linux-kernel - Re: [PATCH v2 1/2] khugepaged: enable collapse pmd for pte-mapped THP

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Message-ID: <619E9EC6-0B6A-4C30-8BA7-D2CA83FFC4E7@fb.com>
Date:   Thu, 1 Aug 2019 17:11:28 +0000
From:   Song Liu <songliubraving@...com>
To:     Oleg Nesterov <oleg@...hat.com>
CC:     lkml <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <matthew.wilcox@...cle.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Kernel Team <Kernel-team@...com>,
        "William Kucharski" <william.kucharski@...cle.com>,
        "srikar@...ux.vnet.ibm.com" <srikar@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 1/2] khugepaged: enable collapse pmd for pte-mapped THP



> On Aug 1, 2019, at 5:43 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> 
> On 07/31, Song Liu wrote:
>> 
>> +void collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long haddr)
>> +{
>> +	struct vm_area_struct *vma = find_vma(mm, haddr);
>> +	pmd_t *pmd = mm_find_pmd(mm, haddr);
>> +	struct page *hpage = NULL;
>> +	unsigned long addr;
>> +	spinlock_t *ptl;
>> +	int count = 0;
>> +	pmd_t _pmd;
>> +	int i;
>> +
>> +	VM_BUG_ON(haddr & ~HPAGE_PMD_MASK);
>> +
>> +	if (!vma || !pmd || pmd_trans_huge(*pmd))
>                            ^^^^^^^^^^^^^^^^^^^^
> 
> mm_find_pmd() returns NULL if pmd_trans_huge()

Good catch! I will simplify this one in v3. 

> 
>> +	/* step 1: check all mapped PTEs are to the right huge page */
>> +	for (i = 0, addr = haddr; i < HPAGE_PMD_NR; i++, addr += PAGE_SIZE) {
>> +		pte_t *pte = pte_offset_map(pmd, addr);
>> +		struct page *page;
>> +
>> +		if (pte_none(*pte))
>> +			continue;
>> +
>> +		page = vm_normal_page(vma, addr, *pte);
>> +
>> +		if (!PageCompound(page))
>> +			return;
>> +
>> +		if (!hpage) {
>> +			hpage = compound_head(page);
>> +			if (hpage->mapping != vma->vm_file->f_mapping)
> 
> Hmm. But how can we know this is still the same vma ?
> 
> If nothing else, why vma->vm_file can't be NULL?

Good point. We should confirm vma->vm_file is not NULL. 

Thanks,
Song