[PATCH 6.6.y] fbdev: defio: Disconnect deferred I/O from the lifetime of struct fb_info

Sasha Levin
2026-05-05T06:00:06Z

From: Thomas Zimmermann <tzimmermann@suse.de>

[ Upstream commit 9ded47ad003f09a94b6a710b5c47f4aa5ceb7429 ]

Hold state of deferred I/O in struct fb_deferred_io_state. Allocate an
instance as part of initializing deferred I/O and remove it only after
the final mapping has been closed. If the fb_info and the contained
deferred I/O meanwhile goes away, clear struct fb_deferred_io_state.info
to invalidate the mapping. Any access will then result in a SIGBUS
signal.

Fixes a long-standing problem, where a device hot-unplug happens while
user space still has an active mapping of the graphics memory. The hot-
unplug frees the instance of struct fb_info. Accessing the memory will
operate on undefined state.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Fixes: 60b59beafba8 ("fbdev: mm: Deferred IO support")
Cc: Helge Deller <deller@gmx.de>
Cc: linux-fbdev@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: stable@vger.kernel.org # v2.6.22+
Signed-off-by: Helge Deller <deller@gmx.de>
[ replaced `kzalloc_obj` with `kzalloc`, and dropped `mutex_destroy(&fbdefio->lock)` ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/video/fbdev/core/fb_defio.c | 179 ++++++++++++++++++++++------
include/linux/fb.h | 4 +-
2 files changed, 145 insertions(+), 38 deletions(-)

diff --git a/drivers/video/fbdev/core/fb_defio.c b/drivers/video/fbdev/core/fb_defio.c
index b9607d5a370d4..f4812a76c3cc0 100644
--- a/drivers/video/fbdev/core/fb_defio.c
+++ b/drivers/video/fbdev/core/fb_defio.c
@@ -23,6 +23,75 @@
#include <linux/rmap.h>
#include <linux/pagemap.h>

+/*
+ * struct fb_deferred_io_state
+ */
+
+struct fb_deferred_io_state {
+ struct kref ref;
+
+ struct mutex lock; /* mutex that protects the pageref list */
+ /* fields protected by lock */
+ struct fb_info *info;
+};
+
+static struct fb_deferred_io_state *fb_deferred_io_state_alloc(void)
+{
+ struct fb_deferred_io_state *fbdefio_state;
+
+ fbdefio_state = kzalloc(sizeof(*fbdefio_state), GFP_KERNEL);
+ if (!fbdefio_state)
+ return NULL;
+
+ kref_init(&fbdefio_state->ref);
+ mutex_init(&fbdefio_state->lock);
+
+ return fbdefio_state;
+}
+
+static void fb_deferred_io_state_release(struct fb_deferred_io_state *fbdefio_state)
+{
+ mutex_destroy(&fbdefio_state->lock);
+
+ kfree(fbdefio_state);
+}
+
+static void fb_deferred_io_state_get(struct fb_deferred_io_state *fbdefio_state)
+{
+ kref_get(&fbdefio_state->ref);
+}
+
+static void __fb_deferred_io_state_release(struct kref *ref)
+{
+ struct fb_deferred_io_state *fbdefio_state =
+ container_of(ref, struct fb_deferred_io_state, ref);
+
+ fb_deferred_io_state_release(fbdefio_state);
+}
+
+static void fb_deferred_io_state_put(struct fb_deferred_io_state *fbdefio_state)
+{
+ kref_put(&fbdefio_state->ref, __fb_deferred_io_state_release);
+}
+
+/*
+ * struct vm_operations_struct
+ */
+
+static void fb_deferred_io_vm_open(struct vm_area_struct *vma)
+{
+ struct fb_deferred_io_state *fbdefio_state = vma->vm_private_data;
+
+ fb_deferred_io_state_get(fbdefio_state);
+}
+
+static void fb_deferred_io_vm_close(struct vm_area_struct *vma)
+{
+ struct fb_deferred_io_state *fbdefio_state = vma->vm_private_data;
+
+ fb_deferred_io_state_put(fbdefio_state);
+}
+
static struct page *fb_deferred_io_page(struct fb_info *info, unsigned long offs)
{
void *screen_base = (void __force *) info->screen_base;
@@ -93,17 +162,31 @@ static void fb_deferred_io_pageref_put(struct fb_deferred_io_pageref *pageref,
/* this is to find and return the vmalloc-ed fb pages */
static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf)
{
+ struct fb_info *info;
unsigned long offset;
struct page *page;
- struct fb_info *info = vmf->vma->vm_private_data;
+ vm_fault_t ret;
+ struct fb_deferred_io_state *fbdefio_state = vmf->vma->vm_private_data;
+
+ mutex_lock(&fbdefio_state->lock);
+
+ info = fbdefio_state->info;
+ if (!info) {
+ ret = VM_FAULT_SIGBUS; /* our device is gone */
+ goto err_mutex_unlock;
+ }

offset = vmf->pgoff << PAGE_SHIFT;
- if (offset >= info->fix.smem_len)
- return VM_FAULT_SIGBUS;
+ if (offset >= info->fix.smem_len) {
+ ret = VM_FAULT_SIGBUS;
+ goto err_mutex_unlock;
+ }

page = fb_deferred_io_page(info, offset);
- if (!page)
- return VM_FAULT_SIGBUS;
+ if (!page) {
+ ret = VM_FAULT_SIGBUS;
+ goto err_mutex_unlock;
+ }

get_page(page);

