Re: [RFC PATCH v2 34/47] hugetlb: userfaultfd: add support for high-granularity UFFDIO_CONTINUE

From: Peter Xu
Date: Thu Nov 17 2022 - 12:00:12 EST

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On Fri, Oct 21, 2022 at 04:36:50PM +0000, James Houghton wrote:
> Changes here are similar to the changes made for hugetlb_no_page.
>
> Pass vmf->real_address to userfaultfd_huge_must_wait because
> vmf->address is rounded down to the hugepage size, and a
> high-granularity page table walk would look up the wrong PTE. Also
> change the call to userfaultfd_must_wait in the same way for
> consistency.
>
> This commit introduces hugetlb_alloc_largest_pte which is used to find
> the appropriate PTE size to map pages with UFFDIO_CONTINUE.
>
> Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
> ---
> fs/userfaultfd.c | 33 +++++++++++++++---
> include/linux/hugetlb.h | 14 +++++++-
> mm/hugetlb.c | 76 +++++++++++++++++++++++++++++++++--------
> mm/userfaultfd.c | 46 +++++++++++++++++--------
> 4 files changed, 135 insertions(+), 34 deletions(-)
>
> diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> index 3a3e9ef74dab..0204108e3882 100644
> --- a/fs/userfaultfd.c
> +++ b/fs/userfaultfd.c
> @@ -245,14 +245,22 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
> struct mm_struct *mm = ctx->mm;
> pte_t *ptep, pte;
> bool ret = true;
> + struct hugetlb_pte hpte;
> + unsigned long sz = vma_mmu_pagesize(vma);
> + unsigned int shift = huge_page_shift(hstate_vma(vma));
>
> mmap_assert_locked(mm);
>
> - ptep = huge_pte_offset(mm, address, vma_mmu_pagesize(vma));
> + ptep = huge_pte_offset(mm, address, sz);
>
> if (!ptep)
> goto out;
>
> + hugetlb_pte_populate(&hpte, ptep, shift, hpage_size_to_level(sz));
> + hugetlb_hgm_walk(mm, vma, &hpte, address, PAGE_SIZE,
> + /*stop_at_none=*/true);

Side note: I had a feeling that we may want a helper function to walk the
whole hugetlb pgtable, that may contain huge_pte_offset() and the hgm walk.
Not really needed for this series, but maybe for the future.

> + ptep = hpte.ptep;
> +
> ret = false;
> pte = huge_ptep_get(ptep);
>
> @@ -498,6 +506,14 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
>
> blocking_state = userfaultfd_get_blocking_state(vmf->flags);
>
> + if (is_vm_hugetlb_page(vmf->vma) && hugetlb_hgm_enabled(vmf->vma))
> + /*
> + * Lock the VMA lock so we can do a high-granularity walk in
> + * userfaultfd_huge_must_wait. We have to grab this lock before
> + * we set our state to blocking.
> + */
> + hugetlb_vma_lock_read(vmf->vma);

Yeah this will help with/without hgm, afaict. Maybe when I rework my other
patchset I'll just take the vma lock unconditionally for this path.

> +
> spin_lock_irq(&ctx->fault_pending_wqh.lock);
> /*
> * After the __add_wait_queue the uwq is visible to userland
> @@ -513,12 +529,15 @@ vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
> spin_unlock_irq(&ctx->fault_pending_wqh.lock);
>
> if (!is_vm_hugetlb_page(vmf->vma))
> - must_wait = userfaultfd_must_wait(ctx, vmf->address, vmf->flags,
> - reason);
> + must_wait = userfaultfd_must_wait(ctx, vmf->real_address,
> + vmf->flags, reason);
> else
> must_wait = userfaultfd_huge_must_wait(ctx, vmf->vma,
> - vmf->address,
> + vmf->real_address,
> vmf->flags, reason);
> +
> + if (is_vm_hugetlb_page(vmf->vma) && hugetlb_hgm_enabled(vmf->vma))
> + hugetlb_vma_unlock_read(vmf->vma);
> mmap_read_unlock(mm);
>
> if (likely(must_wait && !READ_ONCE(ctx->released))) {
> @@ -1463,6 +1482,12 @@ static int userfaultfd_register(struct userfaultfd_ctx *ctx,
> mas_pause(&mas);
> }
> next:
> + if (is_vm_hugetlb_page(vma) && (ctx->features &
> + UFFD_FEATURE_MINOR_HUGETLBFS_HGM)) {
> + ret = enable_hugetlb_hgm(vma);
> + if (ret)
> + break;

[1]

