Re: [PATCH v15 02/23] drm/shmem-helper: Use flag for tracking page count bumped by get_pages_sgt()
On Sat, 2 Sep 2023 21:28:21 +0300
Dmitry Osipenko wrote:
> On 8/28/23 13:55, Boris Brezillon wrote:
> > On Sun, 27 Aug 2023 20:54:28 +0300
> > Dmitry Osipenko wrote:
> >
> >> Use separate flag for tracking page count bumped by shmem->sgt to avoid
> >> imbalanced page counter during of drm_gem_shmem_free() time. It's fragile
> >> to assume that populated shmem->pages at a freeing time means that the
> >> count was bumped by drm_gem_shmem_get_pages_sgt(), using a flag removes
> >> the ambiguity.
> >>
> >> Signed-off-by: Dmitry Osipenko
> >> ---
> >> drivers/gpu/drm/drm_gem_shmem_helper.c | 3 ++-
> >> drivers/gpu/drm/lima/lima_gem.c| 1 +
> >> include/drm/drm_gem_shmem_helper.h | 7 +++
> >> 3 files changed, 10 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> b/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> index 78d9cf2355a5..db20b9123891 100644
> >> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> >> @@ -152,7 +152,7 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object
> >> *shmem)
> >>sg_free_table(shmem->sgt);
> >>kfree(shmem->sgt);
> >>}
> >> - if (shmem->pages)
> >> + if (shmem->got_sgt)
> >>drm_gem_shmem_put_pages(shmem);
> >
> > Can't we just move this drm_gem_shmem_put_pages() call in the
> > if (shmem->sgt) block?
>
> As you've seen in patch #1, the shmem->sgt may belong to imported dmabuf
> and pages aren't referenced in this case.
Unless I'm wrong, you're already in the if (!import_attach) branch
here, so shmem->sgt should not be a dmabuf sgt.
>
> I agree that the freeing code is confusing. The flags make it a better,
> not ideal. Though, the flags+comments solution is good enough to me.
But what's the point of adding a flag when you can just do an
if (!shmem->import_attach && shmem->sgt) check. At best, it just
confuses people as to what these fields mean/are used for (especially
when the field has such a generic name, when what you want is actually
something like ->got_sgt_for_non_imported_object). But the most
problematic aspect is that it adds fields to maintain, and those might
end up being inconsistent with the object state because
new/driver-specific code forgot to update them.
> Please let me know if you have more suggestions, otherwise I'll add
> comment to the code and keep this patch for v16.
I'd definitely prefer adding the following helper
static bool has_implicit_pages_ref(struct drm_gem_shmem_object *shmem)
{
return !shmem->import_attach && shmem->sgt;
}
which provides the same logic without adding a new field/flag.
>
> BTW, I realized that the new flag wasn't placed properly in the Lima
> driver, causing unbalanced page count in the error path. Will correct it
> in v16.
See, that's the sort of subtle bugs I'm talking about. If the state is
inferred from other fields that can't happen.
Re: [PATCH v15 02/23] drm/shmem-helper: Use flag for tracking page count bumped by get_pages_sgt()
On 8/28/23 13:55, Boris Brezillon wrote: > On Sun, 27 Aug 2023 20:54:28 +0300 > Dmitry Osipenko wrote: > >> Use separate flag for tracking page count bumped by shmem->sgt to avoid >> imbalanced page counter during of drm_gem_shmem_free() time. It's fragile >> to assume that populated shmem->pages at a freeing time means that the >> count was bumped by drm_gem_shmem_get_pages_sgt(), using a flag removes >> the ambiguity. >> >> Signed-off-by: Dmitry Osipenko >> --- >> drivers/gpu/drm/drm_gem_shmem_helper.c | 3 ++- >> drivers/gpu/drm/lima/lima_gem.c| 1 + >> include/drm/drm_gem_shmem_helper.h | 7 +++ >> 3 files changed, 10 insertions(+), 1 deletion(-) >> >> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c >> b/drivers/gpu/drm/drm_gem_shmem_helper.c >> index 78d9cf2355a5..db20b9123891 100644 >> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c >> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c >> @@ -152,7 +152,7 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object >> *shmem) >> sg_free_table(shmem->sgt); >> kfree(shmem->sgt); >> } >> -if (shmem->pages) >> +if (shmem->got_sgt) >> drm_gem_shmem_put_pages(shmem); > > Can't we just move this drm_gem_shmem_put_pages() call in the > if (shmem->sgt) block? As you've seen in patch #1, the shmem->sgt may belong to imported dmabuf and pages aren't referenced in this case. I agree that the freeing code is confusing. The flags make it a better, not ideal. Though, the flags+comments solution is good enough to me. Please let me know if you have more suggestions, otherwise I'll add comment to the code and keep this patch for v16. BTW, I realized that the new flag wasn't placed properly in the Lima driver, causing unbalanced page count in the error path. Will correct it in v16. -- Best regards, Dmitry
Re: [PATCH v15 02/23] drm/shmem-helper: Use flag for tracking page count bumped by get_pages_sgt()
On Sun, 27 Aug 2023 20:54:28 +0300
Dmitry Osipenko wrote:
> Use separate flag for tracking page count bumped by shmem->sgt to avoid
> imbalanced page counter during of drm_gem_shmem_free() time. It's fragile
> to assume that populated shmem->pages at a freeing time means that the
> count was bumped by drm_gem_shmem_get_pages_sgt(), using a flag removes
> the ambiguity.
