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Message-ID: <20130409144733.GA3158@phenom.dumpdata.com>
Date: Tue, 9 Apr 2013 10:47:33 -0400
From: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To: Roger Pau Monne <roger.pau@...rix.com>
Cc: linux-kernel@...r.kernel.org, xen-devel@...ts.xen.org
Subject: Re: [PATCH v1 2/7] xen-blkback: use balloon pages for all mappings
On Wed, Mar 27, 2013 at 12:10:38PM +0100, Roger Pau Monne wrote:
> Using balloon pages for all granted pages allows us to simplify the
> logic in blkback, especially in the xen_blkbk_map function, since now
> we can decide if we want to map a grant persistently or not after we
> have actually mapped it. This could not be done before because
> persistent grants used ballooned pages, whereas non-persistent grants
> used pages from the kernel.
>
> This patch also introduces several changes, the first one is that the
> list of free pages is no longer global, now each blkback instance has
> it's own list of free pages that can be used to map grants. Also, a
> run time parameter (max_buffer_pages) has been added in order to tune
> the maximum number of free pages each blkback instance will keep in
> it's buffer.
>
> Signed-off-by: Roger Pau Monné <roger.pau@...rix.com>
> Cc: xen-devel@...ts.xen.org
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Sorry for the late review. Some comments.
> ---
> Changes since RFC:
> * Fix typos in commit message.
> * Minor fixes in code.
> ---
> Documentation/ABI/stable/sysfs-bus-xen-backend | 8 +
> drivers/block/xen-blkback/blkback.c | 265 +++++++++++++-----------
> drivers/block/xen-blkback/common.h | 5 +
> drivers/block/xen-blkback/xenbus.c | 3 +
> 4 files changed, 165 insertions(+), 116 deletions(-)
>
> diff --git a/Documentation/ABI/stable/sysfs-bus-xen-backend b/Documentation/ABI/stable/sysfs-bus-xen-backend
> index 3d5951c..e04afe0 100644
> --- a/Documentation/ABI/stable/sysfs-bus-xen-backend
> +++ b/Documentation/ABI/stable/sysfs-bus-xen-backend
> @@ -73,3 +73,11 @@ KernelVersion: 3.0
> Contact: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> Description:
> Number of sectors written by the frontend.
> +
> +What: /sys/module/xen_blkback/parameters/max_buffer_pages
> +Date: March 2013
> +KernelVersion: 3.10
> +Contact: Roger Pau Monné <roger.pau@...rix.com>
> +Description:
> + Maximum number of free pages to keep in each block
> + backend buffer.
> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> index f7526db..8a1892a 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -63,6 +63,21 @@ static int xen_blkif_reqs = 64;
> module_param_named(reqs, xen_blkif_reqs, int, 0);
> MODULE_PARM_DESC(reqs, "Number of blkback requests to allocate");
>
> +/*
> + * Maximum number of unused free pages to keep in the internal buffer.
> + * Setting this to a value too low will reduce memory used in each backend,
> + * but can have a performance penalty.
> + *
> + * A sane value is xen_blkif_reqs * BLKIF_MAX_SEGMENTS_PER_REQUEST, but can
> + * be set to a lower value that might degrade performance on some intensive
> + * IO workloads.
