linux-kernel - Re: [patch] mm, thp: always specify ineligible vmas as nh in smaps

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Date:   Mon, 24 Sep 2018 12:17:34 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
cc:     Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org,
        Linux-MM layout <linux-mm@...ck.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [patch] mm, thp: always specify ineligible vmas as nh in smaps

On Mon, 24 Sep 2018, Vlastimil Babka wrote:

> > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > --- a/fs/proc/task_mmu.c
> > +++ b/fs/proc/task_mmu.c
> > @@ -653,13 +653,23 @@ static void show_smap_vma_flags(struct seq_file *m, struct vm_area_struct *vma)
> >  #endif
> >  #endif /* CONFIG_ARCH_HAS_PKEYS */
> >  	};
> > +	unsigned long flags = vma->vm_flags;
> >  	size_t i;
> >  
> > +	/*
> > +	 * Disabling thp is possible through both MADV_NOHUGEPAGE and
> > +	 * PR_SET_THP_DISABLE.  Both historically used VM_NOHUGEPAGE.  Since
> > +	 * the introduction of MMF_DISABLE_THP, however, userspace needs the
> > +	 * ability to detect vmas where thp is not eligible in the same manner.
> > +	 */
> > +	if (vma->vm_mm && test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags))
> > +		flags |= VM_NOHUGEPAGE;
> 
> Should it also clear VM_HUGEPAGE? In case MMF_DISABLE_THP overrides a
> madvise(MADV_HUGEPAGE)'d vma? (I expect it does?)
> 

Good point, I think that is should because MMF_DISABLE_THP will override 
VM_HUGEPAGE.  It looks like the Documentation file is still referencing 
both as advise flags and doesn't address PR_SET_THP_DISABLE.  Let me send 
a v2 with a Documentation update and your suggested fix.  Thanks 
Vlastimil!

> > +
> >  	seq_puts(m, "VmFlags: ");
> >  	for (i = 0; i < BITS_PER_LONG; i++) {
> >  		if (!mnemonics[i][0])
> >  			continue;
> > -		if (vma->vm_flags & (1UL << i)) {
> > +		if (flags & (1UL << i)) {
> >  			seq_putc(m, mnemonics[i][0]);
> >  			seq_putc(m, mnemonics[i][1]);
> >  			seq_putc(m, ' ');
> >