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SubjectRe: [PATCH RFC 04/12] xen-blkfront: pre-allocate pages for requests
On Thu, Feb 28, 2013 at 11:28:47AM +0100, Roger Pau Monne wrote:
> This prevents us from having to call alloc_page while we are preparing
> the request. Since blkfront was calling alloc_page with a spinlock
> held we used GFP_ATOMIC, which can fail if we are requesting a lot of
> pages since it is using the emergency memory pools.
>
> Allocating all the pages at init prevents us from having to call
> alloc_page, thus preventing possible failures.
>
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: xen-devel@lists.xen.org
> ---
> drivers/block/xen-blkfront.c | 120 +++++++++++++++++++++++++++--------------
> 1 files changed, 79 insertions(+), 41 deletions(-)
>
> diff --git a/drivers/block/xen-blkfront.c b/drivers/block/xen-blkfront.c
> index 2e39eaf..5ba6b87 100644
> --- a/drivers/block/xen-blkfront.c
> +++ b/drivers/block/xen-blkfront.c
> @@ -165,6 +165,69 @@ static int add_id_to_freelist(struct blkfront_info *info,
> return 0;
> }
>
> +static int fill_grant_buffer(struct blkfront_info *info, int num)
> +{
> + struct page *granted_page;
> + struct grant *gnt_list_entry, *n;
> + int i = 0;
> +
> + while(i < num) {
> + gnt_list_entry = kzalloc(sizeof(struct grant), GFP_NOIO);

GFP_NORMAL ?

> + if (!gnt_list_entry)
> + goto out_of_memory;

Hmm, I guess another patch could be to convert this to a fail-safe
mechanism. Meaning if we fail here, we just cap our maximum amount of
grants we have up to 'i'.


> +
> + granted_page = alloc_page(GFP_NOIO);

GFP_NORMAL

> + if (!granted_page) {
> + kfree(gnt_list_entry);
> + goto out_of_memory;
> + }
> +
> + gnt_list_entry->pfn = page_to_pfn(granted_page);
> + gnt_list_entry->gref = GRANT_INVALID_REF;
> + list_add(&gnt_list_entry->node, &info->persistent_gnts);
> + i++;
> + }
> +
> + return 0;
> +
> +out_of_memory:
> + list_for_each_entry_safe(gnt_list_entry, n,
> + &info->persistent_gnts, node) {
> + list_del(&gnt_list_entry->node);
> + __free_page(pfn_to_page(gnt_list_entry->pfn));
> + kfree(gnt_list_entry);
> + i--;
> + }
> + BUG_ON(i != 0);
> + return -ENOMEM;
> +}
> +
> +static struct grant *get_grant(grant_ref_t *gref_head,
> + struct blkfront_info *info)
> +{
> + struct grant *gnt_list_entry;
> + unsigned long buffer_mfn;
> +
> + BUG_ON(list_empty(&info->persistent_gnts));
> + gnt_list_entry = list_first_entry(&info->persistent_gnts, struct grant,
> + node);
> + list_del(&gnt_list_entry->node);
> +
> + if (gnt_list_entry->gref != GRANT_INVALID_REF) {
> + info->persistent_gnts_c--;
> + return gnt_list_entry;
> + }
> +
> + /* Assign a gref to this page */
> + gnt_list_entry->gref = gnttab_claim_grant_reference(gref_head);
> + BUG_ON(gnt_list_entry->gref == -ENOSPC);
> + buffer_mfn = pfn_to_mfn(gnt_list_entry->pfn);
> + gnttab_grant_foreign_access_ref(gnt_list_entry->gref,
> + info->xbdev->otherend_id,
> + buffer_mfn, 0);
> + return gnt_list_entry;
> +}
> +
> static const char *op_name(int op)
> {
> static const char *const names[] = {
> @@ -306,7 +369,6 @@ static int blkif_queue_request(struct request *req)
> */
> bool new_persistent_gnts;
> grant_ref_t gref_head;
> - struct page *granted_page;
> struct grant *gnt_list_entry = NULL;
> struct scatterlist *sg;
>
> @@ -370,42 +432,9 @@ static int blkif_queue_request(struct request *req)
> fsect = sg->offset >> 9;
> lsect = fsect + (sg->length >> 9) - 1;
>
> - if (info->persistent_gnts_c) {
> - BUG_ON(list_empty(&info->persistent_gnts));
> - gnt_list_entry = list_first_entry(
> - &info->persistent_gnts,
> - struct grant, node);
> - list_del(&gnt_list_entry->node);
> -
> - ref = gnt_list_entry->gref;
> - buffer_mfn = pfn_to_mfn(gnt_list_entry->pfn);
> - info->persistent_gnts_c--;
> - } else {
> - ref = gnttab_claim_grant_reference(&gref_head);
> - BUG_ON(ref == -ENOSPC);
> -
> - gnt_list_entry =
> - kmalloc(sizeof(struct grant),
> - GFP_ATOMIC);
> - if (!gnt_list_entry)
> - return -ENOMEM;
> -
> - granted_page = alloc_page(GFP_ATOMIC);
> - if (!granted_page) {
> - kfree(gnt_list_entry);
> - return -ENOMEM;
> - }
> -
> - gnt_list_entry->pfn =
> - page_to_pfn(granted_page);
> - gnt_list_entry->gref = ref;
> -
> - buffer_mfn = pfn_to_mfn(page_to_pfn(
> - granted_page));
> - gnttab_grant_foreign_access_ref(ref,
> - info->xbdev->otherend_id,
> - buffer_mfn, 0);
> - }
> + gnt_list_entry = get_grant(&gref_head, info);
> + ref = gnt_list_entry->gref;
> + buffer_mfn = pfn_to_mfn(gnt_list_entry->pfn);
>
> info->shadow[id].grants_used[i] = gnt_list_entry;
>
> @@ -803,17 +832,20 @@ static void blkif_free(struct blkfront_info *info, int suspend)
> blk_stop_queue(info->rq);
>
> /* Remove all persistent grants */
> - if (info->persistent_gnts_c) {
> + if (!list_empty(&info->persistent_gnts)) {
> list_for_each_entry_safe(persistent_gnt, n,
> &info->persistent_gnts, node) {
> list_del(&persistent_gnt->node);
> - gnttab_end_foreign_access(persistent_gnt->gref, 0, 0UL);
> + if (persistent_gnt->gref != GRANT_INVALID_REF) {
> + gnttab_end_foreign_access(persistent_gnt->gref,
> + 0, 0UL);
> + info->persistent_gnts_c--;
> + }
> __free_page(pfn_to_page(persistent_gnt->pfn));
> kfree(persistent_gnt);
> - info->persistent_gnts_c--;
> }
> - BUG_ON(info->persistent_gnts_c != 0);
> }
> + BUG_ON(info->persistent_gnts_c != 0);

So if the guest _never_ sent any I/Os and just attached/detached the device - won't
we fail here?.
>
> /* No more gnttab callback work. */
> gnttab_cancel_free_callback(&info->callback);
> @@ -1088,6 +1120,12 @@ again:
> goto destroy_blkring;
> }
>
> + /* Allocate memory for grants */
> + err = fill_grant_buffer(info, BLK_RING_SIZE *
> + BLKIF_MAX_SEGMENTS_PER_REQUEST);
> + if (err)
> + goto out;

That looks to be in the wrong function - talk_to_blkback function is
to talk to the blkback. Not do initialization type operations.

Also I think this means that on resume - we would try to allocate
again the grants?
> +
> xenbus_switch_state(dev, XenbusStateInitialised);
>
> return 0;
> --
> 1.7.7.5 (Apple Git-26)
>
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