On Thu, Feb 28, 2013 at 11:28:51AM +0100, Roger Pau Monne wrote:
> Using balloon pages for all granted pages allows us to simplify the
> logic in blkback, specially in the xen_blkbk_map function, since now
especially
> 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, and non-persistent grants used
^^^whereas
> 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 Monn? <[email protected]>
> Cc: [email protected]
> Cc: Konrad Rzeszutek Wilk <[email protected]>
> ---
> drivers/block/xen-blkback/blkback.c | 278 +++++++++++++++++++----------------
> drivers/block/xen-blkback/common.h | 5 +
> drivers/block/xen-blkback/xenbus.c | 3 +
You also need a Documentation/ABI/sysfs-stable/... file.
> 3 files changed, 159 insertions(+), 127 deletions(-)
>
> diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
> index b5e7495..ba27fc3 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -101,6 +101,21 @@ module_param_named(lru_num_clean, xen_blkif_lru_num_clean, int, 0644);
> MODULE_PARM_DESC(lru_num_clean,
> "Number of persistent grants to unmap when the list is full");
>
> +/*
> + * 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
Should that value be default value then?
> + * be set to a lower value that might degrade performance on some intensive
> + * IO workloads.
> + */
> +
> +static int xen_blkif_max_buffer_pages = 1024;
> +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);
> @@ -120,6 +135,7 @@ struct pending_req {
> int status;
> struct list_head free_list;
> struct persistent_gnt *persistent_gnts[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> + struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> };
>
> #define BLKBACK_INVALID_HANDLE (~0)
> @@ -131,8 +147,6 @@ 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;
> };
> @@ -151,14 +165,66 @@ 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);
> + 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);
> + }
> + 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 remove_free_pages(struct xen_blkif *blkif, int num)
Perhaps 'shrink_free_pagepool'?
> {
> - unsigned long pfn = page_to_pfn(blkbk->pending_page(req, seg));
> - return (unsigned long)pfn_to_kaddr(pfn);
> + /* Remove requested pages in batches of 10 */
> + struct page *page[10];
Hrmp. #define!
> + unsigned long flags;
> + int num_pages = 0;
unsigned int
> +
> + 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 == 10) {
> + 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)])
>
> @@ -178,7 +244,7 @@ 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;
> @@ -194,14 +260,15 @@ 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;
That looks like a seperate bug-fix patch? Especially the pr_alert_ratelimited
part?
> }
> }
>
> /* 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,
> @@ -223,7 +290,8 @@ 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];
> @@ -248,7 +316,7 @@ 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;
> }
>
> @@ -259,7 +327,8 @@ static void free_persistent_gnts(struct rb_root *root, unsigned int num)
> BUG_ON(num != 0);
> }
>
> -static int purge_persistent_gnt(struct rb_root *root, int num)
> +static int purge_persistent_gnt(struct xen_blkif *blkif, struct rb_root *root,
> + int num)
> {
> struct gnttab_unmap_grant_ref unmap[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> struct page *pages[BLKIF_MAX_SEGMENTS_PER_REQUEST];
> @@ -294,7 +363,7 @@ purge_list:
> 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;
> }
>
> @@ -317,7 +386,7 @@ finished:
> if (segs_to_unmap > 0) {
> 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);
> }
> /* Finally remove the "used" flag from all the persistent grants */
> foreach_grant_safe(persistent_gnt, n, root, node) {
> @@ -521,7 +590,7 @@ purge_gnt_list:
> xen_blkif_lru_num_clean;
> rq_purge = rq_purge > blkif->persistent_gnt_c ?
> blkif->persistent_gnt_c : rq_purge;
> - purged = purge_persistent_gnt(
> + purged = purge_persistent_gnt(blkif,
> &blkif->persistent_gnts, rq_purge);
> if (purged != rq_purge)
> pr_debug(DRV_PFX " unable to meet persistent grants purge requirements for device %#x, domain %u, requested %d done %d\n",
> @@ -535,13 +604,17 @@ purge_gnt_list:
> msecs_to_jiffies(xen_blkif_lru_interval);
> }
>
> + remove_free_pages(blkif, xen_blkif_max_buffer_pages);
> +
> if (log_stats && time_after(jiffies, blkif->st_print))
> print_stats(blkif);
> }
>
> + remove_free_pages(blkif, 0);
What purpose does that have?
