Re: [PATCH v3] xen/gntdev: Avoid blocking in unmap_grant_pages()

From: Juergen Gross
Date: Fri Jun 03 2022 - 10:54:42 EST

On 03.06.22 00:53, Demi Marie Obenour wrote:
unmap_grant_pages() currently waits for the pages to no longer be used.
In https://github.com/QubesOS/qubes-issues/issues/7481, this lead to a
deadlock against i915: i915 was waiting for gntdev's MMU notifier to
finish, while gntdev was waiting for i915 to free its pages. I also
believe this is responsible for various deadlocks I have experienced in
the past.

Avoid these problems by making unmap_grant_pages async. This requires
making it return void, as any errors will not be available when the
function returns. Fortunately, the only use of the return value is a
WARN_ON(), which can be replaced by a WARN_ON when the error is
detected. Additionally, a failed call will not prevent further calls
from being made, but this is harmless.

Because unmap_grant_pages is now async, the grant handle will be sent to
INVALID_GRANT_HANDLE too late to prevent multiple unmaps of the same
handle. Instead, a separate bool array is allocated for this purpose.
This wastes memory, but stuffing this information in padding bytes is
too fragile. Furthermore, it is necessary to grab a reference to the
map before making the asynchronous call, and release the reference when
the call returns.

It is also necessary to guard against reentrancy in gntdev_map_put(),
and to handle the case where userspace tries to map a mapping whose
contents have not all been freed yet.

Fixes: 745282256c75 ("xen/gntdev: safely unmap grants in case they are still in use")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Demi Marie Obenour <demi@xxxxxxxxxxxxxxxxxxxxxx>
---
drivers/xen/gntdev-common.h | 7 ++
drivers/xen/gntdev.c | 153 ++++++++++++++++++++++++------------
2 files changed, 109 insertions(+), 51 deletions(-)

diff --git a/drivers/xen/gntdev-common.h b/drivers/xen/gntdev-common.h
index 20d7d059dadb..15c2e3afcc2b 100644
--- a/drivers/xen/gntdev-common.h
+++ b/drivers/xen/gntdev-common.h
@@ -16,6 +16,7 @@
#include <linux/mmu_notifier.h>
#include <linux/types.h>
#include <xen/interface/event_channel.h>
+#include <xen/grant_table.h>
struct gntdev_dmabuf_priv;
@@ -56,6 +57,7 @@ struct gntdev_grant_map {
struct gnttab_unmap_grant_ref *unmap_ops;
struct gnttab_map_grant_ref *kmap_ops;
struct gnttab_unmap_grant_ref *kunmap_ops;
+ bool *being_removed;
struct page **pages;
unsigned long pages_vm_start;
@@ -73,6 +75,11 @@ struct gntdev_grant_map {
/* Needed to avoid allocation in gnttab_dma_free_pages(). */
xen_pfn_t *frames;
#endif
+
+ /* Number of live grants */
+ atomic_long_t live_grants;

Any reason to use atomic_long_t instead of atomic_t?

As the max number of mappings is map->count, which is an int, I don't see why
atomic_t wouldn't work here.

+ /* Needed to avoid allocation in __unmap_grant_pages */
+ struct gntab_unmap_queue_data unmap_data;
};
struct gntdev_grant_map *gntdev_alloc_map(struct gntdev_priv *priv, int count,
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 59ffea800079..e8b83ea1eacd 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -35,6 +35,7 @@
#include <linux/slab.h>
#include <linux/highmem.h>
#include <linux/refcount.h>
+#include <linux/workqueue.h>
#include <xen/xen.h>
#include <xen/grant_table.h>
@@ -60,10 +61,11 @@ module_param(limit, uint, 0644);
MODULE_PARM_DESC(limit,
"Maximum number of grants that may be mapped by one mapping request");
+/* True in PV mode, false otherwise */
static int use_ptemod;
-static int unmap_grant_pages(struct gntdev_grant_map *map,
- int offset, int pages);
+static void unmap_grant_pages(struct gntdev_grant_map *map,
+ int offset, int pages);
static struct miscdevice gntdev_miscdev;
@@ -120,6 +122,7 @@ static void gntdev_free_map(struct gntdev_grant_map *map)
kvfree(map->unmap_ops);
kvfree(map->kmap_ops);
kvfree(map->kunmap_ops);
+ kvfree(map->being_removed);
kfree(map);
}
@@ -140,10 +143,13 @@ struct gntdev_grant_map *gntdev_alloc_map(struct gntdev_priv *priv, int count,
add->unmap_ops = kvmalloc_array(count, sizeof(add->unmap_ops[0]),
GFP_KERNEL);
add->pages = kvcalloc(count, sizeof(add->pages[0]), GFP_KERNEL);
+ add->being_removed =
+ kvcalloc(count, sizeof(add->being_removed[0]), GFP_KERNEL);
if (NULL == add->grants ||
NULL == add->map_ops ||
NULL == add->unmap_ops ||
- NULL == add->pages)
+ NULL == add->pages ||
+ NULL == add->being_removed)
goto err;
if (use_ptemod) {
add->kmap_ops = kvmalloc_array(count, sizeof(add->kmap_ops[0]),
@@ -250,9 +256,34 @@ void gntdev_put_map(struct gntdev_priv *priv, struct gntdev_grant_map *map)
if (!refcount_dec_and_test(&map->users))
return;
- if (map->pages && !use_ptemod)
+ if (map->pages && !use_ptemod) {
+ /*
+ * Increment the reference count. This ensures that the
+ * subsequent call to unmap_grant_pages() will not wind up
+ * re-entering itself. It *can* wind up calling
+ * gntdev_put_map() recursively, but such calls will be with a
+ * nonzero reference count, so they will return before this code
+ * is reached. The recursion depth is thus limited to 1.
+ */
+ refcount_inc(&map->users);
+
+ /*
+ * Unmap the grants. This may or may not be asynchronous, so it
+ * is possible that the reference count is 1 on return, but it
+ * could also be greater than 1.
+ */
unmap_grant_pages(map, 0, map->count);
+ /* Check if the memory now needs to be freed */
+ if (!refcount_dec_and_test(&map->users))
+ return;
+
+ /*
+ * All pages have been returned to the hypervisor, so free the
+ * map. FIXME: this is far too complex.
+ */