> + }
> /*
> * In the vma_merge() successful mprotect-like case 8:
> * the next vma was merged into the current one and
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index e25f97cdd086..00c22a84a1c6 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -250,7 +250,8 @@ unsigned long hugetlb_total_pages(void);
> vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> unsigned long address, unsigned int flags);
> #ifdef CONFIG_USERFAULTFD
> -int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm, pte_t *dst_pte,
> +int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> + struct hugetlb_pte *dst_hpte,
> struct vm_area_struct *dst_vma,
> unsigned long dst_addr,
> unsigned long src_addr,
> @@ -1272,6 +1273,9 @@ static inline enum hugetlb_level hpage_size_to_level(unsigned long sz)
> bool hugetlb_hgm_enabled(struct vm_area_struct *vma);
> bool hugetlb_hgm_eligible(struct vm_area_struct *vma);
> int enable_hugetlb_hgm(struct vm_area_struct *vma);
> +int hugetlb_alloc_largest_pte(struct hugetlb_pte *hpte, struct mm_struct *mm,
> + struct vm_area_struct *vma, unsigned long start,
> + unsigned long end);
> #else
> static inline bool hugetlb_hgm_enabled(struct vm_area_struct *vma)
> {
> @@ -1285,6 +1289,14 @@ static inline int enable_hugetlb_hgm(struct vm_area_struct *vma)
> {
> return -EINVAL;
> }
> +
> +static inline
> +int hugetlb_alloc_largest_pte(struct hugetlb_pte *hpte, struct mm_struct *mm,
> + struct vm_area_struct *vma, unsigned long start,
> + unsigned long end)
> +{
> + return -EINVAL;
> +}
> #endif
>
> static inline spinlock_t *huge_pte_lock(struct hstate *h,
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 6eaec40d66ad..c25d3cd73ac9 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6325,7 +6325,7 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
> * modifications for huge pages.
> */
> int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> - pte_t *dst_pte,
> + struct hugetlb_pte *dst_hpte,
> struct vm_area_struct *dst_vma,
> unsigned long dst_addr,
> unsigned long src_addr,
> @@ -6336,13 +6336,14 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> bool is_continue = (mode == MCOPY_ATOMIC_CONTINUE);
> struct hstate *h = hstate_vma(dst_vma);
> struct address_space *mapping = dst_vma->vm_file->f_mapping;
> - pgoff_t idx = vma_hugecache_offset(h, dst_vma, dst_addr);
> + unsigned long haddr = dst_addr & huge_page_mask(h);
> + pgoff_t idx = vma_hugecache_offset(h, dst_vma, haddr);
> unsigned long size;
> int vm_shared = dst_vma->vm_flags & VM_SHARED;
> pte_t _dst_pte;
> spinlock_t *ptl;
> int ret = -ENOMEM;
> - struct page *page;
> + struct page *page, *subpage;
> int writable;
> bool page_in_pagecache = false;
>
> @@ -6357,12 +6358,12 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> * a non-missing case. Return -EEXIST.
> */
> if (vm_shared &&
> - hugetlbfs_pagecache_present(h, dst_vma, dst_addr)) {
> + hugetlbfs_pagecache_present(h, dst_vma, haddr)) {
> ret = -EEXIST;
> goto out;
> }
>
> - page = alloc_huge_page(dst_vma, dst_addr, 0);
> + page = alloc_huge_page(dst_vma, haddr, 0);
> if (IS_ERR(page)) {
> ret = -ENOMEM;
> goto out;
> @@ -6378,13 +6379,13 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> /* Free the allocated page which may have
> * consumed a reservation.
> */
> - restore_reserve_on_error(h, dst_vma, dst_addr, page);
> + restore_reserve_on_error(h, dst_vma, haddr, page);
> put_page(page);
>
> /* Allocate a temporary page to hold the copied
> * contents.
> */
> - page = alloc_huge_page_vma(h, dst_vma, dst_addr);
> + page = alloc_huge_page_vma(h, dst_vma, haddr);
> if (!page) {
> ret = -ENOMEM;
> goto out;
> @@ -6398,14 +6399,14 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> }
> } else {
> if (vm_shared &&
> - hugetlbfs_pagecache_present(h, dst_vma, dst_addr)) {
> + hugetlbfs_pagecache_present(h, dst_vma, haddr)) {
> put_page(*pagep);
> ret = -EEXIST;
> *pagep = NULL;
> goto out;
> }
>
> - page = alloc_huge_page(dst_vma, dst_addr, 0);
> + page = alloc_huge_page(dst_vma, haddr, 0);
> if (IS_ERR(page)) {
> put_page(*pagep);
> ret = -ENOMEM;
> @@ -6447,7 +6448,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> page_in_pagecache = true;
> }
>
> - ptl = huge_pte_lock(h, dst_mm, dst_pte);
> + ptl = hugetlb_pte_lock(dst_mm, dst_hpte);
>
> ret = -EIO;
> if (PageHWPoison(page))
> @@ -6459,7 +6460,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> * page backing it, then access the page.
> */
> ret = -EEXIST;
> - if (!huge_pte_none_mostly(huge_ptep_get(dst_pte)))
> + if (!huge_pte_none_mostly(huge_ptep_get(dst_hpte->ptep)))
> goto out_release_unlock;
>
> if (page_in_pagecache) {
> @@ -6478,7 +6479,11 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> else
> writable = dst_vma->vm_flags & VM_WRITE;
>
> - _dst_pte = make_huge_pte(dst_vma, page, writable);
> + subpage = hugetlb_find_subpage(h, page, dst_addr);
> + WARN_ON_ONCE(subpage != page && !hugetlb_hgm_enabled(dst_vma));
> +
> + _dst_pte = make_huge_pte_with_shift(dst_vma, subpage, writable,
> + dst_hpte->shift);
> /*
> * Always mark UFFDIO_COPY page dirty; note that this may not be
> * extremely important for hugetlbfs for now since swapping is not
> @@ -6491,12 +6496,12 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> if (wp_copy)
> _dst_pte = huge_pte_mkuffd_wp(_dst_pte);
>
> - set_huge_pte_at(dst_mm, dst_addr, dst_pte, _dst_pte);
> + set_huge_pte_at(dst_mm, dst_addr, dst_hpte->ptep, _dst_pte);
>
> - hugetlb_count_add(pages_per_huge_page(h), dst_mm);
> + hugetlb_count_add(hugetlb_pte_size(dst_hpte) / PAGE_SIZE, dst_mm);
>
> /* No need to invalidate - it was non-present before */
> - update_mmu_cache(dst_vma, dst_addr, dst_pte);
> + update_mmu_cache(dst_vma, dst_addr, dst_hpte->ptep);
>
> spin_unlock(ptl);
> if (!is_continue)
> @@ -7875,6 +7880,47 @@ static unsigned int __shift_for_hstate(struct hstate *h)
> (tmp_h) <= &hstates[hugetlb_max_hstate]; \
> (tmp_h)++)
>
> +/*
> + * Allocate a HugeTLB PTE that maps as much of [start, end) as possible with a
> + * single page table entry. The allocated HugeTLB PTE is returned in @hpte.
> + */
> +int hugetlb_alloc_largest_pte(struct hugetlb_pte *hpte, struct mm_struct *mm,
> + struct vm_area_struct *vma, unsigned long start,
> + unsigned long end)
> +{
> + struct hstate *h = hstate_vma(vma), *tmp_h;
> + unsigned int shift;
> + unsigned long sz;
> + int ret;
> + pte_t *ptep;
> +
> + for_each_hgm_shift(h, tmp_h, shift) {

