From: Rob Clark <[email protected]>
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.
Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
Cc: [email protected]
Signed-off-by: Rob Clark <[email protected]>
---
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
+ * 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);
drm_gem_vm_open(vma);
}
--
2.38.1
On Tue, Nov 29, 2022 at 12:02:42PM -0800, Rob Clark wrote:
> From: Rob Clark <[email protected]>
>
> 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.
>
> Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
> Cc: [email protected]
> Signed-off-by: Rob Clark <[email protected]>
> ---
> 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.
> + * 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);
> }
On Tue, Nov 29, 2022 at 12:32 PM Guenter Roeck <[email protected]> wrote:
>
> On Tue, Nov 29, 2022 at 12:02:42PM -0800, Rob Clark wrote:
> > From: Rob Clark <[email protected]>
> >
> > 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 ;-))
BR,
-R
> >
> > Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
> > Cc: [email protected]
> > Signed-off-by: Rob Clark <[email protected]>
> > ---
> > 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.
>
> > + * 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);
> > }
On 11/29/22 12:47, Rob Clark wrote:
> On Tue, Nov 29, 2022 at 12:32 PM Guenter Roeck <[email protected]> wrote:
>>
>> On Tue, Nov 29, 2022 at 12:02:42PM -0800, Rob Clark wrote:
>>> From: Rob Clark <[email protected]>
>>>
>>> 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: [email protected]
>>> Signed-off-by: Rob Clark <[email protected]>
>>> ---
>>> 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);
>>> }
On Tue, Nov 29, 2022 at 12:47:42PM -0800, Rob Clark wrote:
> On Tue, Nov 29, 2022 at 12:32 PM Guenter Roeck <[email protected]> wrote:
> >
> > On Tue, Nov 29, 2022 at 12:02:42PM -0800, Rob Clark wrote:
> > > From: Rob Clark <[email protected]>
> > >
> > > 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: [email protected]
> > > Signed-off-by: Rob Clark <[email protected]>
> > > ---
> > > 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.
> >
> > > + * 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);
> > > }
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
On Tue, Nov 29, 2022 at 12:02:42PM -0800, Rob Clark wrote:
> From: Rob Clark <[email protected]>
>
> 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.
Please mention hare that the only issue is the mutex_lock_interruptible,
and the only way we've found to hit this is if you send a signal to the
original process in a fork() at _just_ the right time.
With that: Reviewed-by: Daniel Vetter <[email protected]>
>
> Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
> Cc: [email protected]
> Signed-off-by: Rob Clark <[email protected]>
> ---
> 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
> + * 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);
>
> drm_gem_vm_open(vma);
> }
> --
> 2.38.1
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch