linux-kernel - Re: [PATCH] mm/thp: fix call to mmu_notifier in set_pmd_migration

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Date:   Fri, 12 Oct 2018 13:58:00 -0400
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Zi Yan <zi.yan@...rutgers.edu>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        Dave Hansen <dave.hansen@...el.com>,
        David Nellans <dnellans@...dia.com>,
        Ingo Molnar <mingo@...e.hu>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Minchan Kim <minchan@...nel.org>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH] mm/thp: fix call to mmu_notifier in
 set_pmd_migration_entry()

On Fri, Oct 12, 2018 at 01:35:19PM -0400, Jerome Glisse wrote:
> On Fri, Oct 12, 2018 at 01:24:22PM -0400, Andrea Arcangeli wrote:
> > Hello,
> > 
> > On Fri, Oct 12, 2018 at 12:20:54PM -0400, Zi Yan wrote:
> > > On 12 Oct 2018, at 12:09, jglisse@...hat.com wrote:
> > > 
> > > > From: Jérôme Glisse <jglisse@...hat.com>
> > > >
> > > > Inside set_pmd_migration_entry() we are holding page table locks and
> > > > thus we can not sleep so we can not call invalidate_range_start/end()
> > > >
> > > > So remove call to mmu_notifier_invalidate_range_start/end() and add
> > > > call to mmu_notifier_invalidate_range(). Note that we are already
> > 
> > Why the call to mmu_notifier_invalidate_range if we're under
> > range_start and followed by range_end? (it's not _range_only_end, if
> > it was _range_only_end the above would be needed)
> 
> I wanted to be extra safe and accept to over invalidate. You are right
> that it is not strictly necessary. I am fine with removing it.

If it's superfluous, I'd generally prefer strict code unless there's a
very explicit comment about it that says it's actually superfluous.
Otherwise after a while we don't know why it was added there.

> We can remove it. Should i post a v2 without it ?

That's fine with me yes.

Thanks,
Andrea