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SubjectRe: [RFC PATCH 20/26] hugetlb: add support for high-granularity UFFDIO_CONTINUE
On Fri, Jul 15, 2022 at 9:21 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Fri, Jun 24, 2022 at 05:36:50PM +0000, James Houghton wrote:
> > The changes here are very similar to the changes made to
> > hugetlb_no_page, where we do a high-granularity page table walk and
> > do accounting slightly differently because we are mapping only a piece
> > of a page.
> >
> > Signed-off-by: James Houghton <jthoughton@google.com>
> > ---
> > fs/userfaultfd.c | 3 +++
> > include/linux/hugetlb.h | 6 +++--
> > mm/hugetlb.c | 54 +++++++++++++++++++++-----------------
> > mm/userfaultfd.c | 57 +++++++++++++++++++++++++++++++----------
> > 4 files changed, 82 insertions(+), 38 deletions(-)
> >
> > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c
> > index e943370107d0..77c1b8a7d0b9 100644
> > --- a/fs/userfaultfd.c
> > +++ b/fs/userfaultfd.c
> > @@ -245,6 +245,9 @@ static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
> > if (!ptep)
> > goto out;
> >
> > + if (hugetlb_hgm_enabled(vma))
> > + goto out;
> > +
>
> This is weird. It means we'll never wait for sub-page mapping enabled
> vmas. Why?
>

`ret` is true in this case, so we're actually *always* waiting.

> Not to mention hugetlb_hgm_enabled() currently is simply VM_SHARED, so it
> means we'll stop waiting for all shared hugetlbfs uffd page faults..
>
> I'd expect in the in-house postcopy tests you should see vcpu threads
> spinning on the page faults until it's serviced.
>
> IMO we still need to properly wait when the pgtable doesn't have the
> faulted address covered. For sub-page mapping it'll probably need to walk
> into sub-page levels.

Ok, SGTM. I'll do that for the next version. I'm not sure of the
consequences of returning `true` here when we should be returning
`false`.

>
> > ret = false;
> > pte = huge_ptep_get(ptep);
> >
> > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> > index ac4ac8fbd901..c207b1ac6195 100644
> > --- a/include/linux/hugetlb.h
> > +++ b/include/linux/hugetlb.h
> > @@ -221,13 +221,15 @@ 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,
> > enum mcopy_atomic_mode mode,
> > struct page **pagep,
> > - bool wp_copy);
> > + bool wp_copy,
> > + bool new_mapping);
> > #endif /* CONFIG_USERFAULTFD */
> > bool hugetlb_reserve_pages(struct inode *inode, long from, long to,
> > struct vm_area_struct *vma,
> > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > index 0ec2f231524e..09fa57599233 100644
> > --- a/mm/hugetlb.c
> > +++ b/mm/hugetlb.c
> > @@ -5808,6 +5808,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
> > vma_end_reservation(h, vma, haddr);
> > }
> >
> > + /* This lock will get pretty expensive at 4K. */
> > ptl = hugetlb_pte_lock(mm, hpte);
> > ret = 0;
> > /* If pte changed from under us, retry */
> > @@ -6098,24 +6099,26 @@ 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,
> > enum mcopy_atomic_mode mode,
> > struct page **pagep,
> > - bool wp_copy)
> > + bool wp_copy,
> > + bool new_mapping)
> > {
> > bool is_continue = (mode == MCOPY_ATOMIC_CONTINUE);
> > struct hstate *h = hstate_vma(dst_vma);
> > struct address_space *mapping = dst_vma->vm_file->f_mapping;
> > + unsigned long haddr = dst_addr & huge_page_mask(h);
> > pgoff_t idx = vma_hugecache_offset(h, dst_vma, dst_addr);
> > 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;
> >
> > @@ -6130,12 +6133,12 @@ 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;
> > @@ -6151,13 +6154,13 @@ 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;
> > @@ -6171,14 +6174,14 @@ 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)) {
> > ret = -ENOMEM;
> > *pagep = NULL;
> > @@ -6216,8 +6219,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> > page_in_pagecache = true;
> > }
> >
> > - ptl = huge_pte_lockptr(huge_page_shift(h), dst_mm, dst_pte);
> > - spin_lock(ptl);
> > + ptl = hugetlb_pte_lock(dst_mm, dst_hpte);
> >
> > /*
> > * Recheck the i_size after holding PT lock to make sure not
> > @@ -6239,14 +6241,16 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
> > * registered, we firstly wr-protect a none pte which has no page cache
> > * page backing it, then access the page.
> > */
> > - if (!huge_pte_none_mostly(huge_ptep_get(dst_pte)))
> > + if (!hugetlb_pte_none_mostly(dst_hpte))
> > goto out_release_unlock;
> >
> > - if (vm_shared) {
> > - page_dup_file_rmap(page, true);
> > - } else {
> > - ClearHPageRestoreReserve(page);
> > - hugepage_add_new_anon_rmap(page, dst_vma, dst_addr);
> > + if (new_mapping) {
>
> IIUC you wanted to avoid the mapcount accountings when it's the sub-page
> that was going to be mapped.
>
> Is it a must we get this only from the caller? Can we know we're doing
> sub-page mapping already here and make a decision with e.g. dst_hpte?
>
> It looks weird to me to pass this explicitly from the caller, especially
> that's when we don't really have the pgtable lock so I'm wondering about
> possible race conditions too on having stale new_mapping values.