> + */
> +
> +static int xen_blkif_max_buffer_pages = 704;
> +module_param_named(max_buffer_pages, xen_blkif_max_buffer_pages, int, 0644);
> +MODULE_PARM_DESC(max_buffer_pages,
> +"Maximum number of free pages to keep in each block backend buffer");
> +
> /* Run-time switchable: /sys/module/blkback/parameters/ */
> static unsigned int log_stats;
> module_param(log_stats, int, 0644);
> @@ -82,10 +97,14 @@ struct pending_req {
> int status;
> struct list_head free_list;
> DECLARE_BITMAP(unmap_seg, BLKIF_MAX_SEGMENTS_PER_REQUEST);
> + struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> };
>
> #define BLKBACK_INVALID_HANDLE (~0)
>
> +/* Number of free pages to remove on each call to free_xenballooned_pages */
> +#define NUM_BATCH_FREE_PAGES 10
> +
> struct xen_blkbk {
> struct pending_req *pending_reqs;
> /* List of all 'pending_req' available */
> @@ -93,8 +112,6 @@ struct xen_blkbk {
> /* And its spinlock. */
> spinlock_t pending_free_lock;
> wait_queue_head_t pending_free_wq;
> - /* The list of all pages that are available. */
> - struct page **pending_pages;
> /* And the grant handles that are available. */
> grant_handle_t *pending_grant_handles;
> };
> @@ -143,14 +160,66 @@ static inline int vaddr_pagenr(struct pending_req *req, int seg)
> BLKIF_MAX_SEGMENTS_PER_REQUEST + seg;
> }
>
> -#define pending_page(req, seg) pending_pages[vaddr_pagenr(req, seg)]
> +static inline int get_free_page(struct xen_blkif *blkif, struct page **page)
> +{
> + unsigned long flags;
> +
> + spin_lock_irqsave(&blkif->free_pages_lock, flags);
I am curious to why you need to use the irqsave variant one here, as
> + if (list_empty(&blkif->free_pages)) {
> + BUG_ON(blkif->free_pages_num != 0);
> + spin_unlock_irqrestore(&blkif->free_pages_lock, flags);
> + return alloc_xenballooned_pages(1, page, false);
This function is using an mutex.
which would imply it is OK to have an non-irq variant of spinlock?
> + }
> + BUG_ON(blkif->free_pages_num == 0);
> + page[0] = list_first_entry(&blkif->free_pages, struct page, lru);
> + list_del(&page[0]->lru);
> + blkif->free_pages_num--;
> + spin_unlock_irqrestore(&blkif->free_pages_lock, flags);
> +
> + return 0;
> +}
> +
> +static inline void put_free_pages(struct xen_blkif *blkif, struct page **page,
> + int num)
> +{
> + unsigned long flags;
> + int i;
> +
> + spin_lock_irqsave(&blkif->free_pages_lock, flags);
> + for (i = 0; i < num; i++)
> + list_add(&page[i]->lru, &blkif->free_pages);
> + blkif->free_pages_num += num;
> + spin_unlock_irqrestore(&blkif->free_pages_lock, flags);
> +}
>
> -static inline unsigned long vaddr(struct pending_req *req, int seg)
> +static inline void shrink_free_pagepool(struct xen_blkif *blkif, int num)
> {
> - unsigned long pfn = page_to_pfn(blkbk->pending_page(req, seg));
> - return (unsigned long)pfn_to_kaddr(pfn);
> + /* Remove requested pages in batches of NUM_BATCH_FREE_PAGES */
> + struct page *page[NUM_BATCH_FREE_PAGES];
> + unsigned long flags;
> + unsigned int num_pages = 0;
> +
> + spin_lock_irqsave(&blkif->free_pages_lock, flags);
> + while (blkif->free_pages_num > num) {
> + BUG_ON(list_empty(&blkif->free_pages));
> + page[num_pages] = list_first_entry(&blkif->free_pages,
> + struct page, lru);
> + list_del(&page[num_pages]->lru);
> + blkif->free_pages_num--;
> + if (++num_pages == NUM_BATCH_FREE_PAGES) {
> + spin_unlock_irqrestore(&blkif->free_pages_lock, flags);
> + free_xenballooned_pages(num_pages, page);
> + spin_lock_irqsave(&blkif->free_pages_lock, flags);
> + num_pages = 0;
