linux-kernel - Re: [patch v3] mm, thp: always specify disabled vmas as nh in smaps

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Message-ID: <20180926061247.GB18685@dhcp22.suse.cz>
Date:   Wed, 26 Sep 2018 08:12:47 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-api@...r.kernel.org
Subject: Re: [patch v3] mm, thp: always specify disabled vmas as nh in smaps

On Tue 25-09-18 14:50:52, David Rientjes wrote:
[...]
Let's put my general disagreement with the approach asside for a while.
If this is really the best way forward the is the implementation really
correct?

> +	/*
> +	 * 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_HUGEPAGE;
> +		flags |= VM_NOHUGEPAGE;
> +	}

Do we want to report all vmas nh? Shouldn't we limit that to THP-able
mappings? It seems quite strange that an application started without
PR_SET_THP_DISABLE wouldn't report nh for most mappings while it would
otherwise. Also when can we have vma->vm_mm == NULL?

> +
>  	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, ' ');

-- 
Michal Hocko
SUSE Labs