On Wed, 27 Sep 2017 12:41:52 -0700 Eric Anholt <e...@anholt.net> wrote:
> Boris Brezillon <boris.brezil...@free-electrons.com> writes: > > > On Wed, 27 Sep 2017 10:15:23 -0700 > > Eric Anholt <e...@anholt.net> wrote: > > > >> Boris Brezillon <boris.brezil...@free-electrons.com> writes: > >> > >> > On Wed, 27 Sep 2017 15:24:16 +0100 > >> > Chris Wilson <ch...@chris-wilson.co.uk> wrote: > >> > > >> >> Quoting Boris Brezillon (2017-09-27 15:06:53) > >> >> > On Wed, 27 Sep 2017 14:50:10 +0100 > >> >> > Chris Wilson <ch...@chris-wilson.co.uk> wrote: > >> >> > > >> >> > > Quoting Boris Brezillon (2017-09-27 14:45:17) > >> >> > > > static struct vc4_bo * > >> >> > > > vc4_bo_from_cache(struct vc4_screen *screen, uint32_t size, > >> >> > > > const char *name) > >> >> > > > { > >> >> > > > @@ -111,6 +121,11 @@ vc4_bo_from_cache(struct vc4_screen *screen, > >> >> > > > uint32_t size, const char *name) > >> >> > > > return NULL; > >> >> > > > } > >> >> > > > > >> >> > > > + if (vc4_bo_purgeable(bo, false)) { > >> >> > > > + mtx_unlock(&cache->lock); > >> >> > > > + return NULL; > >> >> > > > >> >> > > So this would just mean that the bo was purged in the meantime. Why > >> >> > > not > >> >> > > just try to use the next one in the cache or allocate afresh? > >> >> > > >> >> > No, this means the BO was purged and the kernel failed to allocate the > >> >> > memory back. We don't care about the retained status here, because we > >> >> > don't need to restore BO's content, that's why we're not checking > >> >> > arg.retained in vc4_bo_purgeable(). Allocating a fresh BO is likely to > >> >> > fail with the same ENOMEM error because both path use the CMA mem. > >> >> > > >> >> > >> >> Hmm, you don't treat purging as permanent. But you do track the lose of > >> >> contents, so retained is false? > >> > > >> > vc4_bo_purgeable() is not reporting the retained status, it just > >> > reports whether the BO can be used or not. I can change > >> > vc4_bo_purgeable() semantic to return 1 if the BO content was retained, > >> > 0 if it was purged and -1 if you the ioctl returned an error (ENOMEM) > >> > if you prefer, but in the end, all I'll check here is > >> > 'vc4_bo_purgeable() >= 0' because I don't don't care about the retained > >> > status in this specific use case, all I care about is whether the BO can > >> > be re-used or not (IOW, is there a valid CMA region attached to the BO). > >> > > >> >> > >> >> I took a harder line, and said that userspace should recreate the object > >> >> from scratch after it was purged. I thought that would be easier > >> >> overall. But maybe not.:) > >> > > >> > Well, maybe I'm wrong in how I implemented this > >> > DRM_IOCTL_VC4_GEM_MADVISE ioctl, but right now, when the BO has been > >> > purged and someone marks it back as unpurgeable I'm trying to > >> > re-allocate BO's buffer in the ioctl path, and if the CMA allocation > >> > fails I return -ENOMEM. I could move the allocation in the fault > >> > handler, but this would result in pretty much the same behavior except > >> > it would require an extra page-fault to realize the memory is not > >> > available or force us to check the retained status and decide to > >> > release the BO object from the BO cache. > >> > >> Hmm. The downside I see to this plan is if we eventually decide to have > >> the purge operation not clear all the BOs, then we would probably rather > >> have userspace freeing objects that had been purged until it finds one > >> in the cache that hadn't been purged, rather than forcing reallocation > >> of this BO now (and possibly then purging something from elsewhere in > >> the cache). > > > > Okay, that's a good reason to move dma_alloc_wc() in the page-fault > > path. I need to change a bit the implementation to check cma_gem->vaddr > > value instead of checking bo->madv != __VC4_MADV_PURGED, otherwise we > > might pass a non-allocated BO to the GPU/Display-Engine. > > Huh, allocation in the page-fault path? We would need the storage to be > definitely be available at the point that we've set it back to WILLNEED. > Otherwise I'll "allocate" the BO from the cache, go to fill it through > my mapping, and sigbus when CMA says we're out of memory. Yep, I find that weird too, but that's unfortunately the only way we can achieve what you want to do. The only solution to know the ->retained status is by asking the the DRM driver to put the BO in WILLNEED or DONTNEED state. If you send ->madv = DONTNEED, and the kernel returns ->retained = true, this ->retained state may not be valid anymore when you get back to the application, because someone else may have triggered a purge. If you send ->madv = WILLNEED then the ->retained state is guaranteed to be valid until you explicitly switch back to DONTNEED, but that also means the driver has already allocated the memory if ->retained is false, so it's already too late to do what you were suggesting (evict the BO from the userspace cache to avoid purging other purgeable BOs). _______________________________________________ mesa-dev mailing list mesa-dev@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/mesa-dev