linux-kernel - Re: [PATCH v4] mm: Add MREMAP

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Date:   Fri, 14 Feb 2020 03:36:40 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Brian Geffon <bgeffon@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Linux API <linux-api@...r.kernel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Will Deacon <will@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Sonny Rao <sonnyrao@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Yu Zhao <yuzhao@...gle.com>,
        Jesse Barnes <jsbarnes@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Florian Weimer <fweimer@...hat.com>
Subject: Re: [PATCH v4] mm: Add MREMAP_DONTUNMAP to mremap().

On Thu, Feb 13, 2020 at 10:20:44AM -0800, Brian Geffon wrote:
> Hi Kirill,
> 
> > But if you do the operation for the VM_LOCKED vma, you'll have two locked
> > VMA's now, right? Where do you account the old locked vma you left behind?
> 
> You bring up a good point. In a previous iteration of my patch I had
> it clearing the locked flags on the old VMA as technically the locked
> pages had migrated. I talked myself out of that but the more I think
> about it we should probably do that. Something along the lines of:
> 
> +    if (vm_flags & VM_LOCKED) {
> +      /* Locked pages would have migrated to the new VMA */
> +      vma->vm_flags &= VM_LOCKED_CLEAR_MASK;
> +      if (new_len > old_len)
> +              mm->locked_vm += (new_len - old_len) >> PAGE_SHIFT;
> +   }
> 
> I feel that this is correct. The only other possible option would be
> to clear only the VM_LOCKED flag on the old vma leaving VM_LOCKONFAULT
> to handle the MCL_ONFAULT mlocked situation, thoughts? Regardless I'll
> have to mail a new patch because that part where I'm incrementing the
> mm->locked_vm lost the check on VM_LOCKED during patch versions.

Note, that we account mlock limit on per-VMA basis, not per page. Even for
VM_LOCKONFAULT.

> Thanks again for taking the time to review.

I believe the right approach is to strip VM_LOCKED[ONFAULT] from the vma
you left behind. Or the new vma. It is a policy decision.

JFYI, we do not inherit VM_LOCKED on fork(), so it's common practice to
strip VM_LOCKED on vma duplication.

Other option is to leave VM_LOCKED on both VMAs and fail the operation if
we are over the limit. But we need to have a good reason to take this
path. It makes the interface less flexible.

-- 
 Kirill A. Shutemov