linux-kernel - Re: [RFC PATCH 2/2] mm/khugepaged: Remove compound

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Message-ID: <ZRVB49PruFvT7+43@casper.infradead.org>
Date:   Thu, 28 Sep 2023 10:05:39 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Yang Shi <shy828301@...il.com>
Cc:     "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
        linux-mm@...ck.org, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 2/2] mm/khugepaged: Remove compound_pagelist

On Tue, Sep 26, 2023 at 03:07:18PM -0700, Yang Shi wrote:
> On Fri, Sep 22, 2023 at 9:33 PM Vishal Moola (Oracle)
> <vishal.moola@...il.com> wrote:
> >
> > Currently, khugepaged builds a compound_pagelist while scanning, which
> > is used to properly account for compound pages. We can now account
> > for a compound page as a singular folio instead, so remove this list.
> >
> > Large folios are guaranteed to have consecutive ptes and addresses, so
> > once the first pte of a large folio is found skip over the rest.
> 
> The address space may just map a partial folio, for example, in the
> extreme case the HUGE_PMD size range may have HUGE_PMD_NR folios with
> mapping one subpage from each folio per PTE. So assuming the PTE
> mapped folio is mapped consecutively may be wrong.

How?  You can do that with two VMAs, but this is limited to scanning
within a single VMA.  If we've COWed a large folio, we currently do
so as a single page folio, and I'm not seeing any demand to change that.
If we did COW as a large folio, we'd COW every page in that folio.
How do we interleave two large folios in the same VMA?

> Please refer to collapse_compound_extreme() in
> tools/testing/selftests/mm/khugepaged.c.

I agree that running that part of the test-suite would be useful, but
could you point to which test specifically would create a problem here?