The fallback to PAGE_SIZE (in __shift_for_hstate()) is not obvious. Would
it be clearer we handle PAGE_SIZE explicitly here?

> + sz = 1UL << shift;
> +
> + if (!IS_ALIGNED(start, sz) || start + sz > end)
> + continue;
> + goto found;
> + }
> + return -EINVAL;
> +found:
> + ptep = huge_pte_alloc(mm, vma, start, huge_page_size(h));
> + if (!ptep)
> + return -ENOMEM;
> +
> + hugetlb_pte_populate(hpte, ptep, huge_page_shift(h),
> + hpage_size_to_level(huge_page_size(h)));
> +
> + ret = hugetlb_hgm_walk(mm, vma, hpte, start, 1L << shift,
> + /*stop_at_none=*/false);
> + if (ret)
> + return ret;
> +
> + if (hpte->shift > shift)
> + return -EEXIST;
> +
> + return 0;
> +}
> +
> #endif /* CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING */
>
> /*
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index e24e8a47ce8a..c4a8e6666ea6 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -315,14 +315,16 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> {
> int vm_shared = dst_vma->vm_flags & VM_SHARED;
> ssize_t err;
> - pte_t *dst_pte;
> unsigned long src_addr, dst_addr;
> long copied;
> struct page *page;
> - unsigned long vma_hpagesize;
> + unsigned long vma_hpagesize, target_pagesize;
> pgoff_t idx;
> u32 hash;
> struct address_space *mapping;
> + bool use_hgm = uffd_ctx_has_hgm(&dst_vma->vm_userfaultfd_ctx) &&
> + mode == MCOPY_ATOMIC_CONTINUE;
> + struct hstate *h = hstate_vma(dst_vma);
>
> /*
> * There is no default zero huge page for all huge page sizes as
> @@ -340,12 +342,13 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> copied = 0;
> page = NULL;
> vma_hpagesize = vma_kernel_pagesize(dst_vma);
> + target_pagesize = use_hgm ? PAGE_SIZE : vma_hpagesize;

Nit: "target_pagesize" is slightly misleading? Because hgm can do e.g. 2M
on 1G too. I feel like what you want to check here is the minimum
requirement, hence.. "min_pagesize"?