> +
> /* 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));
> @@ -571,6 +644,7 @@ static void xen_blkbk_unmap(struct pending_req *req)
> struct persistent_gnt *persistent_gnt;
> 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++) {
> @@ -581,17 +655,18 @@ static void xen_blkbk_unmap(struct pending_req *req)
> 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,
> @@ -606,7 +681,6 @@ 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;
> @@ -632,69 +706,17 @@ static int xen_blkbk_map(struct blkif_request *req,
> * We are using persistent grants and
> * the grant is already mapped
> */
> - new_map = false;
> persistent_gnt->flags |= PERSISTENT_GNT_ACTIVE;
> - } else if (use_persistent_gnts &&
> - blkif->persistent_gnt_c < xen_blkif_max_pgrants) {
> - /*
> - * 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;
> - persistent_gnt->flags = PERSISTENT_GNT_ACTIVE;
> -
> - 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,
> - xen_blkif_max_pgrants);
> - } 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_debug(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) {
> 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,
> @@ -714,54 +736,71 @@ static int xen_blkbk_map(struct blkif_request *req,
> * the page from the other domain.
> */
> 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;
> 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;
> }
> + pending_handle(pending_req, i) = map[j].handle;
> }
> - 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 < xen_blkif_max_pgrants) {
> + /*
> + * 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;
> - }
> - pending_handle(pending_req, i) =
> - persistent_gnts[i]->handle;
> -
> - if (ret)
> - continue;
> -
> - seg[i].buf = pfn_to_mfn(page_to_pfn(
> - persistent_gnts[i]->page)) << PAGE_SHIFT |
> - (req->u.rw.seg[i].first_sect << 9);
> - } else {
> - pending_handle(pending_req, i) = map[j].handle;
> -
> - if (ret) {
> j++;
> - continue;
> + goto next;
> }
> -
> - seg[i].buf = map[j++].dev_bus_addr |
> - (req->u.rw.seg[i].first_sect << 9);
> + persistent_gnt->gnt = map[j].ref;
> + persistent_gnt->handle = map[j].handle;
> + persistent_gnt->flags = PERSISTENT_GNT_ACTIVE;
> + persistent_gnt->page = pages[i];
> + if (add_persistent_gnt(&blkif->persistent_gnts,
> + persistent_gnt)) {
> + kfree(persistent_gnt);
> + goto next;
> + }
> + blkif->persistent_gnt_c++;
> + persistent_gnts[i] = persistent_gnt;
> + pr_debug(DRV_PFX " grant %u added to the tree of persistent grants, using %u/%u\n",
> + persistent_gnt->gnt, blkif->persistent_gnt_c,
> + xen_blkif_max_pgrants);
> + 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);
> }
> + j++;
> +next:
> + seg[i].buf = pfn_to_mfn(page_to_pfn(pages[i])) << PAGE_SHIFT |
> + (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,
> @@ -962,7 +1001,7 @@ 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:
> @@ -1193,22 +1232,14 @@ 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)
> @@ -1233,13 +1264,6 @@ 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 bd44d75..604bd30 100644
> --- a/drivers/block/xen-blkback/common.h
> +++ b/drivers/block/xen-blkback/common.h
> @@ -209,6 +209,11 @@ struct xen_blkif {
> unsigned int persistent_gnt_c;
> unsigned long next_lru;
>
> + /* 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;
> int st_rd_req;
> diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
> index abb399a..d7926ec 100644
> --- a/drivers/block/xen-blkback/xenbus.c
> +++ b/drivers/block/xen-blkback/xenbus.c
> @@ -119,6 +119,9 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
> blkif->next_lru = 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)
>
On 04/03/13 21:22, Konrad Rzeszutek Wilk wrote:
[...]
>> @@ -535,13 +604,17 @@ purge_gnt_list:
>> msecs_to_jiffies(xen_blkif_lru_interval);
>> }
>>
>> + remove_free_pages(blkif, xen_blkif_max_buffer_pages);
>> +
>> if (log_stats && time_after(jiffies, blkif->st_print))
>> print_stats(blkif);
>> }
>>
>> + remove_free_pages(blkif, 0);
>
> What purpose does that have?
This removes all the pages from the pool before closing down.
On 04/03/13 21:22, Konrad Rzeszutek Wilk wrote:
>> @@ -194,14 +260,15 @@ 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;
>
> That looks like a seperate bug-fix patch? Especially the pr_alert_ratelimited
> part?
Not really, the way we added granted frames before this patch, it was
never possible to add a persistent grant with the same gref twice.
With the changes introduced in this patch we first map the grants and
then we try to make them persistent by adding them to the tree. So it is
possible for a frontend to craft a malicious request that has the same
gref in all segments, and when we try to add them to the tree of
persistent grants we would hit the BUG, that's why we need to ratelimit
the alert (to prevent flooding), and return EINVAL instead of crashing.