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SubjectRe: [RFC PATCH 13/18] mm: add try_to_free_user_pte() helper
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On 2022/4/29 9:35 PM, Qi Zheng wrote:
> Normally, the percpu_ref of the user PTE page table page is in
> percpu mode. This patch add try_to_free_user_pte() to switch
> the percpu_ref to atomic mode and check if it is 0. If the
> percpu_ref is 0, which means that no one is using the user PTE
> page table page, then we can safely reclaim it.
>
> Signed-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
> ---
> include/linux/pte_ref.h | 7 +++
> mm/pte_ref.c | 99 ++++++++++++++++++++++++++++++++++++++++-
> 2 files changed, 104 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/pte_ref.h b/include/linux/pte_ref.h
> index bfe620038699..379c3b45a6ab 100644
> --- a/include/linux/pte_ref.h
> +++ b/include/linux/pte_ref.h
> @@ -16,6 +16,8 @@ void free_user_pte(struct mm_struct *mm, pmd_t *pmd, unsigned long addr);
> bool pte_tryget(struct mm_struct *mm, pmd_t *pmd, unsigned long addr);
> void __pte_put(pgtable_t page);
> void pte_put(pte_t *ptep);
> +void try_to_free_user_pte(struct mm_struct *mm, pmd_t *pmd, unsigned long addr,
> + bool switch_back);
>
> #else /* !CONFIG_FREE_USER_PTE */
>
> @@ -47,6 +49,11 @@ static inline void pte_put(pte_t *ptep)
> {
> }
>
> +static inline void try_to_free_user_pte(struct mm_struct *mm, pmd_t *pmd,
> + unsigned long addr, bool switch_back)
> +{
> +}
> +
> #endif /* CONFIG_FREE_USER_PTE */
>
> #endif /* _LINUX_PTE_REF_H */
> diff --git a/mm/pte_ref.c b/mm/pte_ref.c
> index 5b382445561e..bf9629272c71 100644
> --- a/mm/pte_ref.c
> +++ b/mm/pte_ref.c
> @@ -8,6 +8,9 @@
> #include <linux/pte_ref.h>
> #include <linux/percpu-refcount.h>
> #include <linux/slab.h>
> +#include <linux/hugetlb.h>
> +#include <asm/tlbflush.h>
> +#include <asm/pgalloc.h>
>
> #ifdef CONFIG_FREE_USER_PTE
>
> @@ -44,8 +47,6 @@ void pte_ref_free(pgtable_t pte)
> kfree(ref);
> }
>
> -void free_user_pte(struct mm_struct *mm, pmd_t *pmd, unsigned long addr) {}
> -
> /*
> * pte_tryget - try to get the pte_ref of the user PTE page table page
> * @mm: pointer the target address space
> @@ -102,4 +103,98 @@ void pte_put(pte_t *ptep)
> }
> EXPORT_SYMBOL(pte_put);
>
> +#ifdef CONFIG_DEBUG_VM
> +void pte_free_debug(pmd_t pmd)
> +{
> + pte_t *ptep = (pte_t *)pmd_page_vaddr(pmd);
> + int i = 0;
> +
> + for (i = 0; i < PTRS_PER_PTE; i++)
> + BUG_ON(!pte_none(*ptep++));
> +}
> +#else
> +static inline void pte_free_debug(pmd_t pmd)
> +{
> +}
> +#endif
> +
> +static inline void pte_free_rcu(struct rcu_head *rcu)
> +{
> + struct page *page = container_of(rcu, struct page, rcu_head);
> +
> + pgtable_pte_page_dtor(page);
> + __free_page(page);
> +}
> +
> +/*
> + * free_user_pte - free the user PTE page table page
> + * @mm: pointer the target address space
> + * @pmd: pointer to a PMD
> + * @addr: start address of the tlb range to be flushed
> + *
> + * Context: The pmd range has been unmapped and TLB purged. And the user PTE
> + * page table page will be freed by rcu handler.
> + */
> +void free_user_pte(struct mm_struct *mm, pmd_t *pmd, unsigned long addr)
> +{
> + struct vm_area_struct vma = TLB_FLUSH_VMA(mm, 0);
> + spinlock_t *ptl;
> + pmd_t pmdval;
> +
> + ptl = pmd_lock(mm, pmd);
> + pmdval = *pmd;
> + if (pmd_none(pmdval) || pmd_leaf(pmdval)) {
> + spin_unlock(ptl);
> + return;
> + }
> + pmd_clear(pmd);
> + flush_tlb_range(&vma, addr, addr + PMD_SIZE);
> + spin_unlock(ptl);
> +
> + pte_free_debug(pmdval);
> + mm_dec_nr_ptes(mm);
> + call_rcu(&pmd_pgtable(pmdval)->rcu_head, pte_free_rcu);
> +}
> +
> +/*
> + * try_to_free_user_pte - try to free the user PTE page table page
> + * @mm: pointer the target address space
> + * @pmd: pointer to a PMD
> + * @addr: virtual address associated with pmd
> + * @switch_back: indicates if switching back to percpu mode is required
> + */
> +void try_to_free_user_pte(struct mm_struct *mm, pmd_t *pmd, unsigned long addr,
> + bool switch_back)
> +{
> + pgtable_t pte;
> +
> + if (&init_mm == mm)
> + return;
> +
> + if (!pte_tryget(mm, pmd, addr))
> + return;
> + pte = pmd_pgtable(*pmd);
> + percpu_ref_switch_to_atomic_sync(pte->pte_ref);
> + rcu_read_lock();
> + /*
> + * Here we can safely put the pte_ref because we already hold the rcu
> + * lock, which guarantees that the user PTE page table page will not
> + * be released.
> + */
> + __pte_put(pte);
> + if (percpu_ref_is_zero(pte->pte_ref)) {
> + rcu_read_unlock();
> + free_user_pte(mm, pmd, addr & PMD_MASK);
> + return;
> + }
> + rcu_read_unlock();
> +
> + if (switch_back) {
> + if (pte_tryget(mm, pmd, addr)) {
> + percpu_ref_switch_to_percpu(pte->pte_ref);
> + __pte_put(pte);
> + }
> + }

We shouldn't switch back to percpu mode here, it will drastically reduce
performance.

> +}
> +
> #endif /* CONFIG_FREE_USER_PTE */

--
Thanks,
Qi

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