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Re: [PATCH v3 12/20] xen/balloon: Don't rely on the page granularity is the same for Xen and Linux

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Hi Stefano,

On 10/08/15 12:18, Stefano Stabellini wrote:
>>  			/* Link back into the page tables if not highmem. */
>> @@ -396,14 +413,15 @@ static enum bp_state increase_reservation(unsigned long nr_pages)
>>  static enum bp_state decrease_reservation(unsigned long nr_pages, gfp_t gfp)
>>  {
>>  	enum bp_state state = BP_DONE;
>> -	unsigned long  pfn, i;
>> +	unsigned long i;
>>  	struct page   *page;
>>  	int ret;
>>  	struct xen_memory_reservation reservation = {
>>  		.address_bits = 0,
>> -		.extent_order = 0,
>> +		.extent_order = EXTENT_ORDER,
>>  		.domid        = DOMID_SELF
>>  	};
>> +	static struct page *pages[ARRAY_SIZE(frame_list)];
> 
> This array can be rather large: I would try to avoid it, see below.

[..]

> 
> I would simply and avoid introducing a new array:
>     pfn = (frame_list[i] << XEN_PAGE_SHIFT) >> PAGE_SHIFT;
>     page = pfn_to_page(pfn);

Which won't work because the frame_list contains a gfn and not a pfn.
We need to translate back the gfn into a pfn and the into a page.

The cost of the translation may be big and I wanted to avoid anymore
XEN_PAGE_SHIFT in the code. In general we should avoid to deal with 4KB
PFN when it's not necessary, it make the code more confusing to read.

If your only concern is the size of the array, we could decrease the
number of frames by batch. Or allocation the variable once a boot time.