Do you have an idea how to simplify this?

If yes, I'm fine with the comment. If no, I'd rather drop the "FIXME".

+ }
+
if (map->notify.flags & UNMAP_NOTIFY_SEND_EVENT) {
notify_remote_via_evtchn(map->notify.event);
evtchn_put(map->notify.event);
@@ -283,6 +314,7 @@ static int find_grant_ptes(pte_t *pte, unsigned long addr, void *data)
int gntdev_map_grant_pages(struct gntdev_grant_map *map)
{
+ size_t alloced = 0;
int i, err = 0;
if (!use_ptemod) {
@@ -331,97 +363,114 @@ int gntdev_map_grant_pages(struct gntdev_grant_map *map)
map->count);
for (i = 0; i < map->count; i++) {
- if (map->map_ops[i].status == GNTST_okay)
+ if (map->map_ops[i].status == GNTST_okay) {
map->unmap_ops[i].handle = map->map_ops[i].handle;
- else if (!err)
+ if (!use_ptemod)
+ alloced++;
+ } else if (!err)
err = -EINVAL;
if (map->flags & GNTMAP_device_map)
map->unmap_ops[i].dev_bus_addr = map->map_ops[i].dev_bus_addr;
if (use_ptemod) {
- if (map->kmap_ops[i].status == GNTST_okay)
+ if (map->kmap_ops[i].status == GNTST_okay) {
+ if (map->map_ops[i].status == GNTST_okay)
+ alloced++;
map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
- else if (!err)
+ } else if (!err) {
+ /* FIXME: should this be a WARN()? */

I don't think a WARN() would be appropriate here, as the page is basically
selctable via user code. In case the caller is passing e.g. a user address
which is already mapping a foreign frame, this would result in an error here.

So just returning an error is better than flooding the console with messages.

err = -EINVAL;
+ }
}
}
+ atomic_long_add(alloced, &map->live_grants);
return err;
}
-static int __unmap_grant_pages(struct gntdev_grant_map *map, int offset,
- int pages)
+static void __unmap_grant_pages_done(int result,
+ struct gntab_unmap_queue_data *data)
{
- int i, err = 0;
- struct gntab_unmap_queue_data unmap_data;
-
- if (map->notify.flags & UNMAP_NOTIFY_CLEAR_BYTE) {
- int pgno = (map->notify.addr >> PAGE_SHIFT);
- if (pgno >= offset && pgno < offset + pages) {
- /* No need for kmap, pages are in lowmem */
- uint8_t *tmp = pfn_to_kaddr(page_to_pfn(map->pages[pgno]));
- tmp[map->notify.addr & (PAGE_SIZE-1)] = 0;
- map->notify.flags &= ~UNMAP_NOTIFY_CLEAR_BYTE;
- }
- }
-
- unmap_data.unmap_ops = map->unmap_ops + offset;
- unmap_data.kunmap_ops = use_ptemod ? map->kunmap_ops + offset : NULL;
- unmap_data.pages = map->pages + offset;
- unmap_data.count = pages;
-
- err = gnttab_unmap_refs_sync(&unmap_data);
- if (err)
- return err;
+ unsigned int i;
+ struct gntdev_grant_map *map = data->data;
+ unsigned int offset = data->unmap_ops - map->unmap_ops;
+ atomic_long_sub(data->count, &map->live_grants);

Shouldn't this be done only after the last usage of map (i.e. after the
follwing loop)? Otherwise gntdev_mmap() would no longer be blocked from
resuing map.