The only way to know what the correct value for `new_mapping` should
be is to know if we had to change the hstate-level P*D to non-none to
service this UFFDIO_CONTINUE request. I'll see if there is a nice way
to do that check in `hugetlb_mcopy_atomic_pte`. Right now there is no
race, because we synchronize on the per-hpage mutex.

>
> > + if (vm_shared) {
> > + page_dup_file_rmap(page, true);
> > + } else {
> > + ClearHPageRestoreReserve(page);
> > + hugepage_add_new_anon_rmap(page, dst_vma, haddr);
> > + }
> > }
> >
> > /*
> > @@ -6258,7 +6262,11 @@ 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);
> > + if (subpage != page)
> > + BUG_ON(!hugetlb_hgm_enabled(dst_vma));
> > +
> > + _dst_pte = make_huge_pte(dst_vma, subpage, writable);
> > /*
> > * Always mark UFFDIO_COPY page dirty; note that this may not be
> > * extremely important for hugetlbfs for now since swapping is not
> > @@ -6271,14 +6279,14 @@ 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);
> >
> > - (void)huge_ptep_set_access_flags(dst_vma, dst_addr, dst_pte, _dst_pte,
> > - dst_vma->vm_flags & VM_WRITE);
> > - hugetlb_count_add(pages_per_huge_page(h), dst_mm);
> > + (void)huge_ptep_set_access_flags(dst_vma, dst_addr, dst_hpte->ptep,
> > + _dst_pte, dst_vma->vm_flags & VM_WRITE);
> > + 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)
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index 4f4892a5f767..ee40d98068bf 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -310,14 +310,16 @@ 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, vma_altpagesize;
> > pgoff_t idx;
> > u32 hash;
> > struct address_space *mapping;
> > + bool use_hgm = hugetlb_hgm_enabled(dst_vma) &&
> > + mode == MCOPY_ATOMIC_CONTINUE;
> > + struct hstate *h = hstate_vma(dst_vma);
> >
> > /*
> > * There is no default zero huge page for all huge page sizes as
> > @@ -335,12 +337,16 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> > copied = 0;
> > page = NULL;
> > vma_hpagesize = vma_kernel_pagesize(dst_vma);
> > + if (use_hgm)
> > + vma_altpagesize = PAGE_SIZE;
>
> Do we need to check the "len" to know whether we should use sub-page
> mapping or original hpage size? E.g. any old UFFDIO_CONTINUE code will
> still want the old behavior I think.