>
> Signed-off-by: Dmitry Osipenko
> ---
> drivers/gpu/drm/drm_gem_shmem_helper.c | 3 ++-
> drivers/gpu/drm/lima/lima_gem.c| 1 +
> include/drm/drm_gem_shmem_helper.h | 7 +++
> 3 files changed, 10 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c
> b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 78d9cf2355a5..db20b9123891 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -152,7 +152,7 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object
> *shmem)
> sg_free_table(shmem->sgt);
> kfree(shmem->sgt);
> }
> - if (shmem->pages)
> + if (shmem->got_sgt)
> drm_gem_shmem_put_pages(shmem);
Can't we just move this drm_gem_shmem_put_pages() call in the
if (shmem->sgt) block?
>
> drm_WARN_ON(obj->dev, shmem->pages_use_count);
> @@ -687,6 +687,7 @@ static struct sg_table
> *drm_gem_shmem_get_pages_sgt_locked(struct drm_gem_shmem_
> if (ret)
> goto err_free_sgt;
>
> + shmem->got_sgt = true;
> shmem->sgt = sgt;
>
> return sgt;
> diff --git a/drivers/gpu/drm/lima/lima_gem.c b/drivers/gpu/drm/lima/lima_gem.c
> index 4f9736e5f929..28602302c281 100644
> --- a/drivers/gpu/drm/lima/lima_gem.c
> +++ b/drivers/gpu/drm/lima/lima_gem.c
> @@ -89,6 +89,7 @@ int lima_heap_alloc(struct lima_bo *bo, struct lima_vm *vm)
> }
>
> *bo->base.sgt = sgt;
> + bo->base.got_sgt = true;
>
> if (vm) {
> ret = lima_vm_map_bo(vm, bo, old_size >> PAGE_SHIFT);
> diff --git a/include/drm/drm_gem_shmem_helper.h
> b/include/drm/drm_gem_shmem_helper.h
> index ec70a98a8fe1..f87124629bb5 100644
> --- a/include/drm/drm_gem_shmem_helper.h
> +++ b/include/drm/drm_gem_shmem_helper.h
> @@ -73,6 +73,13 @@ struct drm_gem_shmem_object {
>*/
> unsigned int vmap_use_count;
>
> + /**
> + * @got_sgt:
> + *
> + * True if SG table was retrieved using drm_gem_shmem_get_pages_sgt()
> + */
> + bool got_sgt : 1;
> +
> /**
>* @imported_sgt:
>*
[PATCH v15 02/23] drm/shmem-helper: Use flag for tracking page count bumped by get_pages_sgt()
Use separate flag for tracking page count bumped by shmem->sgt to avoid
imbalanced page counter during of drm_gem_shmem_free() time. It's fragile
to assume that populated shmem->pages at a freeing time means that the
count was bumped by drm_gem_shmem_get_pages_sgt(), using a flag removes
the ambiguity.
Signed-off-by: Dmitry Osipenko
---
drivers/gpu/drm/drm_gem_shmem_helper.c | 3 ++-
drivers/gpu/drm/lima/lima_gem.c| 1 +
include/drm/drm_gem_shmem_helper.h | 7 +++
3 files changed, 10 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c
b/drivers/gpu/drm/drm_gem_shmem_helper.c
index 78d9cf2355a5..db20b9123891 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -152,7 +152,7 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
sg_free_table(shmem->sgt);
kfree(shmem->sgt);
}
- if (shmem->pages)
+ if (shmem->got_sgt)
drm_gem_shmem_put_pages(shmem);
drm_WARN_ON(obj->dev, shmem->pages_use_count);
@@ -687,6 +687,7 @@ static struct sg_table
*drm_gem_shmem_get_pages_sgt_locked(struct drm_gem_shmem_
if (ret)
goto err_free_sgt;
+ shmem->got_sgt = true;
shmem->sgt = sgt;
return sgt;
diff --git a/drivers/gpu/drm/lima/lima_gem.c b/drivers/gpu/drm/lima/lima_gem.c
index 4f9736e5f929..28602302c281 100644
--- a/drivers/gpu/drm/lima/lima_gem.c
+++ b/drivers/gpu/drm/lima/lima_gem.c
@@ -89,6 +89,7 @@ int lima_heap_alloc(struct lima_bo *bo, struct lima_vm *vm)
}
*bo->base.sgt = sgt;
+ bo->base.got_sgt = true;
if (vm) {
ret = lima_vm_map_bo(vm, bo, old_size >> PAGE_SHIFT);
diff --git a/include/drm/drm_gem_shmem_helper.h
b/include/drm/drm_gem_shmem_helper.h
index ec70a98a8fe1..f87124629bb5 100644
--- a/include/drm/drm_gem_shmem_helper.h
+++ b/include/drm/drm_gem_shmem_helper.h
@@ -73,6 +73,13 @@ struct drm_gem_shmem_object {
*/
unsigned int vmap_use_count;
+ /**
+* @got_sgt:
+*
+* True if SG table was retrieved using drm_gem_shmem_get_pages_sgt()
+*/
+ bool got_sgt : 1;
+
/**
* @imported_sgt:
*
--
2.41.0
