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Re: [Qemu-devel] [PATCH v2 for-xen-4.5] xen_disk: fix unmapping of persi
From: |
Stefano Stabellini |
Subject: |
Re: [Qemu-devel] [PATCH v2 for-xen-4.5] xen_disk: fix unmapping of persistent grants |
Date: |
Thu, 13 Nov 2014 15:36:08 +0000 |
User-agent: |
Alpine 2.02 (DEB 1266 2009-07-14) |
On Thu, 13 Nov 2014, Roger Pau Monne wrote:
> This patch fixes two issues with persistent grants and the disk PV backend
> (Qdisk):
>
> - Keep track of memory regions where persistent grants have been mapped
> since we need to unmap them as a whole. It is not possible to unmap a
> single grant if it has been batch-mapped.
> - Unmap persistent grants before switching to the closed state, so the
> frontend can also free them.
>
> Signed-off-by: Roger Pau Monné <address@hidden>
> Reported-by: George Dunlap <address@hidden>
> Cc: Stefano Stabellini <address@hidden>
> Cc: Kevin Wolf <address@hidden>
> Cc: Stefan Hajnoczi <address@hidden>
> Cc: George Dunlap <address@hidden>
> Cc: Konrad Rzeszutek Wilk <address@hidden>
> ---
> Xen release exception: this is obviously an important bug-fix that should be
> included in 4.5. Without this fix, trying to do a block-detach of a Qdisk
> from a running guest can lead to guest crash depending on the blkfront
> implementation.
> ---
> Changes since v1:
> - Instead of disabling batch mappings when using persistent grants, keep
> track of the persistently mapped regions and unmap them on disconnection.
> This prevents unmapping single persistent grants, but the current
> implementation doesn't require it.
>
> This new version is slightly faster than the previous one, the following
> test results are in IOPS from 20 runs of a block-attach, fio run,
> block-detach test loop:
>
> x batch
> + no_batch
> +----------------------------------------------------------------------+
> | + + x x + + + x xx x |
> |+ + x x+ *+++ * +x* +++x* x xx x *|
> | |_____________A_____M______||
> | |________________A_____M___________| |
> +----------------------------------------------------------------------+
> N Min Max Median Avg Stddev
> x 20 52941 63822 62396 61180.15 2673.6497
> + 20 49967 63849 60322 59116.9 3456.3455
> Difference at 95.0% confidence
> -2063.25 +/- 1977.66
> -3.37242% +/- 3.23252%
> (Student's t, pooled s = 3089.88)
Not too bad! Thanks!
> hw/block/xen_disk.c | 77
> ++++++++++++++++++++++++++++++++++++++++++++---------
> 1 file changed, 64 insertions(+), 13 deletions(-)
>
> diff --git a/hw/block/xen_disk.c b/hw/block/xen_disk.c
> index 231e9a7..75bdc16 100644
> --- a/hw/block/xen_disk.c
> +++ b/hw/block/xen_disk.c
> @@ -59,6 +59,13 @@ struct PersistentGrant {
>
> typedef struct PersistentGrant PersistentGrant;
>
> +struct PersistentRegion {
> + void *addr;
> + int num;
> +};
> +
> +typedef struct PersistentRegion PersistentRegion;
> +
> struct ioreq {
> blkif_request_t req;
> int16_t status;
> @@ -118,6 +125,7 @@ struct XenBlkDev {
> gboolean feature_discard;
> gboolean feature_persistent;
> GTree *persistent_gnts;
> + GSList *persistent_regions;
> unsigned int persistent_gnt_count;
> unsigned int max_grants;
>
> @@ -164,19 +172,41 @@ static gint int_cmp(gconstpointer a, gconstpointer b,
> gpointer user_data)
> static void destroy_grant(gpointer pgnt)
> {
> PersistentGrant *grant = pgnt;
> - XenGnttab gnt = grant->blkdev->xendev.gnttabdev;
>
> - if (xc_gnttab_munmap(gnt, grant->page, 1) != 0) {
> - xen_be_printf(&grant->blkdev->xendev, 0,
> - "xc_gnttab_munmap failed: %s\n",
> - strerror(errno));
> - }
> grant->blkdev->persistent_gnt_count--;
> xen_be_printf(&grant->blkdev->xendev, 3,
> - "unmapped grant %p\n", grant->page);
> + "removed grant %p\n", grant->page);
> g_free(grant);
> }
>
> +static void add_persistent_region(struct XenBlkDev *blkdev, void *addr, int
> num)
> +{
> + PersistentRegion *region;
> +
> + region = g_malloc0(sizeof(*region));
> + region->addr = addr;
> + region->num = num;
> + blkdev->persistent_regions = g_slist_append(blkdev->persistent_regions,
> + region);
> +}
> +
> +static void remove_persistent_region(gpointer data, gpointer dev)
> +{
> + PersistentRegion *region = data;
> + struct XenBlkDev *blkdev = dev;
> + XenGnttab gnt = blkdev->xendev.gnttabdev;
> +
> + if (xc_gnttab_munmap(gnt, region->addr, region->num) != 0) {
> + xen_be_printf(&blkdev->xendev, 0,
> + "xc_gnttab_munmap region %p failed: %s\n",
> + region->addr, strerror(errno));
> + }
> + xen_be_printf(&blkdev->xendev, 3,
> + "unmapped grant region %p with %d pages\n",
> + region->addr, region->num);
> + g_free(region);
> +}
> +
> static struct ioreq *ioreq_start(struct XenBlkDev *blkdev)
> {
> struct ioreq *ioreq = NULL;
> @@ -421,7 +451,17 @@ static int ioreq_map(struct ioreq *ioreq)
> }
> }
> }
> - if (ioreq->blkdev->feature_persistent) {
> + if (ioreq->blkdev->feature_persistent && new_maps &&
> + (!batch_maps || (ioreq->blkdev->persistent_gnt_count + new_maps <=
> + ioreq->blkdev->max_grants))) {
Do we really need this change? If so, could you please write something
about it in the commit message?
