[RFC] drm: property: use vzalloc() instead of kvzalloc() for large blobs

[Rob Clark]

On Wed, Mar 8, 2023 at 1:23 PM Ville Syrjälä
<ville.syrjala at linux.intel.com> wrote:
>
> On Wed, Mar 08, 2023 at 12:02:42PM -0800, Abhinav Kumar wrote:
> > For DRM property blobs created by user mode using
> > drm_property_create_blob(), if the blob value needs to be updated the
> > only way is to destroy the previous blob and create a new one instead.
> >
> > For some of the property blobs, if the size of the blob is more
> > than one page size, kvzalloc() can slow down system as it will first
> > try to allocate physically contiguous memory but upon failure will
> > fall back to non-contiguous (vmalloc) allocation.
> >
> > If the blob property being used is bigger than one page size, in a
> > heavily loaded system, this causes performance issues because
> > some of the blobs are updated on a per-frame basis.
> >
> > To mitigate the performance impact of kvzalloc(), use it only when
> > the size of allocation is less than a page size when creating property
> > blobs
>
> Not sure how badly this will eat into the vmalloc area.

Normally I wouldn't expect this to be much of a problem, but we don't
appear to restrict CREATEBLOBPROP to DRM_MASTER, which seems like it
might have been a mistake.. so perhaps we want to either restrict
CREATEBLOBPROP or put an upper threshold limit on total size of all
allocated blob props using vmalloc area?

BR,
-R

> Is there no GFP flag to avoid the expensive stuff instead?
>
> >
> > Signed-off-by: Abhinav Kumar <quic_abhinavk at quicinc.com>
> > ---
> >  drivers/gpu/drm/drm_property.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/drm_property.c b/drivers/gpu/drm/drm_property.c
> > index dfec479830e4..40c2a3142038 100644
> > --- a/drivers/gpu/drm/drm_property.c
> > +++ b/drivers/gpu/drm/drm_property.c
> > @@ -561,7 +561,11 @@ drm_property_create_blob(struct drm_device *dev, size_t length,
> >       if (!length || length > INT_MAX - sizeof(struct drm_property_blob))
> >               return ERR_PTR(-EINVAL);
> >
> > -     blob = kvzalloc(sizeof(struct drm_property_blob)+length, GFP_KERNEL);
> > +     if (sizeof(struct drm_property_blob) + length > PAGE_SIZE)
> > +             blob = vzalloc(sizeof(struct drm_property_blob)+length);
> > +     else
> > +             blob = kvzalloc(sizeof(struct drm_property_blob)+length, GFP_KERNEL);
> > +
> >       if (!blob)
> >               return ERR_PTR(-ENOMEM);
> >
> > --
> > 2.7.4
>
> --
> Ville Syrjälä
> Intel