On Tue, Nov 29, 2022 at 12:47:42PM -0800, Rob Clark wrote:
On Tue, Nov 29, 2022 at 12:32 PM Guenter Roeck linux@roeck-us.net wrote:
On Tue, Nov 29, 2022 at 12:02:42PM -0800, Rob Clark wrote:
From: Rob Clark robdclark@chromium.org
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 ;-))
Yhea vm_ops->open should really be called vm_ops->dupe or ->copy or something like that ... -Daniel
BR, -R
Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects") Cc: stable@vger.kernel.org Signed-off-by: Rob Clark robdclark@chromium.org
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 grabsshould or guaranteed ? This sounds a bit weaker than the commit description.
* an additional reference for the new mm the vma is getting* copied into.*/WARN_ON_ONCE(!shmem->pages_use_count);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);}