From: Dan Williams <dan.j.williams@intel.com>
To: Vivek Goyal <vgoyal@redhat.com>
Cc: linux-fsdevel <linux-fsdevel@vger.kernel.org>,
virtio-fs@redhat.com, linux-nvdimm <linux-nvdimm@lists.01.org>
Subject: Re: [PATCH] dax: dax_layout_busy_page() should not unmap cow pages
Date: Fri, 2 Aug 2019 12:37:59 -0700 [thread overview]
Message-ID: <CAPcyv4jxknEGq9FzGpsMJ6E7jC51d1W9KbNg4HX6Cj6vqt7dqg@mail.gmail.com> (raw)
In-Reply-To: <20190802192956.GA3032@redhat.com>
On Fri, Aug 2, 2019 at 12:30 PM Vivek Goyal <vgoyal@redhat.com> wrote:
>
> As of now dax_layout_busy_page() calls unmap_mapping_range() with last
> argument as 1, which says even unmap cow pages. I am wondering who needs
> to get rid of cow pages as well.
>
> I noticed one interesting side affect of this. I mount xfs with -o dax and
> mmaped a file with MAP_PRIVATE and wrote some data to a page which created
> cow page. Then I called fallocate() on that file to zero a page of file.
> fallocate() called dax_layout_busy_page() which unmapped cow pages as well
> and then I tried to read back the data I wrote and what I get is old
> data from persistent memory. I lost the data I had written. This
> read basically resulted in new fault and read back the data from
> persistent memory.
>
> This sounds wrong. Are there any users which need to unmap cow pages
> as well? If not, I am proposing changing it to not unmap cow pages.
>
> I noticed this while while writing virtio_fs code where when I tried
> to reclaim a memory range and that corrupted the executable and I
> was running from virtio-fs and program got segment violation.
>
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> ---
> fs/dax.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> Index: rhvgoyal-linux/fs/dax.c
> ===================================================================
> --- rhvgoyal-linux.orig/fs/dax.c 2019-08-01 17:03:10.574675652 -0400
> +++ rhvgoyal-linux/fs/dax.c 2019-08-02 14:32:28.809639116 -0400
> @@ -600,7 +600,7 @@ struct page *dax_layout_busy_page(struct
> * guaranteed to either see new references or prevent new
> * references from being established.
> */
> - unmap_mapping_range(mapping, 0, 0, 1);
> + unmap_mapping_range(mapping, 0, 0, 0);
Good find, yes, this looks correct to me and should also go to -stable.
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next prev parent reply other threads:[~2019-08-02 19:40 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-02 19:29 [PATCH] dax: dax_layout_busy_page() should not unmap cow pages Vivek Goyal
2019-08-02 19:37 ` Dan Williams [this message]
2019-08-05 11:53 ` Boaz Harrosh
2019-08-05 18:49 ` Vivek Goyal
2019-08-05 19:16 ` Boaz Harrosh
2019-08-05 20:11 ` Dan Williams
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