On 04/09/2024 11:09, Dev Jain wrote:
> Introduce do_huge_zero_wp_pmd() to handle wp-fault on a hugezeropage and
> replace it with a PMD-mapped THP. Change the helpers introduced in the
> previous patch to flush TLB entry corresponding to the hugezeropage,
> and preserve PMD uffd-wp marker. In case of failure, fallback to
> splitting the PMD.
>
> Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
> ---
> include/linux/huge_mm.h | 6 ++++
> mm/huge_memory.c | 79 +++++++++++++++++++++++++++++++++++------
> mm/memory.c | 5 +--
> 3 files changed, 78 insertions(+), 12 deletions(-)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index e25d9ebfdf89..fdd2cf473a3c 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -9,6 +9,12 @@
> #include <linux/kobject.h>
>
> vm_fault_t do_huge_pmd_anonymous_page(struct vm_fault *vmf);
> +vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
> + unsigned long haddr, struct folio **foliop,
> + unsigned long addr);
> +void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
> + struct vm_area_struct *vma, unsigned long haddr,
> + pgtable_t pgtable);
I don't think you are using either of these outside of huge_memory.c, so not
sure you need to declare them here or make them non-static?
> int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> pmd_t *dst_pmd, pmd_t *src_pmd, unsigned long addr,
> struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma);
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 58125fbcc532..150163ad77d3 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -943,9 +943,9 @@ unsigned long thp_get_unmapped_area(struct file *filp, unsigned long addr,
> }
> EXPORT_SYMBOL_GPL(thp_get_unmapped_area);
>
> -static vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
> - unsigned long haddr, struct folio **foliop,
> - unsigned long addr)
> +vm_fault_t thp_fault_alloc(gfp_t gfp, int order, struct vm_area_struct *vma,
> + unsigned long haddr, struct folio **foliop,
> + unsigned long addr)
> {
> struct folio *folio = vma_alloc_folio(gfp, order, vma, haddr, true);
>
> @@ -984,21 +984,29 @@ static void __thp_fault_success_stats(struct vm_area_struct *vma, int order)
> count_memcg_event_mm(vma->vm_mm, THP_FAULT_ALLOC);
> }
>
> -static void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
> - struct vm_area_struct *vma, unsigned long haddr,
> - pgtable_t pgtable)
> +void map_pmd_thp(struct folio *folio, struct vm_fault *vmf,
> + struct vm_area_struct *vma, unsigned long haddr,
> + pgtable_t pgtable)
> {
> - pmd_t entry;
> + pmd_t entry, old_pmd;
> + bool is_pmd_none = pmd_none(*vmf->pmd);
>
> entry = mk_huge_pmd(&folio->page, vma->vm_page_prot);
> entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
> folio_add_new_anon_rmap(folio, vma, haddr, RMAP_EXCLUSIVE);
> folio_add_lru_vma(folio, vma);
> - pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
> + if (!is_pmd_none) {
> + old_pmd = pmdp_huge_clear_flush(vma, haddr, vmf->pmd);
> + if (pmd_uffd_wp(old_pmd))
> + entry = pmd_mkuffd_wp(entry);
I don't really get this; entry is writable, so I wouldn't expect to also be
setting uffd-wp here? That combination is not allowed and is checked for in
page_table_check_pte_flags().
It looks like you expect to get here in the uffd-wp-async case, which used to
cause the pmd to be split to ptes. I'm guessing (but don't know for sure) that
would cause the uffd-wp bit to be set in each of the new ptes, then during
fallback to handling the wp fault on the pte, uffd would handle it?
> + }
> + if (pgtable)
> + pgtable_trans_huge_deposit(vma->vm_mm, vmf->pmd, pgtable);
Should this call be moved outside of here? It doesn't really feel like it
belongs. Could it be called before calling map_pmd_thp() for the site that has a
pgtable?