> + }
> + }
> + spin_unlock_irqrestore(&blkif->free_pages_lock, flags);
> + if (num_pages != 0)
> + free_xenballooned_pages(num_pages, page);
> }
>
> +#define vaddr(page) ((unsigned long)pfn_to_kaddr(page_to_pfn(page)))
> +
> #define pending_handle(_req, _seg) \
> (blkbk->pending_grant_handles[vaddr_pagenr(_req, _seg)])
>
> @@ -170,7 +239,7 @@ static void make_response(struct xen_blkif *blkif, u64 id,
> (n) = (&(pos)->node != NULL) ? rb_next(&(pos)->node) : NULL)
>
>
> -static void add_persistent_gnt(struct rb_root *root,
> +static int add_persistent_gnt(struct rb_root *root,
> struct persistent_gnt *persistent_gnt)
> {
> struct rb_node **new = &(root->rb_node), *parent = NULL;
> @@ -186,14 +255,15 @@ static void add_persistent_gnt(struct rb_root *root,
> else if (persistent_gnt->gnt > this->gnt)
> new = &((*new)->rb_right);
> else {
> - pr_alert(DRV_PFX " trying to add a gref that's already in the tree\n");
> - BUG();
> + pr_alert_ratelimited(DRV_PFX " trying to add a gref that's already in the tree\n");
> + return -EINVAL;
> }
> }
>
> /* Add new node and rebalance tree. */
> rb_link_node(&(persistent_gnt->node), parent, new);
> rb_insert_color(&(persistent_gnt->node), root);
> + return 0;
> }
>
> static struct persistent_gnt *get_persistent_gnt(struct rb_root *root,
> @@ -215,7 +285,8 @@ static struct persistent_gnt *get_persistent_gnt(struct rb_root *root,
> return NULL;
> }
>
> -static void free_persistent_gnts(struct rb_root *root, unsigned int num)
> +static void free_persistent_gnts(struct xen_blkif *blkif, struct rb_root *root,
> + unsigned int num)
> {
> struct gnttab_unmap_grant_ref unmap[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> @@ -240,7 +311,7 @@ static void free_persistent_gnts(struct rb_root *root, unsigned int num)
> ret = gnttab_unmap_refs(unmap, NULL, pages,
> segs_to_unmap);
> BUG_ON(ret);
> - free_xenballooned_pages(segs_to_unmap, pages);
> + put_free_pages(blkif, pages, segs_to_unmap);
> segs_to_unmap = 0;
> }
>
> @@ -422,13 +493,17 @@ int xen_blkif_schedule(void *arg)
> if (do_block_io_op(blkif))
> blkif->waiting_reqs = 1;
>
> + shrink_free_pagepool(blkif, xen_blkif_max_buffer_pages);
That threw me off the first time I saw it. I somehow thought it meant
to shrink all xen_blkif_max_buffer_pages, not the value "above" it.
Might need a comment saying: "/* Shrink if we have more than xen_blkif_max_buffer_pages. */"
> +
> if (log_stats && time_after(jiffies, blkif->st_print))
> print_stats(blkif);
> }
>
> + shrink_free_pagepool(blkif, 0);
Add a little comment by the zero please that says: /* All */
> +
> /* Free all persistent grant pages */
> if (!RB_EMPTY_ROOT(&blkif->persistent_gnts))
> - free_persistent_gnts(&blkif->persistent_gnts,
> + free_persistent_gnts(blkif, &blkif->persistent_gnts,
> blkif->persistent_gnt_c);
>
> BUG_ON(!RB_EMPTY_ROOT(&blkif->persistent_gnts));
> @@ -457,23 +532,25 @@ static void xen_blkbk_unmap(struct pending_req *req)
> struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> unsigned int i, invcount = 0;
> grant_handle_t handle;
> + struct xen_blkif *blkif = req->blkif;
> int ret;
>
> for (i = 0; i < req->nr_pages; i++) {
> if (!test_bit(i, req->unmap_seg))
> continue;
> handle = pending_handle(req, i);
> + pages[invcount] = req->pages[i];
> if (handle == BLKBACK_INVALID_HANDLE)
> continue;
> - gnttab_set_unmap_op(&unmap[invcount], vaddr(req, i),
> + gnttab_set_unmap_op(&unmap[invcount], vaddr(pages[invcount]),
> GNTMAP_host_map, handle);
> pending_handle(req, i) = BLKBACK_INVALID_HANDLE;
