linux-kernel - Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and PMD migration entry

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Message-ID: <20181106003509.GA27283@brain-police>
Date:   Tue, 6 Nov 2018 00:35:10 +0000
From:   Will Deacon <will.deacon@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Zi Yan <zi.yan@...rutgers.edu>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kirill.shutemov@...ux.intel.com,
        akpm@...ux-foundation.org, mhocko@...e.com,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>
Subject: Re: [PATCH] mm/thp: Correctly differentiate between mapped THP and
 PMD migration entry

On Fri, Nov 02, 2018 at 11:45:00AM +0530, Anshuman Khandual wrote:
> On 10/17/2018 07:39 AM, Andrea Arcangeli wrote:
> > What we need to do during split is an invalidate of the huge TLB.
> > There's no pmd_trans_splitting anymore, so we only clear the present
> > bit in the PTE despite pmd_present still returns true (just like
> > PROT_NONE, nothing new in this respect). pmd_present never meant the
> 
> On arm64, the problem is that pmd_present() is tied with pte_present() which
> checks for PTE_VALID (also PTE_PROT_NONE) but which gets cleared during PTE
> invalidation. pmd_present() returns false just after the first step of PMD
> splitting. So pmd_present() needs to be decoupled from PTE_VALID which is
> same as PMD_SECT_VALID and instead should depend upon a pte bit which sticks
> around like PAGE_PSE as in case of x86. I am working towards a solution.

Could we not just go via a PROT_NONE mapping during the split, instead of
having to allocate a new software bit to treat these invalid ptes as
present?

Will