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

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Date:   Thu, 1 Aug 2019 16:50:32 +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 v2 1/2] khugepaged: enable collapse pmd for pte-mapped THP

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() ?


> +	spin_lock(&khugepaged_mm_lock);
> +	mm_slot = get_mm_slot(mm);
> +	if (likely(mm_slot && mm_slot->nr_pte_mapped_thp < MAX_PTE_MAPPED_THP))
> +		mm_slot->pte_mapped_thp[mm_slot->nr_pte_mapped_thp++] = addr;

if get_mm_slot() returns mm_slot != NULL we can safely modify ->pte_mapped_thp.
We do not care even if this task has already passed __mmput/__khugepaged_exit,
this slot can't go away.

No?

Oleg.