> > +                             has_i_mmap_sem = false;
> > +                             i_mmap_unlock_read(mapping);
> > +                     }
> >                       continue;
> >               }
> >
> >               /* vaddr may not be aligned to PAGE_SIZE */
> > -             refs = min3(pages_per_huge_page(h) - pfn_offset, remainder,
> > +             refs = min3(pages_per_hpte - pfn_offset, remainder,
> >                   (vma->vm_end - ALIGN_DOWN(vaddr, PAGE_SIZE)) >> PAGE_SHIFT);
> >
> >               if (pages || vmas)
> > @@ -6447,6 +6482,10 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> >                       if (WARN_ON_ONCE(!try_grab_folio(pages[i], refs,
> >                                                        flags))) {
> >                               spin_unlock(ptl);
> > +                             if (has_i_mmap_sem) {
> > +                                     has_i_mmap_sem = false;
> > +                                     i_mmap_unlock_read(mapping);
> > +                             }
> >                               remainder = 0;
> >                               err = -ENOMEM;
> >                               break;
> > @@ -6458,8 +6497,13 @@ long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma,
> >               i += refs;
> >
> >               spin_unlock(ptl);
> > +             if (has_i_mmap_sem) {
> > +                     has_i_mmap_sem = false;
> > +                     i_mmap_unlock_read(mapping);
> > +             }
> >       }
> >       *nr_pages = remainder;
> > +     BUG_ON(has_i_mmap_sem);
> >       /*
> >        * setting position is actually required only if remainder is
> >        * not zero but it's faster not to add a "if (remainder)"
>
> Thanks
>
> Manish Mishra
>