On 11/29/22 12:47, Rob Clark wrote:
On Tue, Nov 29, 2022 at 12:32 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
On Tue, Nov 29, 2022 at 12:02:42PM -0800, Rob Clark wrote:
From: Rob Clark <robdclark@xxxxxxxxxxxx>
vm_open() is not allowed to fail. Fortunately we are guaranteed that
the pages are already pinned, and only need to increment the refcnt. So
just increment it directly.
I don't know anything about drm or gem, but I am wondering _how_
this would be guaranteed. Would it be through the pin function ?
Just wondering, because that function does not seem to be mandatory.
We've pinned the pages already in mmap.. vm->open() is perhaps not the
best name for the callback function, but it is called for copying an
existing vma into a new process (and for some other cases which do not
apply here because VM_DONTEXPAND).
(Other drivers pin pages in the fault handler, where there is actually
potential to return an error, but that change was a bit more like
re-writing shmem helper ;-))
Maybe add a bit of that (where the pinning happened) to the commit description
and to the patch itself ?
BR,
-R
Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Rob Clark <robdclark@xxxxxxxxxxxx>
---
drivers/gpu/drm/drm_gem_shmem_helper.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)
diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index 110a9eac2af8..9885ba64127f 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -571,12 +571,20 @@ static void drm_gem_shmem_vm_open(struct vm_area_struct *vma)
{
struct drm_gem_object *obj = vma->vm_private_data;
struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
- int ret;
WARN_ON(shmem->base.import_attach);
- ret = drm_gem_shmem_get_pages(shmem);
- WARN_ON_ONCE(ret != 0);
+ mutex_lock(&shmem->pages_lock);
+
+ /*
+ * We should have already pinned the pages, vm_open() just grabs
should or guaranteed ? This sounds a bit weaker than the commit
description.
like ... the pages were already pinned in (mmap function).
+ * an additional reference for the new mm the vma is getting
+ * copied into.
+ */
+ WARN_ON_ONCE(!shmem->pages_use_count);
If the code can't be trusted and still needs the warning, how about
something like the following ?
if (WARN_ON_ONCE(!shmem->pages_use_count)) {
mutex_unlock(&shmem->pages_lock);
return;
}
Thanks,
Guenter
+
+ shmem->pages_use_count++;
+ mutex_unlock(&shmem->pages_lock);
The previous code, in that situation, would not increment pages_use_count,
and it would not set not set shmem->pages. Hopefully, it would not try to
do anything with the pages it was unable to get. The new code assumes that
shmem->pages is valid even if pages_use_count is 0, while at the same time
taking into account that this can possibly happen (or the WARN_ON_ONCE
would not be needed).
Again, I don't know anything about gem and drm, but it seems to me that
there might now be a severe problem later on if the WARN_ON_ONCE()
ever triggers.
Thanks,
Guenter
drm_gem_vm_open(vma);
}
