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

> + 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

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