> set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
> update_mmu_cache_pmd(vma, vmf->address, vmf->pmd);
> add_mm_counter(vma->vm_mm, MM_ANONPAGES, HPAGE_PMD_NR);
> - mm_inc_nr_ptes(vma->vm_mm);
> + if (is_pmd_none)
> + mm_inc_nr_ptes(vma->vm_mm);
> }
>
> static vm_fault_t __do_huge_pmd_anonymous_page(struct vm_fault *vmf)
> @@ -1576,6 +1584,50 @@ void huge_pmd_set_accessed(struct vm_fault *vmf)
> spin_unlock(vmf->ptl);
> }
>
> +static vm_fault_t do_huge_zero_wp_pmd_locked(struct vm_fault *vmf,
> + unsigned long haddr,
> + struct folio *folio)
> +{
> + struct vm_area_struct *vma = vmf->vma;
> + vm_fault_t ret = 0;
> +
> + ret = check_stable_address_space(vma->vm_mm);
> + if (ret)
> + goto out;
> + map_pmd_thp(folio, vmf, vma, haddr, NULL);
> +out:
> + return ret;
> +}
> +
> +static vm_fault_t do_huge_zero_wp_pmd(struct vm_fault *vmf, unsigned long haddr)
> +{
> + struct vm_area_struct *vma = vmf->vma;
> + gfp_t gfp = vma_thp_gfp_mask(vma);
> + struct mmu_notifier_range range;
> + struct folio *folio = NULL;
> + vm_fault_t ret = 0;
> +
> + ret = thp_fault_alloc(gfp, HPAGE_PMD_ORDER, vma, haddr, &folio,
> + vmf->address);
Just checking: the PTE table was already allocated during the read fault, right?
So we don't have to allocate it here.
> + if (ret)
> + goto out;
> +
> + mmu_notifier_range_init(&range, MMU_NOTIFY_CLEAR, 0, vma->vm_mm, haddr,
> + haddr + HPAGE_PMD_SIZE);
> + mmu_notifier_invalidate_range_start(&range);
> + vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
> + if (unlikely(!pmd_same(pmdp_get(vmf->pmd), vmf->orig_pmd)))
> + goto unlock;
> + ret = do_huge_zero_wp_pmd_locked(vmf, haddr, folio);
> + if (!ret)
> + __thp_fault_success_stats(vma, HPAGE_PMD_ORDER);
> +unlock:
> + spin_unlock(vmf->ptl);
> + mmu_notifier_invalidate_range_end(&range);
I'll confess I don't understand all the mmu notifier rules. But the doc at
Documentation/mm/mmu_notifier.rst implies that the notification must be done
while holding the PTL. Although that's not how wp_page_copy(). Are you confident
what you have done is correct?
Thanks,
Ryan
> +out:
> + return ret;
> +}
> +
> vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
> {
> const bool unshare = vmf->flags & FAULT_FLAG_UNSHARE;
> @@ -1588,8 +1640,15 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
> vmf->ptl = pmd_lockptr(vma->vm_mm, vmf->pmd);
> VM_BUG_ON_VMA(!vma->anon_vma, vma);
>
> - if (is_huge_zero_pmd(orig_pmd))
> + if (is_huge_zero_pmd(orig_pmd)) {
> + vm_fault_t ret = do_huge_zero_wp_pmd(vmf, haddr);
> +
> + if (!(ret & VM_FAULT_FALLBACK))
> + return ret;
> +
> + /* Fallback to splitting PMD if THP cannot be allocated */
> goto fallback;
> + }
>
> spin_lock(vmf->ptl);
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 3c01d68065be..c081a25f5173 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5409,9 +5409,10 @@ static inline vm_fault_t wp_huge_pmd(struct vm_fault *vmf)
> if (vma_is_anonymous(vma)) {
> if (likely(!unshare) &&
> userfaultfd_huge_pmd_wp(vma, vmf->orig_pmd)) {
> - if (userfaultfd_wp_async(vmf->vma))
> + if (!userfaultfd_wp_async(vmf->vma))
> + return handle_userfault(vmf, VM_UFFD_WP);
> + if (!is_huge_zero_pmd(vmf->orig_pmd))
> goto split;
> - return handle_userfault(vmf, VM_UFFD_WP);
> }
> return do_huge_pmd_wp_page(vmf);
> }