> - pages[invcount] = virt_to_page(vaddr(req, i));
> invcount++;
> }
>
> ret = gnttab_unmap_refs(unmap, NULL, pages, invcount);
> BUG_ON(ret);
> + put_free_pages(blkif, pages, invcount);
> }
>
> static int xen_blkbk_map(struct blkif_request *req,
> @@ -488,7 +565,6 @@ static int xen_blkbk_map(struct blkif_request *req,
> struct xen_blkif *blkif = pending_req->blkif;
> phys_addr_t addr = 0;
> int i, j;
> - bool new_map;
> int nseg = req->u.rw.nr_segments;
> int segs_to_map = 0;
> int ret = 0;
> @@ -517,68 +593,16 @@ static int xen_blkbk_map(struct blkif_request *req,
> * We are using persistent grants and
> * the grant is already mapped
> */
> - new_map = false;
> - } else if (use_persistent_gnts &&
> - blkif->persistent_gnt_c <
> - max_mapped_grant_pages(blkif->blk_protocol)) {
> - /*
> - * We are using persistent grants, the grant is
> - * not mapped but we have room for it
> - */
> - new_map = true;
> - persistent_gnt = kmalloc(
> - sizeof(struct persistent_gnt),
> - GFP_KERNEL);
> - if (!persistent_gnt)
> - return -ENOMEM;
> - if (alloc_xenballooned_pages(1, &persistent_gnt->page,
> - false)) {
> - kfree(persistent_gnt);
> - return -ENOMEM;
> - }
> - persistent_gnt->gnt = req->u.rw.seg[i].gref;
> - persistent_gnt->handle = BLKBACK_INVALID_HANDLE;
> -
> - pages_to_gnt[segs_to_map] =
> - persistent_gnt->page;
> - addr = (unsigned long) pfn_to_kaddr(
> - page_to_pfn(persistent_gnt->page));
> -
> - add_persistent_gnt(&blkif->persistent_gnts,
> - persistent_gnt);
> - blkif->persistent_gnt_c++;
> - pr_debug(DRV_PFX " grant %u added to the tree of persistent grants, using %u/%u\n",
> - persistent_gnt->gnt, blkif->persistent_gnt_c,
> - max_mapped_grant_pages(blkif->blk_protocol));
> - } else {
> - /*
> - * We are either using persistent grants and
> - * hit the maximum limit of grants mapped,
> - * or we are not using persistent grants.
> - */
> - if (use_persistent_gnts &&
> - !blkif->vbd.overflow_max_grants) {
> - blkif->vbd.overflow_max_grants = 1;
> - pr_alert(DRV_PFX " domain %u, device %#x is using maximum number of persistent grants\n",
> - blkif->domid, blkif->vbd.handle);
> - }
> - new_map = true;
> - pages[i] = blkbk->pending_page(pending_req, i);
> - addr = vaddr(pending_req, i);
> - pages_to_gnt[segs_to_map] =
> - blkbk->pending_page(pending_req, i);
> - }
> -
> - if (persistent_gnt) {
Nice :-)
> pages[i] = persistent_gnt->page;
> persistent_gnts[i] = persistent_gnt;
> } else {
> + if (get_free_page(blkif, &pages[i]))
> + goto out_of_memory;
> + addr = vaddr(pages[i]);
> + pages_to_gnt[segs_to_map] = pages[i];
> persistent_gnts[i] = NULL;
> - }
> -
> - if (new_map) {
> flags = GNTMAP_host_map;
> - if (!persistent_gnt &&
> + if (!use_persistent_gnts &&
> (pending_req->operation != BLKIF_OP_READ))
> flags |= GNTMAP_readonly;
> gnttab_set_map_op(&map[segs_to_map++], addr,
> @@ -599,47 +623,71 @@ static int xen_blkbk_map(struct blkif_request *req,
> */
> bitmap_zero(pending_req->unmap_seg, BLKIF_MAX_SEGMENTS_PER_REQUEST);
> for (i = 0, j = 0; i < nseg; i++) {
> - if (!persistent_gnts[i] ||
> - persistent_gnts[i]->handle == BLKBACK_INVALID_HANDLE) {
> + if (!persistent_gnts[i]) {
> /* This is a newly mapped grant */
> BUG_ON(j >= segs_to_map);
> if (unlikely(map[j].status != 0)) {
> pr_debug(DRV_PFX "invalid buffer -- could not remap it\n");
> - map[j].handle = BLKBACK_INVALID_HANDLE;
> + pending_handle(pending_req, i) =
> + BLKBACK_INVALID_HANDLE;
You can make that on one line. The 80 character limit is not that ... strict anymore.