@@ -115,8 +198,15 @@ static vm_fault_t fb_deferred_io_fault(struct vm_fault *vmf)
BUG_ON(!page->mapping);
page->index = vmf->pgoff; /* for page_mkclean() */

+ mutex_unlock(&fbdefio_state->lock);
+
vmf->page = page;
+
return 0;
+
+err_mutex_unlock:
+ mutex_unlock(&fbdefio_state->lock);
+ return ret;
}

int fb_deferred_io_fsync(struct file *file, loff_t start, loff_t end, int datasync)
@@ -143,15 +233,24 @@ EXPORT_SYMBOL_GPL(fb_deferred_io_fsync);
* Adds a page to the dirty list. Call this from struct
* vm_operations_struct.page_mkwrite.
*/
-static vm_fault_t fb_deferred_io_track_page(struct fb_info *info, unsigned long offset,
- struct page *page)
+static vm_fault_t fb_deferred_io_track_page(struct fb_deferred_io_state *fbdefio_state,
+ unsigned long offset, struct page *page)
{
- struct fb_deferred_io *fbdefio = info->fbdefio;
+ struct fb_info *info;
+ struct fb_deferred_io *fbdefio;
struct fb_deferred_io_pageref *pageref;
vm_fault_t ret;

/* protect against the workqueue changing the page list */
- mutex_lock(&fbdefio->lock);
+ mutex_lock(&fbdefio_state->lock);
+
+ info = fbdefio_state->info;
+ if (!info) {
+ ret = VM_FAULT_SIGBUS; /* our device is gone */
+ goto err_mutex_unlock;
+ }
+
+ fbdefio = info->fbdefio;

pageref = fb_deferred_io_pageref_get(info, offset, page);
if (WARN_ON_ONCE(!pageref)) {
@@ -169,50 +268,38 @@ static vm_fault_t fb_deferred_io_track_page(struct fb_info *info, unsigned long
*/
lock_page(pageref->page);

- mutex_unlock(&fbdefio->lock);
+ mutex_unlock(&fbdefio_state->lock);

/* come back after delay to process the deferred IO */
schedule_delayed_work(&info->deferred_work, fbdefio->delay);
return VM_FAULT_LOCKED;

err_mutex_unlock:
- mutex_unlock(&fbdefio->lock);
+ mutex_unlock(&fbdefio_state->lock);
return ret;
}

-/*
- * fb_deferred_io_page_mkwrite - Mark a page as written for deferred I/O
- * @fb_info: The fbdev info structure
- * @vmf: The VM fault
- *
- * This is a callback we get when userspace first tries to
- * write to the page. We schedule a workqueue. That workqueue
- * will eventually mkclean the touched pages and execute the
- * deferred framebuffer IO. Then if userspace touches a page
- * again, we repeat the same scheme.
- *
- * Returns:
- * VM_FAULT_LOCKED on success, or a VM_FAULT error otherwise.
- */
-static vm_fault_t fb_deferred_io_page_mkwrite(struct fb_info *info, struct vm_fault *vmf)
+static vm_fault_t fb_deferred_io_page_mkwrite(struct fb_deferred_io_state *fbdefio_state,
+ struct vm_fault *vmf)
{
unsigned long offset = vmf->pgoff << PAGE_SHIFT;
struct page *page = vmf->page;

file_update_time(vmf->vma->vm_file);

- return fb_deferred_io_track_page(info, offset, page);
+ return fb_deferred_io_track_page(fbdefio_state, offset, page);
}

-/* vm_ops->page_mkwrite handler */
static vm_fault_t fb_deferred_io_mkwrite(struct vm_fault *vmf)
{
- struct fb_info *info = vmf->vma->vm_private_data;
+ struct fb_deferred_io_state *fbdefio_state = vmf->vma->vm_private_data;

