linux-kernel - Re: [LSF/MM TOPIC] Non standard size THP

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Message-ID: <20190213132800.dekg525rhrjn3cmj@kshutemo-mobl1>
Date:   Wed, 13 Feb 2019 16:28:00 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        lsf-pc@...ts.linux-foundation.org,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [LSF/MM TOPIC] Non standard size THP

On Wed, Feb 13, 2019 at 05:06:47AM -0800, Matthew Wilcox wrote:
> On Tue, Feb 12, 2019 at 11:33:31AM +0300, Kirill A. Shutemov wrote:
> > To consider it seriously we need to understand what it means for
> > split_huge_p?d()/split_huge_page()? How khugepaged will deal with this?
> > 
> > In particular, I'm worry to expose (to user or CPU) page table state in
> > the middle of conversion (huge->small or small->huge). Handling this on
> > page table level provides a level atomicity that you will not have.
> 
> We could do an RCU-style trick where (eg) for merging 16 consecutive
> entries together, we allocate a new PTE leaf, take the mmap_sem for write,
> copy the page table over, update the new entries, then put the new leaf
> into the PMD level.  Then iterate over the old PTE leaf again, and set
> any dirty bits in the new leaf which were set during the race window.
> 
> Does that cover all the problems?

Probably, but it will kill scalability. Taking mmap_sem for write to
handle page fault or MADV_DONTNEED will not make anybody happy.

-- 
 Kirill A. Shutemov