Here is what Ingo said about it:
https://lkml.org/lkml/2012/2/3/101
> ret |= 1;
> - if (persistent_gnts[i]) {
> - rb_erase(&persistent_gnts[i]->node,
> - &blkif->persistent_gnts);
> - blkif->persistent_gnt_c--;
> - kfree(persistent_gnts[i]);
> - persistent_gnts[i] = NULL;
> - }
> + j++;
> + continue;
The old code had abit of different logic for the non-persistent error path.
It would do:
598 if (!persistent_gnts[i] ||
..
602 if (unlikely(map[j].status != 0)) {
605 ret |= 1;
.. then later.. for the !persisten_gnts case:
634 } else {
635 pending_handle(pending_req, i) = map[j].handle;
636 bitmap_set(pending_req->unmap_seg, i, 1);
637
638 if (ret) {
639 j++;
640 continue;
641 }
> }
> + pending_handle(pending_req, i) = map[j].handle;
Which means that for this code, we skip the 635 (which is OK as you have done the
"pending_handle(pending_req, i) = BLKBACK_INVALID_HANDLE", but what the 636 case
(that is the bitmap_set)?
It presumarily is OK, as the 'pending_handle(pending_req, i)' ends up being
set to BLKBACK_INVALID_HANDLE, so the loop in xen_blkbk_unmap will still skip
over it?
This I think warrants a little comment saying: "We can skip the bitmap_set' as
xen_blkbk_unmap can handle BLKBACK_INVALID_HANDLE'.
But then that begs the question - why do we even need the bitmap_set code path anymore?
> }
> - if (persistent_gnts[i]) {
> - if (persistent_gnts[i]->handle ==
> - BLKBACK_INVALID_HANDLE) {
> + if (persistent_gnts[i])
> + goto next;
> + if (use_persistent_gnts &&
> + blkif->persistent_gnt_c <
> + max_mapped_grant_pages(blkif->blk_protocol)) {
> + /*
> + * We are using persistent grants, the grant is
> + * not mapped but we have room for it
> + */
> + persistent_gnt = kmalloc(sizeof(struct persistent_gnt),
> + GFP_KERNEL);
> + if (!persistent_gnt) {
> /*
> - * If this is a new persistent grant
> - * save the handler
> + * If we don't have enough memory to
> + * allocate the persistent_gnt struct
> + * map this grant non-persistenly
> */
> - persistent_gnts[i]->handle = map[j++].handle;
> + j++;
> + goto next;
So you are doing this by assuming that get_persistent_gnt in the earlier loop
failed, which means you have in effect done this:
map[segs_to_map++]
Doing the next label will set:
seg[i].offset = (req->u.rw.seg[i].first_sect << 9);
OK, that sounds right. Is this then:
bitmap_set(pending_req->unmap_seg, i, 1);
even needed? The "pending_handle(pending_req, i) = map[j].handle;" had already been
done in the /* This is a newly mapped grant */ if case, so we are set there.
Perhaps you could update the comment from saying 'map this grant' (which
implies doing it NOW as opposed to have done it already), and say:
/*
.. continue using the grant non-persistently. Note that
we mapped it in the earlier loop and the earlier if conditional
sets pending_handle(pending_req, i) = map[j].handle.
*/
> }
> - pending_handle(pending_req, i) =
> - persistent_gnts[i]->handle;
> -
> - if (ret)
> - continue;
> - } else {
> - pending_handle(pending_req, i) = map[j++].handle;
> - bitmap_set(pending_req->unmap_seg, i, 1);
> -
> - if (ret)
> - continue;
> + persistent_gnt->gnt = map[j].ref;
> + persistent_gnt->handle = map[j].handle;
> + persistent_gnt->page = pages[i];
Oh boy, that is a confusing. i and j. Keep loosing track which one is which.
It lookis right.
> + if (add_persistent_gnt(&blkif->persistent_gnts,
> + persistent_gnt)) {
> + kfree(persistent_gnt);
I would also say 'persisten_gnt = NULL' for extra measure of safety
> + j++;
Perhaps the 'j' variable can be called 'map_idx' ? By this point I am pretty
sure I know what the 'i' and 'j' variables are used for, but if somebody new
is trying to grok this code they might spend some 5 minutes trying to figure
this out.
