On Mon, 12 Jan 2026 17:10:12 +0000 Steven Price <[email protected]> wrote:
> On 12/01/2026 16:59, Boris Brezillon wrote: > > On Mon, 12 Jan 2026 16:49:33 +0000 > > Steven Price <[email protected]> wrote: > > > >> On 12/01/2026 14:39, Boris Brezillon wrote: > >>> On Mon, 12 Jan 2026 12:33:33 +0000 > >>> Steven Price <[email protected]> wrote: > >>> > >>>> On 09/01/2026 13:08, Boris Brezillon wrote: > >>>>> This will be used to order things by reclaimability. > >>>>> > >>>>> Signed-off-by: Boris Brezillon <[email protected]> > >>>>> --- > >>>>> drivers/gpu/drm/panthor/panthor_gem.c | 44 +++++++++++++++++++++++++-- > >>>>> drivers/gpu/drm/panthor/panthor_gem.h | 3 ++ > >>>>> 2 files changed, 45 insertions(+), 2 deletions(-) > >>>>> > >>>>> diff --git a/drivers/gpu/drm/panthor/panthor_gem.c > >>>>> b/drivers/gpu/drm/panthor/panthor_gem.c > >>>>> index 44f05bd957e7..458d22380e96 100644 > >>>>> --- a/drivers/gpu/drm/panthor/panthor_gem.c > >>>>> +++ b/drivers/gpu/drm/panthor/panthor_gem.c > >>>>> @@ -484,6 +484,7 @@ static void panthor_gem_print_info(struct > >>>>> drm_printer *p, unsigned int indent, > >>>>> drm_printf_indent(p, indent, "vmap_use_count=%u\n", > >>>>> refcount_read(&bo->cmap.vaddr_use_count)); > >>>>> drm_printf_indent(p, indent, "vaddr=%p\n", bo->cmap.vaddr); > >>>>> + drm_printf_indent(p, indent, "mmap_count=%u\n", > >>>>> refcount_read(&bo->cmap.mmap_count)); > >>>>> } > >>>>> > >>>>> static int panthor_gem_pin_locked(struct drm_gem_object *obj) > >>>>> @@ -600,6 +601,13 @@ static int panthor_gem_mmap(struct drm_gem_object > >>>>> *obj, struct vm_area_struct *v > >>>>> if (is_cow_mapping(vma->vm_flags)) > >>>>> return -EINVAL; > >>>>> > >>>>> + if (!refcount_inc_not_zero(&bo->cmap.mmap_count)) { > >>>>> + dma_resv_lock(obj->resv, NULL); > >>>>> + if (!refcount_inc_not_zero(&bo->cmap.mmap_count)) > >>>>> + refcount_set(&bo->cmap.mmap_count, 1); > >>>>> + dma_resv_unlock(obj->resv); > >>>>> + } > >>>>> + > >>>>> vm_flags_set(vma, VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP); > >>>>> vma->vm_page_prot = vm_get_page_prot(vma->vm_flags); > >>>>> if (should_map_wc(bo)) > >>>>> @@ -732,10 +740,42 @@ static vm_fault_t panthor_gem_fault(struct > >>>>> vm_fault *vmf) > >>>>> return blocking_page_setup(vmf, bo, page_offset, true); > >>>>> } > >>>>> > >>>>> +static void panthor_gem_vm_open(struct vm_area_struct *vma) > >>>>> +{ > >>>>> + struct panthor_gem_object *bo = > >>>>> to_panthor_bo(vma->vm_private_data); > >>>>> + > >>>>> + /* mmap_count must have been incremented at mmap time, so it > >>>>> can't be > >>>>> + * zero here. > >>>>> + */ > >>>>> + if (!drm_gem_is_imported(&bo->base)) > >>>>> + drm_WARN_ON(bo->base.dev, > >>>>> !refcount_inc_not_zero(&bo->cmap.mmap_count)); > >>>>> + > >>>>> + drm_gem_vm_open(vma); > >>>>> +} > >>>>> + > >>>>> +static void panthor_gem_vm_close(struct vm_area_struct *vma) > >>>>> +{ > >>>>> + struct panthor_gem_object *bo = > >>>>> to_panthor_bo(vma->vm_private_data); > >>>>> + > >>>>> + if (drm_gem_is_imported(&bo->base)) > >>>>> + goto out; > >>>>> + > >>>>> + if (refcount_dec_not_one(&bo->cmap.mmap_count)) > >>>>> + goto out; > >>>>> + > >>>>> + dma_resv_lock(bo->base.resv, NULL); > >>>>> + if (!refcount_dec_not_one(&bo->cmap.mmap_count)) > >>>>> + refcount_set(&bo->cmap.mmap_count, 0); > >>>>> + dma_resv_unlock(bo->base.resv); > >>>> > >>>> I don't think this logic is safe. Holding the resv_lock doesn't protect > >>>> against another thread doing a refcount_inc_not_zero() without holding > >>>> the lock. > >>>> > >>>> I think you can just replace the if() part with a refcount_dec() call, > >>>> the lock AFAICT is needed because the following patch wants to be sure > >>>> that !!mmap_count is stable when resv_lock is held. > >>> > >>> I wish I could, but refcount_dec() doesn't let me do the 1 -> 0 without > >>> complaining :P. > >> > >> Yeah, I misread the refcount_dec() code the first time I looked at it. > >> > >>>> > >>>> I also feel you should invert the conditino for refcount_dec_not_one, > >>>> leading to the following which I feel is easier to read: > >>>> > >>>> static void panthor_gem_vm_close(struct vm_area_struct *vma) > >>>> { > >>>> [...] > >>>> > >>>> if (!refcount_dec_not_one(&bo->cmap.mmap_count)) { > >>>> dma_resv_lock(bo->base.resv, NULL); > >>>> refcount_dec(&bo->cmap.mmap_count); > >>>> dma_resv_unlock(bo->base.resv); > >>>> } > >>> > >>> The best I can do is: > >>> > >>> if (!refcount_dec_not_one(&bo->cmap.mmap_count)) { > >>> dma_resv_lock(bo->base.resv, NULL); > >>> if (!refcount_dec_not_one(&bo->cmap.mmap_count)) > >> > >> The problem is here - if another thread does an increment from 1 to 2 at > >> this point then we lose a reference count. And since you don't have to > >> hold the lock for that we have a problem. > > > > Okay, I see what you mean now. I was considering 0 -> 1, not 1 -> 2. So > > we really need a > > > > if (refcount_dec_and_test(&bo->cmap.mmap_count)) > > /* Do nothing */; > > Yes - I was just writing a longer email to say that ;) Note that as soon as the shrinker is introduced, there's stuff to put in those if (refcount_dec_and_test(&bo->cmap.mmap_count)) blocks, so it's not that terrible in the end.
