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Date:   Tue, 28 Sep 2021 13:06:39 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Song Liu <song@kernel.org>
Cc:     Rongwei Wang <rongwei.wang@linux.alibaba.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        William Kucharski <william.kucharski@oracle.com>,
        Hugh Dickins <hughd@google.com>
Subject: Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page
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On Mon, Sep 27, 2021 at 03:24:47PM -0700, Song Liu wrote:
> OTOH, does it make sense to block writes within khugepaged, like:
> @@ -1652,6 +1653,11 @@ static void collapse_file(struct mm_struct *mm,
>         /* Only allocate from the target node */
>         gfp = alloc_hugepage_khugepaged_gfpmask() | __GFP_THISNODE;
> 
> +       if (deny_write_access(file)) {
> +               result = SCAN_BUSY_WRITE;
> +               return;
> +       }

The problem is that it denies, rather than blocking.  That means that the
writer gets ETXTBSY instead of waiting until khugepaged is done.