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From:   Shakeel Butt <shakeelb@google.com>
Date:   Wed, 7 Jul 2021 13:53:26 -0700
Message-ID: <CALvZod4m9iCD9cmfDqRPOqz=i5AkBdLQoJqYz2HQ2Av=mAt8yA@mail.gmail.com>
Subject: Re: [PATCH 2/4] mm/rmap: fix old bug: munlocking THP missed other mlocks
To:     Hugh Dickins <hughd@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Alistair Popple <apopple@nvidia.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Christoph Hellwig <hch@lst.de>, Yang Shi <shy828301@gmail.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>
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On Wed, Jul 7, 2021 at 1:08 PM Hugh Dickins <hughd@google.com> wrote:
>
> The kernel recovers in due course from missing Mlocked pages: but there
> was no point in calling page_mlock() (formerly known as try_to_munlock())
> on a THP, because nothing got done even when it was found to be mapped in
> another VM_LOCKED vma.
>
> It's true that we need to be careful: Mlocked accounting of pte-mapped
> THPs is too difficult (so consistently avoided); but Mlocked accounting
> of only-pmd-mapped THPs is supposed to work, even when multiple mappings
> are mlocked and munlocked or munmapped.  Refine the tests.
>
> There is already a VM_BUG_ON_PAGE(PageDoubleMap) in page_mlock(), so
> page_mlock_one() does not even have to worry about that complication.
>
> (I said the kernel recovers: but would page reclaim be likely to split
> THP before rediscovering that it's VM_LOCKED? I've not followed that up.)

I think, yes, page reclaim will split the THP prematurely in this case.

>
> Fixes: 9a73f61bdb8a ("thp, mlock: do not mlock PTE-mapped file huge pages")
> Signed-off-by: Hugh Dickins <hughd@google.com>

Reviewed-by: Shakeel Butt <shakeelb@google.com>