linux-kernel - Re: [PATCH 4/5] Hot-remove implementation for arm64

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Message-Id: <20170421100500.GB20029@samekh>
Date:   Fri, 21 Apr 2017 11:05:01 +0100
From:   Andrea Reale <ar@...ux.vnet.ibm.com>
To:     Laura Abbott <labbott@...hat.com>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        m.bielski@...tualopensystems.com, scott.branden@...adcom.com,
        Will Deacon <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Xishi Qiu <qiuxishi@...wei.com>
Subject: Re: [PATCH 4/5] Hot-remove implementation for arm64

Hi all,

thanks for taking the time to comment. Replies in-line.

On Wed, Apr 19, 2017 at 08:53:13AM -0700, Laura Abbott wrote:
> On 04/18/2017 11:48 AM, Ard Biesheuvel wrote:
> >On 18 April 2017 at 19:21, Mark Rutland <mark.rutland@....com> wrote:
> >>On Fri, Apr 14, 2017 at 03:01:58PM +0100, Andrea Reale wrote:

[...]

> >>
> >> From a quick scan, I see that it's necessary to use pgtable_page_ctor()
> >>for pages that will be used for userspace page tables, but it's not
> >>clear to me if it's ever necessary for pages used for kernel page
> >>tables.
> >>
> >>If it is, we appear to have a bug on arm64.
> >>
> >>Laura, Ard, thoughts?
> >>
> >
> >The generic apply_to_page_range() will expect the PTE lock to be
> >initialized for page table pages that are not part of init_mm. For
> >arm64, that is precisely efi_mm as far as I am aware. For EFI, the
> >locking is unnecessary but does no harm (the permissions are set once
> >via apply_to_page_range() at boot), so I added this call when adding
> >support for strict permissions in EFI rt services mappings.
> >
> >So I think it is appropriate for create_pgd_mapping() to be in charge
> >of calling the ctor(). We simply have no destroy_pgd_mapping()
> >counterpart that would be the place for the dtor() call, given that we
> >never take down EFI rt services mappi >
> >Whether it makes sense or not to lock/unlock in apply_to_page_range()
> >is something I did not spend any brain cycles on at the time.
> >
> 
> Agreed there shouldn't be a problem right now. I do think the locking is
> appropriate in apply_to_page_range given what other functions also get
> locked.
> 
> I really wish this were less asymmetrical though since it get hard
> to reason about. It looks like hotplug_paging will call the ctor,
> so is there an issue with calling hot-remove on memory that was once
> hot-added or is that not a concern?
> 
> Thanks,
> Laura

I think the confusion comes from the fact that, in hotplug_paging, we are
passing pgd_pgtable_alloc as the page allocator for __create_pgd_mapping,
which always calls the ctor.

If I got things right (but, please, correct me if I am wrong), we don't
need to get the pte_lock that the ctor gets since - in hotplug - we are
adding to init_mm.

Moreover, I am just realizing that calling the dtor while hot-removing
might create problems when removing memory that *was not* previously
hotplugged, as we are calling a dtor on something that was never
ctor'ed. Is that what you were hinting at, Laura?

Thanks and best regards,
Andrea