linux-kernel - Re: [PATCH] khugepaged: collapse_pte_mapped

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Message-ID: <20200802210738.i4trrpq5b77yaxzw@box>
Date:   Mon, 3 Aug 2020 00:07:38 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Song Liu <songliubraving@...com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] khugepaged: collapse_pte_mapped_thp() flush the right
 range

On Sun, Aug 02, 2020 at 12:12:42PM -0700, Hugh Dickins wrote:
> pmdp_collapse_flush() should be given the start address at which the huge
> page is mapped, haddr: it was given addr, which at that point has been
> used as a local variable, incremented to the end address of the extent.
> 
> Found by source inspection while chasing a hugepage locking bug, which
> I then could not explain by this. At first I thought this was very bad;
> then saw that all of the page translations that were not flushed would
> actually still point to the right pages afterwards, so harmless; then
> realized that I know nothing of how different architectures and models
> cache intermediate paging structures, so maybe it matters after all -
> particularly since the page table concerned is immediately freed.
> 
> Much easier to fix than to think about.
> 
> Fixes: 27e1f8273113 ("khugepaged: enable collapse pmd for pte-mapped THP")
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> Cc: stable@...r.kernel.org # v5.4+

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
 Kirill A. Shutemov