linux-kernel - Re: [PATCH uprobe, thp 1/4] mm, thp: allow preallocate pgtable for split_huge_pmd

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Date:   Thu, 30 May 2019 14:14:16 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Song Liu <songliubraving@...com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org, namit@...are.com,
        peterz@...radead.org, oleg@...hat.com, rostedt@...dmis.org,
        mhiramat@...nel.org, matthew.wilcox@...cle.com,
        kirill.shutemov@...ux.intel.com, kernel-team@...com,
        william.kucharski@...cle.com, chad.mynhier@...cle.com,
        mike.kravetz@...cle.com
Subject: Re: [PATCH uprobe, thp 1/4] mm, thp: allow preallocate pgtable for
 split_huge_pmd_address()

On Thu, May 30, 2019 at 02:10:15PM +0300, Kirill A. Shutemov wrote:
> On Wed, May 29, 2019 at 02:20:46PM -0700, Song Liu wrote:
> > @@ -2133,10 +2133,15 @@ static void __split_huge_pmd_locked(struct vm_area_struct *vma, pmd_t *pmd,
> >  	VM_BUG_ON_VMA(vma->vm_end < haddr + HPAGE_PMD_SIZE, vma);
> >  	VM_BUG_ON(!is_pmd_migration_entry(*pmd) && !pmd_trans_huge(*pmd)
> >  				&& !pmd_devmap(*pmd));
> > +	/* only file backed vma need preallocate pgtable*/
> > +	VM_BUG_ON(vma_is_anonymous(vma) && prealloc_pgtable);
> >  
> >  	count_vm_event(THP_SPLIT_PMD);
> >  
> > -	if (!vma_is_anonymous(vma)) {
> > +	if (prealloc_pgtable) {
> > +		pgtable_trans_huge_deposit(mm, pmd, prealloc_pgtable);
> > +		mm_inc_nr_pmds(mm);
> > +	} else if (!vma_is_anonymous(vma)) {
> >  		_pmd = pmdp_huge_clear_flush_notify(vma, haddr, pmd);
> >  		/*
> >  		 * We are going to unmap this huge page. So
> 
> Nope. This going to leak a page table for architectures where
> arch_needs_pgtable_deposit() is true.

And I don't there's correct handling of dirty bit.

And what about DAX? Will it blow up? I think so.

-- 
 Kirill A. Shutemov