>
> /*
> - * Validate alignment based on huge page size
> + * Validate alignment based on the targeted page size.
> */
> err = -EINVAL;
> - if (dst_start & (vma_hpagesize - 1) || len & (vma_hpagesize - 1))
> + if (dst_start & (target_pagesize - 1) || len & (target_pagesize - 1))
> goto out_unlock;
>
> retry:
> @@ -362,6 +365,8 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> err = -EINVAL;
> if (vma_hpagesize != vma_kernel_pagesize(dst_vma))
> goto out_unlock;
> + if (use_hgm && !hugetlb_hgm_enabled(dst_vma))
> + goto out_unlock;

Nit: this seems not needed, because enabling the hgm feature for uffd
requires to enable it in hugetlb already when register uffd above [1].

>
> vm_shared = dst_vma->vm_flags & VM_SHARED;
> }
> @@ -376,13 +381,15 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> }
>
> while (src_addr < src_start + len) {
> + struct hugetlb_pte hpte;
> + pte_t *dst_pte;
> BUG_ON(dst_addr >= dst_start + len);
>
> /*
> * Serialize via vma_lock and hugetlb_fault_mutex.
> - * vma_lock ensures the dst_pte remains valid even
> - * in the case of shared pmds. fault mutex prevents
> - * races with other faulting threads.
> + * vma_lock ensures the hpte.ptep remains valid even
> + * in the case of shared pmds and page table collapsing.
> + * fault mutex prevents races with other faulting threads.
> */
> idx = linear_page_index(dst_vma, dst_addr);
> mapping = dst_vma->vm_file->f_mapping;
> @@ -390,23 +397,33 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> mutex_lock(&hugetlb_fault_mutex_table[hash]);
> hugetlb_vma_lock_read(dst_vma);
>
> - err = -ENOMEM;
> + err = 0;
> dst_pte = huge_pte_alloc(dst_mm, dst_vma, dst_addr, vma_hpagesize);
> - if (!dst_pte) {
> + if (!dst_pte)
> + err = -ENOMEM;
> + else {
> + hugetlb_pte_populate(&hpte, dst_pte, huge_page_shift(h),
> + hpage_size_to_level(huge_page_size(h)));
> + if (use_hgm)
> + err = hugetlb_alloc_largest_pte(&hpte,
> + dst_mm, dst_vma, dst_addr,
> + dst_start + len);

dst_addr, not dst_start?

> + }
> + if (err) {
> hugetlb_vma_unlock_read(dst_vma);
> mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> goto out_unlock;
> }
>
> if (mode != MCOPY_ATOMIC_CONTINUE &&
> - !huge_pte_none_mostly(huge_ptep_get(dst_pte))) {
> + !huge_pte_none_mostly(huge_ptep_get(hpte.ptep))) {
> err = -EEXIST;
> hugetlb_vma_unlock_read(dst_vma);
> mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> goto out_unlock;
> }
>
> - err = hugetlb_mcopy_atomic_pte(dst_mm, dst_pte, dst_vma,
> + err = hugetlb_mcopy_atomic_pte(dst_mm, &hpte, dst_vma,
> dst_addr, src_addr, mode, &page,
> wp_copy);
>
> @@ -418,6 +435,7 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> if (unlikely(err == -ENOENT)) {
> mmap_read_unlock(dst_mm);
> BUG_ON(!page);
> + BUG_ON(hpte.shift != huge_page_shift(h));
>
> err = copy_huge_page_from_user(page,
> (const void __user *)src_addr,
> @@ -435,9 +453,9 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> BUG_ON(page);
>
> if (!err) {
> - dst_addr += vma_hpagesize;
> - src_addr += vma_hpagesize;
> - copied += vma_hpagesize;
> + dst_addr += hugetlb_pte_size(&hpte);
> + src_addr += hugetlb_pte_size(&hpte);
> + copied += hugetlb_pte_size(&hpte);
>
> if (fatal_signal_pending(current))
> err = -EINTR;
> --
> 2.38.0.135.g90850a2211-goog
>

--
Peter Xu

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