I think that's a fair point; however, if we enable HGM and the address
and len happen to be hstate-aligned, we basically do the same thing as
if HGM wasn't enabled. It could be a minor performance optimization to
do `vma_altpagesize=vma_hpagesize` in that case, but in terms of how
the page tables are set up, the end result would be the same.

>
> > + else
> > + vma_altpagesize = vma_hpagesize;
> >
> > /*
> > * Validate alignment based on huge page size
> > */
> > err = -EINVAL;
> > - if (dst_start & (vma_hpagesize - 1) || len & (vma_hpagesize - 1))
> > + if (dst_start & (vma_altpagesize - 1) || len & (vma_altpagesize - 1))
> > goto out_unlock;
> >
> > retry:
> > @@ -361,6 +367,8 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> > vm_shared = dst_vma->vm_flags & VM_SHARED;
> > }
> >
> > + BUG_ON(!vm_shared && use_hgm);
> > +
> > /*
> > * If not shared, ensure the dst_vma has a anon_vma.
> > */
> > @@ -371,11 +379,13 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> > }
> >
> > while (src_addr < src_start + len) {
> > + struct hugetlb_pte hpte;
> > + bool new_mapping;
> > BUG_ON(dst_addr >= dst_start + len);
> >
> > /*
> > * Serialize via i_mmap_rwsem and hugetlb_fault_mutex.
> > - * i_mmap_rwsem ensures the dst_pte remains valid even
> > + * i_mmap_rwsem ensures the hpte.ptep remains valid even
> > * in the case of shared pmds. fault mutex prevents
> > * races with other faulting threads.
> > */
> > @@ -383,27 +393,47 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
> > i_mmap_lock_read(mapping);
> > idx = linear_page_index(dst_vma, dst_addr);
> > hash = hugetlb_fault_mutex_hash(mapping, idx);
> > + /* This lock will get expensive at 4K. */
> > mutex_lock(&hugetlb_fault_mutex_table[hash]);
> >
> > - err = -ENOMEM;
> > - dst_pte = huge_pte_alloc(dst_mm, dst_vma, dst_addr, vma_hpagesize);
> > - if (!dst_pte) {
> > + err = 0;
> > +
> > + pte_t *ptep = huge_pte_alloc(dst_mm, dst_vma, dst_addr,
> > + vma_hpagesize);
> > + if (!ptep)
> > + err = -ENOMEM;
> > + else {
> > + hugetlb_pte_populate(&hpte, ptep,
> > + huge_page_shift(h));
> > + /*
> > + * If the hstate-level PTE is not none, then a mapping
> > + * was previously established.
> > + * The per-hpage mutex prevents double-counting.
> > + */
> > + new_mapping = hugetlb_pte_none(&hpte);
> > + if (use_hgm)
> > + err = hugetlb_alloc_largest_pte(&hpte, dst_mm, dst_vma,
> > + dst_addr,
> > + dst_start + len);
> > + }
> > +
> > + if (err) {
> > mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> > i_mmap_unlock_read(mapping);
> > goto out_unlock;
> > }
> >
> > if (mode != MCOPY_ATOMIC_CONTINUE &&
> > - !huge_pte_none_mostly(huge_ptep_get(dst_pte))) {
> > + !hugetlb_pte_none_mostly(&hpte)) {
> > err = -EEXIST;
> > mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> > i_mmap_unlock_read(mapping);
> > 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);
> > + wp_copy, new_mapping);
> >
> > mutex_unlock(&hugetlb_fault_mutex_table[hash]);
> > i_mmap_unlock_read(mapping);
> > @@ -413,6 +443,7 @@ 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,
> > @@ -430,9 +461,9 @@ 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;
> > --
> > 2.37.0.rc0.161.g10f37bed90-goog
> >
>
> --
> Peter Xu
>

Thanks, Peter! :)

- James

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