On Mon, 12 Jan 2026 16:19:52 +0100 Alice Ryhl <[email protected]> wrote:
> On Mon, Jan 12, 2026 at 3:40 PM Boris Brezillon > <[email protected]> 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. > > I'm pretty sure that refcount_dec() is fine with 1->0. That's not what [1] says. refcount_dec_and_test() is okay though, but it'd force us to do a (void)refcount_dec_and_test() and detail why it's okay to ignore the returned value. Not too sure which one is better. > Did you mean > refcount_inc() and 0->1? > > > > 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)) > > refcount_set(&bo->cmap.mmap_count, 0); > > dma_resv_unlock(bo->base.resv); > > } > > > > so we only take the lock when absolutely needed, but the 1 -> 0 > > transition still has to be done with "if (dec_not_one) set(0)". > > Why not just use atomic_t and use the atomic inc/dec operations? They > don't have saturation, but also do not require treating zero > specially. I had suggested using atomics back when I was reviewing the shmem-shrinker stuff to avoid this exact same issue. I can't find the thread anymore and I can't remember the rationale either (probably that saturation detection was useful, still), but the decision was to use a refcount_t. I don't mind using atomics here, but I'd rather not be blocked on that when/if I try to move that code into a common lib. [1]https://elixir.bootlin.com/linux/v6.19-rc4/source/include/linux/refcount.h#L460
