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

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Message-ID: <36D3C0F0-17CE-42B9-9661-B376D608FA7D@fb.com>
Date:   Thu, 1 Aug 2019 17:37:03 +0000
From:   Song Liu <songliubraving@...com>
To:     Oleg Nesterov <oleg@...hat.com>
CC:     lkml <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "matthew.wilcox@...cle.com" <matthew.wilcox@...cle.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        Kernel Team <Kernel-team@...com>,
        "william.kucharski@...cle.com" <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 7:50 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> 
> On 07/31, Song Liu wrote:
>> 
>> +static int khugepaged_add_pte_mapped_thp(struct mm_struct *mm,
>> +					 unsigned long addr)
>> +{
>> +	struct mm_slot *mm_slot;
>> +	int ret = 0;
>> +
>> +	/* hold mmap_sem for khugepaged_test_exit() */
>> +	VM_BUG_ON_MM(!rwsem_is_locked(&mm->mmap_sem), mm);
>> +	VM_BUG_ON(addr & ~HPAGE_PMD_MASK);
>> +
>> +	if (unlikely(khugepaged_test_exit(mm)))
>> +		return 0;
>> +
>> +	if (!test_bit(MMF_VM_HUGEPAGE, &mm->flags) &&
>> +	    !test_bit(MMF_DISABLE_THP, &mm->flags)) {
>> +		ret = __khugepaged_enter(mm);
>> +		if (ret)
>> +			return ret;
>> +	}
> 
> could you explain why do we need mm->mmap_sem, khugepaged_test_exit() check
> and __khugepaged_enter() ?

If the mm doesn't have a mm_slot, we would like to create one here (by 
calling __khugepaged_enter()). 

This happens when the THP is created by another mm, or by tmpfs with 
"huge=always"; and then page table of this mm got split by split_huge_pmd(). 
With current kernel, this happens when we attach/detach uprobe to a file 
in tmpfs with huge=always. 

Does this answer your question?

Thanks,
Song