linux-kernel - Re: [PATCH v4 1/2] khugepaged: enable collapse pmd for pte-mapped THP

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Message-ID: <20190806100256.GA21454@redhat.com>
Date:   Tue, 6 Aug 2019 12:02:57 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Song Liu <songliubraving@...com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, matthew.wilcox@...cle.com,
        kirill.shutemov@...ux.intel.com, kernel-team@...com,
        william.kucharski@...cle.com, srikar@...ux.vnet.ibm.com
Subject: Re: [PATCH v4 1/2] khugepaged: enable collapse pmd for pte-mapped THP

On 08/02, Song Liu wrote:
>
> +void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr)
> +{
> +	unsigned long haddr = addr & HPAGE_PMD_MASK;
> +	struct vm_area_struct *vma = find_vma(mm, haddr);
> +	pmd_t *pmd = mm_find_pmd(mm, haddr);
> +	struct page *hpage = NULL;
> +	spinlock_t *ptl;
> +	int count = 0;
> +	pmd_t _pmd;
> +	int i;
> +
> +	if (!vma || !vma->vm_file || !pmd ||
> +	    vma->vm_start > haddr || vma->vm_end < haddr + HPAGE_PMD_SIZE)
> +		return;

I still can't understand why is it safe to blindly use mm_find_pmd().

Say, what pmd_offset(pud, address) will return to this function if
pud_huge() == T? IIUC, this is possible if is_file_hugepages(vm_file).
How the code below can use this result?

I think you need something like hugepage_vma_check() or even
hugepage_vma_revalidate().

Oleg.