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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