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SubjectRe: [PATCH v3 3/3] mm/memory.c: Add memory read privilege before filling PTE entry
On Mon, 18 May 2020 13:08:49 +0800 Bibo Mao <maobibo@loongson.cn> wrote:

> On mips platform, hw PTE entry valid bit is set in pte_mkyoung
> function, it is used to set physical page with readable privilege.

pte_mkyoung() seems to be a strange place to set the pte's valid bit.
Why is it done there? Can it be done within mips's mk_pte()?

> Here add pte_mkyoung function to make page readable on MIPS platform
> during page fault handling. This patch improves page fault latency
> about 10% on my MIPS machine with lmbench lat_pagefault case.
>
> ...
>
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2704,6 +2704,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
> }
> flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
> entry = mk_pte(new_page, vma->vm_page_prot);
> + entry = pte_mkyoung(entry);
> entry = maybe_mkwrite(pte_mkdirty(entry), vma);

What is the effect on non-mips machines? If it's only "additional
overhead" then it would be better to add a new pte_mkvalid() (or
whatever) and arrange for it to be a no-op on all but mips?

> /*
> * Clear the pte entry and flush it first, before updating the
> @@ -3378,6 +3379,7 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
> __SetPageUptodate(page);
>
> entry = mk_pte(page, vma->vm_page_prot);
> + entry = pte_mkyoung(entry);
> if (vma->vm_flags & VM_WRITE)
> entry = pte_mkwrite(pte_mkdirty(entry));
>
> @@ -3660,6 +3662,7 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
>
> flush_icache_page(vma, page);
> entry = mk_pte(page, vma->vm_page_prot);
> + entry = pte_mkyoung(entry);
> if (write)
> entry = maybe_mkwrite(pte_mkdirty(entry), vma);
> /* copy-on-write page */
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 494192ca..673f1cd 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -131,6 +131,8 @@ static unsigned long change_pte_range(struct vm_area_struct *vma, pmd_t *pmd,
> ptent = pte_clear_uffd_wp(ptent);
> }
>
> + if (vma->vm_flags & VM_READ)
> + ptent = pte_mkyoung(ptent);
> /* Avoid taking write faults for known dirty pages */
> if (dirty_accountable && pte_dirty(ptent) &&
> (pte_soft_dirty(ptent) ||

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