> + goto next;
> + }
> + blkif->persistent_gnt_c++;
> + pr_debug(DRV_PFX " grant %u added to the tree of persistent grants, using %u/%u\n",
> + persistent_gnt->gnt, blkif->persistent_gnt_c,
> + max_mapped_grant_pages(blkif->blk_protocol));
> + j++;
> + goto next;
> }
> + if (use_persistent_gnts && !blkif->vbd.overflow_max_grants) {
> + blkif->vbd.overflow_max_grants = 1;
> + pr_debug(DRV_PFX " domain %u, device %#x is using maximum number of persistent grants\n",
> + blkif->domid, blkif->vbd.handle);
> + }
> + bitmap_set(pending_req->unmap_seg, i, 1);
> + j++;
> +next:
> seg[i].offset = (req->u.rw.seg[i].first_sect << 9);
> }
> return ret;
> +
> +out_of_memory:
> + pr_alert(DRV_PFX "%s: out of memory\n", __func__);
> + put_free_pages(blkif, pages_to_gnt, segs_to_map);
> + return -ENOMEM;
> }
>
> static int dispatch_discard_io(struct xen_blkif *blkif,
> @@ -863,7 +911,7 @@ static int dispatch_rw_block_io(struct xen_blkif *blkif,
> int operation;
> struct blk_plug plug;
> bool drain = false;
> - struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> + struct page **pages = pending_req->pages;
>
> switch (req->operation) {
> case BLKIF_OP_READ:
> @@ -1090,22 +1138,14 @@ static int __init xen_blkif_init(void)
> xen_blkif_reqs, GFP_KERNEL);
> blkbk->pending_grant_handles = kmalloc(sizeof(blkbk->pending_grant_handles[0]) *
> mmap_pages, GFP_KERNEL);
> - blkbk->pending_pages = kzalloc(sizeof(blkbk->pending_pages[0]) *
> - mmap_pages, GFP_KERNEL);
>
> - if (!blkbk->pending_reqs || !blkbk->pending_grant_handles ||
> - !blkbk->pending_pages) {
> + if (!blkbk->pending_reqs || !blkbk->pending_grant_handles) {
> rc = -ENOMEM;
> goto out_of_memory;
> }
>
> for (i = 0; i < mmap_pages; i++) {
> blkbk->pending_grant_handles[i] = BLKBACK_INVALID_HANDLE;
> - blkbk->pending_pages[i] = alloc_page(GFP_KERNEL);
> - if (blkbk->pending_pages[i] == NULL) {
> - rc = -ENOMEM;
> - goto out_of_memory;
> - }
> }
> rc = xen_blkif_interface_init();
> if (rc)
> @@ -1130,13 +1170,6 @@ static int __init xen_blkif_init(void)
> failed_init:
> kfree(blkbk->pending_reqs);
> kfree(blkbk->pending_grant_handles);
> - if (blkbk->pending_pages) {
> - for (i = 0; i < mmap_pages; i++) {
> - if (blkbk->pending_pages[i])
> - __free_page(blkbk->pending_pages[i]);
> - }
> - kfree(blkbk->pending_pages);
> - }
> kfree(blkbk);
> blkbk = NULL;
> return rc;
> diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
> index 60103e2..6c73c38 100644
> --- a/drivers/block/xen-blkback/common.h
> +++ b/drivers/block/xen-blkback/common.h
> @@ -220,6 +220,11 @@ struct xen_blkif {
> struct rb_root persistent_gnts;
> unsigned int persistent_gnt_c;
>
> + /* buffer of free pages to map grant refs */
> + spinlock_t free_pages_lock;
> + int free_pages_num;
> + struct list_head free_pages;
> +
> /* statistics */
> unsigned long st_print;
> unsigned long long st_rd_req;
> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> index 8bfd1bc..24f7f6d 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -118,6 +118,9 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
> blkif->st_print = jiffies;
> init_waitqueue_head(&blkif->waiting_to_free);
> blkif->persistent_gnts.rb_node = NULL;
> + spin_lock_init(&blkif->free_pages_lock);
> + INIT_LIST_HEAD(&blkif->free_pages);
> + blkif->free_pages_num = 0;
>
> return blkif;
> }
> --
> 1.7.7.5 (Apple Git-26)
>
--
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