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Message-ID: <2ab85f0c-2605-c401-ca59-7afca5807349@suse.com>
Date: Fri, 3 Jun 2022 16:54:33 +0200
From: Juergen Gross <jgross@...e.com>
To: Demi Marie Obenour <demi@...isiblethingslab.com>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Stefano Stabellini <sstabellini@...nel.org>,
Jennifer Herbert <jennifer.herbert@...rix.com>
Cc: xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
stable@...r.kernel.org
Subject: Re: [PATCH v3] xen/gntdev: Avoid blocking in unmap_grant_pages()
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@...r.kernel.org
> Signed-off-by: Demi Marie Obenour <demi@...isiblethingslab.com>
> ---
> 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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