Regards,

-- 
Julien Grall
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[PATCH v3 00/20] xen/arm64: Add support for 64KB page Julien Grall <julien.grall@citrix.com> - 2015-08-07 18:50 +0200
  [PATCH v3 09/20] xen/biomerge: Don't allow biovec to be merge when Linux is not using 4KB page Julien Grall <julien.grall@citrix.com> - 2015-08-07 18:50 +0200
    Re: [PATCH v3 09/20] xen/biomerge: Don't allow biovec to be merge  when Linux is not using 4KB page Stefano Stabellini <stefano.stabellini@eu.citrix.com> - 2015-08-10 13:00 +0200
      Re: [Xen-devel] [PATCH v3 09/20] xen/biomerge: Don't allow biovec  to be merge when Linux is not using 4KB page Julien Grall <julien.grall@citrix.com> - 2015-08-10 13:30 +0200
        Re: [Xen-devel] [PATCH v3 09/20] xen/biomerge: Don't allow biovec  to be merge when Linux is not using 4KB page Stefano Stabellini <stefano.stabellini@eu.citrix.com> - 2015-08-10 13:30 +0200
          Re: [Xen-devel] [PATCH v3 09/20] xen/biomerge: Don't allow biovec  to be merge when Linux is not using 4KB page Julien Grall <julien.grall@citrix.com> - 2015-08-10 13:40 +0200
            Re: [Xen-devel] [PATCH v3 09/20] xen/biomerge: Don't allow biovec  to be merge when Linux is not using 4KB page David Vrabel <david.vrabel@citrix.com> - 2015-08-10 14:50 +0200
  [PATCH v3 07/20] block/xen-blkfront: Store a page rather a pfn in the grant structure Julien Grall <julien.grall@citrix.com> - 2015-08-07 18:50 +0200
  [PATCH v3 06/20] block/xen-blkfront: Split blkif_queue_request in 2 Julien Grall <julien.grall@citrix.com> - 2015-08-07 18:50 +0200
  [PATCH v3 03/20] xen: Add Xen specific page definition Julien Grall <julien.grall@citrix.com> - 2015-08-07 19:00 +0200
    Re: [PATCH v3 03/20] xen: Add Xen specific page definition Stefano Stabellini <stefano.stabellini@eu.citrix.com> - 2015-08-10 12:50 +0200
  [PATCH v3 01/20] net/xen-netback: xenvif_gop_frag_copy: move GSO check out of the loop Julien Grall <julien.grall@citrix.com> - 2015-08-07 19:00 +0200
    Re: [PATCH v3 01/20] net/xen-netback: xenvif_gop_frag_copy: move GSO  check out of the loop Wei Liu <wei.liu2@citrix.com> - 2015-08-08 16:00 +0200
  [PATCH v3 14/20] xen/grant-table: Make it running on 64KB granularity Julien Grall <julien.grall@citrix.com> - 2015-08-07 19:20 +0200
  Re: [Xen-devel] [PATCH v3 00/20] xen/arm64: Add support for 64KB  page Julien Grall <julien.grall@citrix.com> - 2015-08-07 19:20 +0200
  [PATCH v3 10/20] xen/xenbus: Use Xen page definition Julien Grall <julien.grall@citrix.com> - 2015-08-07 19:20 +0200
  [PATCH v3 17/20] net/xen-netfront: Make it running on 64KB page granularity Julien Grall <julien.grall@citrix.com> - 2015-08-07 19:20 +0200
  [PATCH v3 11/20] tty/hvc: xen: Use xen page definition Julien Grall <julien.grall@citrix.com> - 2015-08-07 19:20 +0200
  [PATCH v3 12/20] xen/balloon: Don't rely on the page granularity is the same for Xen and Linux Julien Grall <julien.grall@citrix.com> - 2015-08-07 19:20 +0200
    Re: [PATCH v3 12/20] xen/balloon: Don't rely on the page granularity  is the same for Xen and Linux Stefano Stabellini <stefano.stabellini@eu.citrix.com> - 2015-08-10 13:20 +0200
      Re: [PATCH v3 12/20] xen/balloon: Don't rely on the page granularity  is the same for Xen and Linux Julien Grall <julien.grall@citrix.com> - 2015-08-10 13:40 +0200
        Re: [PATCH v3 12/20] xen/balloon: Don't rely on the page granularity  is the same for Xen and Linux Stefano Stabellini <stefano.stabellini@eu.citrix.com> - 2015-08-10 15:00 +0200
          Re: [PATCH v3 12/20] xen/balloon: Don't rely on the page granularity  is the same for Xen and Linux Julien Grall <julien.grall@citrix.com> - 2015-08-10 15:40 +0200
  [PATCH v3 13/20] xen/events: fifo: Make it running on 64KB granularity Julien Grall <julien.grall@citrix.com> - 2015-08-07 19:20 +0200
  [PATCH v3 19/20] xen/privcmd: Add support for Linux 64KB page granularity Julien Grall <julien.grall@citrix.com> - 2015-08-07 19:20 +0200
    Re: [PATCH v3 19/20] xen/privcmd: Add support for Linux 64KB page  granularity Stefano Stabellini <stefano.stabellini@eu.citrix.com> - 2015-08-10 14:10 +0200
      Re: [Xen-devel] [PATCH v3 19/20] xen/privcmd: Add support for Linux  64KB page granularity David Vrabel <david.vrabel@citrix.com> - 2015-08-10 14:20 +0200
        Re: [Xen-devel] [PATCH v3 19/20] xen/privcmd: Add support for Linux  64KB page granularity Stefano Stabellini <stefano.stabellini@eu.citrix.com> - 2015-08-10 15:00 +0200
          Re: [Xen-devel] [PATCH v3 19/20] xen/privcmd: Add support for Linux  64KB page granularity Julien Grall <julien.grall@citrix.com> - 2015-08-10 15:30 +0200
  [PATCH v3 18/20] net/xen-netback: Make it running on 64KB page granularity Julien Grall <julien.grall@citrix.com> - 2015-08-07 19:20 +0200
    Re: [PATCH v3 18/20] net/xen-netback: Make it running on 64KB page  granularity Wei Liu <wei.liu2@citrix.com> - 2015-08-08 17:00 +0200
      Re: [PATCH v3 18/20] net/xen-netback: Make it running on 64KB page  granularity Julien Grall <julien.grall@citrix.com> - 2015-08-10 12:00 +0200
        Re: [PATCH v3 18/20] net/xen-netback: Make it running on 64KB page  granularity Wei Liu <wei.liu2@citrix.com> - 2015-08-10 13:40 +0200
          Re: [PATCH v3 18/20] net/xen-netback: Make it running on 64KB page  granularity Julien Grall <julien.grall@citrix.com> - 2015-08-10 14:10 +0200
  [PATCH v3 15/20] block/xen-blkfront: Make it running on 64KB page granularity Julien Grall <julien.grall@citrix.com> - 2015-08-07 19:20 +0200
  [PATCH v3 16/20] block/xen-blkback: Make it running on 64KB page granularity Julien Grall <julien.grall@citrix.com> - 2015-08-07 19:20 +0200
  [PATCH v3 20/20] arm/xen: Add support for 64KB page granularity Julien Grall <julien.grall@citrix.com> - 2015-08-07 19:20 +0200
    Re: [PATCH v3 20/20] arm/xen: Add support for 64KB page  granularity Stefano Stabellini <stefano.stabellini@eu.citrix.com> - 2015-08-10 15:00 +0200

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