> + /*
> + * If we are using persistent grants and batch mappings only
> + * add the new maps to the list of persistent grants if the whole
> + * area can be persistently mapped.
> + */
Is this the reason for the change above?
> + if (batch_maps && new_maps) {
> + add_persistent_region(ioreq->blkdev, ioreq->pages, new_maps);
> + }
>
> while ((ioreq->blkdev->persistent_gnt_count <
> ioreq->blkdev->max_grants)
> && new_maps) {
> /* Go through the list of newly mapped grants and add as many
> @@ -437,6 +477,7 @@ static int ioreq_map(struct ioreq *ioreq)
> grant->page = ioreq->pages + (new_maps) * XC_PAGE_SIZE;
> } else {
> grant->page = ioreq->page[new_maps];
> + add_persistent_region(ioreq->blkdev, ioreq->page[new_maps],
> 1);
Basically in the !batch_maps case we are duplicating persistent_gnts
into persistent_regions. Couldn't we just avoid calling
add_persistent_region at all in that case and rely on the old
destroy_grant function? In fact I think we could just pass NULL as
GDestroyNotify function when creating persistent_gnts if batch_maps and
the old unmodified destroy_grant if !batch_maps.
> }
> grant->blkdev = ioreq->blkdev;
> xen_be_printf(&ioreq->blkdev->xendev, 3,
> @@ -447,6 +488,7 @@ static int ioreq_map(struct ioreq *ioreq)
> grant);
> ioreq->blkdev->persistent_gnt_count++;
> }
> + assert(!batch_maps || new_maps == 0);
Shouldn't new_maps be == 0 even in the !batch_maps case?
> }
> for (i = 0; i < ioreq->v.niov; i++) {
> ioreq->v.iov[i].iov_base += (uintptr_t)page[i];
> @@ -972,6 +1014,7 @@ static int blk_connect(struct XenDevice *xendev)
> blkdev->persistent_gnts = g_tree_new_full((GCompareDataFunc)int_cmp,
> NULL, NULL,
> (GDestroyNotify)destroy_grant);
> + blkdev->persistent_regions = NULL;
> blkdev->persistent_gnt_count = 0;
> }
>
> @@ -1000,6 +1043,19 @@ static void blk_disconnect(struct XenDevice *xendev)
> blkdev->cnt_map--;
> blkdev->sring = NULL;
> }
> +
> + /*
> + * Unmap persistent grants before switching to the closed state
> + * so the frontend can free them.
> + */
> + if (blkdev->feature_persistent) {
> + g_tree_destroy(blkdev->persistent_gnts);
> + assert(blkdev->persistent_gnt_count == 0);
> + g_slist_foreach(blkdev->persistent_regions,
> + (GFunc)remove_persistent_region, blkdev);
> + g_slist_free(blkdev->persistent_regions);
> + blkdev->feature_persistent = false;
> + }
> }
>
> static int blk_free(struct XenDevice *xendev)
> @@ -1011,11 +1067,6 @@ static int blk_free(struct XenDevice *xendev)
> blk_disconnect(xendev);
> }
>
> - /* Free persistent grants */
> - if (blkdev->feature_persistent) {
> - g_tree_destroy(blkdev->persistent_gnts);
> - }
> -
> while (!QLIST_EMPTY(&blkdev->freelist)) {
> ioreq = QLIST_FIRST(&blkdev->freelist);
> QLIST_REMOVE(ioreq, list);
> --
> 1.9.3 (Apple Git-50)
>