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Re: [Xen-devel] ADs over dom0 iSCSI = high page_count()

  • To: Joshua Nicholas <jnicholas@xxxxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
  • Date: Fri, 05 Dec 2008 17:25:59 +0000
  • Cc:
  • Delivery-date: Fri, 05 Dec 2008 09:26:59 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AclW/onHynMF04j6aU2tBRRgqDyFRw==
  • Thread-topic: [Xen-devel] ADs over dom0 iSCSI = high page_count()

On 05/12/2008 16:51, "Joshua Nicholas" <jnicholas@xxxxxxxxxxxxxxx> wrote:

> Also in netback.c in net_tx_action_dealloc() after make_tx_response() I see:
>     /* Ready for next use. */
>     gnttab_reset_grant_page()
> Sure this resets the page_mapcount() back to 0, but it also
> sets the page_count() to 1 regardless of the current value.
> QUESTION 2:  Why does the page_count() have to be set to 1?

A page is only acted on by net_tx_action_dealloc() after it is queued by
netif_page_release(), and that is called from free_pages(), which means
page_count() must be zero at that time (i.e., page is unused).
gnttab_reset_grant_page() is simply setting up the page for its next use.

If the page is *not* actually finished with by the net or blocks layers,
that would be very bad indeed. :-)

 -- Keir

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