- for (i = 0; i < pages; i++) {
- if (map->unmap_ops[offset+i].status)
- err = -EINVAL;
+ for (i = 0; i < data->count; i++) {
+ WARN_ON(map->unmap_ops[offset+i].status);
pr_debug("unmap handle=%d st=%d\n",
map->unmap_ops[offset+i].handle,
map->unmap_ops[offset+i].status);
map->unmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
if (use_ptemod) {
- if (map->kunmap_ops[offset+i].status)
- err = -EINVAL;
+ WARN_ON(map->kunmap_ops[offset+i].status);
pr_debug("kunmap handle=%u st=%d\n",
map->kunmap_ops[offset+i].handle,
map->kunmap_ops[offset+i].status);
map->kunmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
}
}
- return err;
+
+ /* Release reference taken by __unmap_grant_pages */
+ gntdev_put_map(NULL, map);
}
-static int unmap_grant_pages(struct gntdev_grant_map *map, int offset,
- int pages)
+static void __unmap_grant_pages(struct gntdev_grant_map *map, int offset,
+ int pages)
{
- int range, err = 0;
+ if (map->notify.flags & UNMAP_NOTIFY_CLEAR_BYTE) {
+ int pgno = (map->notify.addr >> PAGE_SHIFT);
+
+ if (pgno >= offset && pgno < offset + pages) {
+ /* No need for kmap, pages are in lowmem */
+ uint8_t *tmp = pfn_to_kaddr(page_to_pfn(map->pages[pgno]));
+
+ tmp[map->notify.addr & (PAGE_SIZE-1)] = 0;
+ map->notify.flags &= ~UNMAP_NOTIFY_CLEAR_BYTE;
+ }
+ }
+
+ map->unmap_data.unmap_ops = map->unmap_ops + offset;
+ map->unmap_data.kunmap_ops = use_ptemod ? map->kunmap_ops + offset : NULL;
+ map->unmap_data.pages = map->pages + offset;
+ map->unmap_data.count = pages;
+ map->unmap_data.done = __unmap_grant_pages_done;
+ map->unmap_data.data = map;
+ refcount_inc(&map->users); /* to keep map alive during async call below */
+
+ gnttab_unmap_refs_async(&map->unmap_data);
+}
+
+static void unmap_grant_pages(struct gntdev_grant_map *map, int offset,
+ int pages)
+{
+ int range;
+
+ if (atomic_long_read(&map->live_grants) == 0)
+ return; /* Nothing to do */
pr_debug("unmap %d+%d [%d+%d]\n", map->index, map->count, offset, pages);
/* It is possible the requested range will have a "hole" where we
* already unmapped some of the grants. Only unmap valid ranges.
*/
- while (pages && !err) {
- while (pages &&
- map->unmap_ops[offset].handle == INVALID_GRANT_HANDLE) {
+ while (pages) {
+ while (pages && map->being_removed[offset]) {
offset++;
pages--;
}
range = 0;
while (range < pages) {
- if (map->unmap_ops[offset + range].handle ==
- INVALID_GRANT_HANDLE)
+ if (map->being_removed[offset + range])
break;
+ map->being_removed[offset + range] = true;
range++;
}
- err = __unmap_grant_pages(map, offset, range);
+ if (range)
+ __unmap_grant_pages(map, offset, range);
offset += range;
pages -= range;
}
-
- return err;
}
/* ------------------------------------------------------------------ */
@@ -473,7 +522,6 @@ static bool gntdev_invalidate(struct mmu_interval_notifier *mn,
struct gntdev_grant_map *map =
container_of(mn, struct gntdev_grant_map, notifier);
unsigned long mstart, mend;
- int err;
if (!mmu_notifier_range_blockable(range))
return false;
@@ -494,10 +542,9 @@ static bool gntdev_invalidate(struct mmu_interval_notifier *mn,
map->index, map->count,
map->vma->vm_start, map->vma->vm_end,
range->start, range->end, mstart, mend);
- err = unmap_grant_pages(map,
+ unmap_grant_pages(map,
(mstart - map->vma->vm_start) >> PAGE_SHIFT,
(mend - mstart) >> PAGE_SHIFT);
- WARN_ON(err);
return true;
}
@@ -985,6 +1032,10 @@ static int gntdev_mmap(struct file *flip, struct vm_area_struct *vma)
goto unlock_out;
if (use_ptemod && map->vma)
goto unlock_out;
+ if (atomic_long_read(&map->live_grants)) {
+ err = -EAGAIN;
+ goto unlock_out;
+ }
refcount_inc(&map->users);
vma->vm_ops = &gntdev_vmops;


Juergen

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