- return fb_deferred_io_page_mkwrite(info, vmf);
+ return fb_deferred_io_page_mkwrite(fbdefio_state, vmf);
}

static const struct vm_operations_struct fb_deferred_io_vm_ops = {
+ .open = fb_deferred_io_vm_open,
+ .close = fb_deferred_io_vm_close,
.fault = fb_deferred_io_fault,
.page_mkwrite = fb_deferred_io_mkwrite,
};
@@ -227,7 +314,10 @@ int fb_deferred_io_mmap(struct fb_info *info, struct vm_area_struct *vma)
vm_flags_set(vma, VM_DONTEXPAND | VM_DONTDUMP);
if (!(info->flags & FBINFO_VIRTFB))
vm_flags_set(vma, VM_IO);
- vma->vm_private_data = info;
+ vma->vm_private_data = info->fbdefio_state;
+
+ fb_deferred_io_state_get(info->fbdefio_state); /* released in vma->vm_ops->close() */
+
return 0;
}
EXPORT_SYMBOL_GPL(fb_deferred_io_mmap);
@@ -238,9 +328,10 @@ static void fb_deferred_io_work(struct work_struct *work)
struct fb_info *info = container_of(work, struct fb_info, deferred_work.work);
struct fb_deferred_io_pageref *pageref, *next;
struct fb_deferred_io *fbdefio = info->fbdefio;
+ struct fb_deferred_io_state *fbdefio_state = info->fbdefio_state;

/* here we mkclean the pages, then do all deferred IO */
- mutex_lock(&fbdefio->lock);
+ mutex_lock(&fbdefio_state->lock);
list_for_each_entry(pageref, &fbdefio->pagereflist, list) {
struct page *cur = pageref->page;
lock_page(cur);
@@ -255,12 +346,13 @@ static void fb_deferred_io_work(struct work_struct *work)
list_for_each_entry_safe(pageref, next, &fbdefio->pagereflist, list)
fb_deferred_io_pageref_put(pageref, info);

- mutex_unlock(&fbdefio->lock);
+ mutex_unlock(&fbdefio_state->lock);
}

int fb_deferred_io_init(struct fb_info *info)
{
struct fb_deferred_io *fbdefio = info->fbdefio;
+ struct fb_deferred_io_state *fbdefio_state;
struct fb_deferred_io_pageref *pagerefs;
unsigned long npagerefs, i;
int ret;
@@ -270,7 +362,11 @@ int fb_deferred_io_init(struct fb_info *info)
if (WARN_ON(!info->fix.smem_len))
return -EINVAL;

- mutex_init(&fbdefio->lock);
+ fbdefio_state = fb_deferred_io_state_alloc();
+ if (!fbdefio_state)
+ return -ENOMEM;
+ fbdefio_state->info = info;
+
INIT_DELAYED_WORK(&info->deferred_work, fb_deferred_io_work);
INIT_LIST_HEAD(&fbdefio->pagereflist);
if (fbdefio->delay == 0) /* set a default of 1 s */
@@ -289,10 +385,12 @@ int fb_deferred_io_init(struct fb_info *info)
info->npagerefs = npagerefs;
info->pagerefs = pagerefs;

+ info->fbdefio_state = fbdefio_state;
+
return 0;

err:
- mutex_destroy(&fbdefio->lock);
+ fb_deferred_io_state_release(fbdefio_state);
return ret;
}
EXPORT_SYMBOL_GPL(fb_deferred_io_init);
@@ -333,11 +431,18 @@ EXPORT_SYMBOL_GPL(fb_deferred_io_release);

void fb_deferred_io_cleanup(struct fb_info *info)
{
- struct fb_deferred_io *fbdefio = info->fbdefio;
+ struct fb_deferred_io_state *fbdefio_state = info->fbdefio_state;

fb_deferred_io_lastclose(info);

+ info->fbdefio_state = NULL;
+
+ mutex_lock(&fbdefio_state->lock);
+ fbdefio_state->info = NULL;
+ mutex_unlock(&fbdefio_state->lock);
+
+ fb_deferred_io_state_put(fbdefio_state);
+
kvfree(info->pagerefs);
- mutex_destroy(&fbdefio->lock);
}
EXPORT_SYMBOL_GPL(fb_deferred_io_cleanup);
diff --git a/include/linux/fb.h b/include/linux/fb.h
index 322b4d20afa55..8a9d949cc7e2d 100644
--- a/include/linux/fb.h
+++ b/include/linux/fb.h
@@ -214,11 +214,12 @@ struct fb_deferred_io {
unsigned long delay;
bool sort_pagereflist; /* sort pagelist by offset */
int open_count; /* number of opened files; protected by fb_info lock */
- struct mutex lock; /* mutex that protects the pageref list */
struct list_head pagereflist; /* list of pagerefs for touched pages */
/* callback */
void (*deferred_io)(struct fb_info *info, struct list_head *pagelist);
};
+
+struct fb_deferred_io_state;
#endif

/*
@@ -476,6 +477,7 @@ struct fb_info {
unsigned long npagerefs;
struct fb_deferred_io_pageref *pagerefs;
struct fb_deferred_io *fbdefio;
+ struct fb_deferred_io_state *fbdefio_state;
#endif

const struct fb_ops *fbops;
--
2.53.0