Re: [PATCH v15 02/23] drm/shmem-helper: Use flag for tracking page count bumped by get_pages_sgt()

[Boris Brezillon]

On Sat, 2 Sep 2023 21:28:21 +0300
Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> wrote:

> On 8/28/23 13:55, Boris Brezillon wrote:
> > On Sun, 27 Aug 2023 20:54:28 +0300
> > Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx> 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 <dmitry.osipenko@xxxxxxxxxxxxx>
> >> ---
> >>  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.