On Sun, Mar 21, 2021 at 07:45:29PM +0100, Thomas Hellström (Intel) wrote:
To block fast gup we need to make sure TTM ptes are always special. With MIXEDMAP we, on architectures that don't support pte_special, insert normal ptes, but OTOH on those architectures, fast is not supported. At the same time, the function documentation to vm_normal_page() suggests that ptes pointing to system memory pages of MIXEDMAP vmas are always normal, but that doesn't seem consistent with what's implemented in vmf_insert_mixed(). I'm thus not entirely sure this patch is actually needed.
But to make sure and to avoid also normal (non-fast) gup, make all TTM vmas PFNMAP. With PFNMAP we can't allow COW mappings anymore so make is_cow_mapping() available and use it to reject COW mappigs at mmap time.
There was previously a comment in the code that WC mappings together with x86 PAT + PFNMAP was bad for performance. However from looking at vmf_insert_mixed() it looks like in the current code PFNMAP and MIXEDMAP are handled the same for architectures that support pte_special. This means there should not be a performance difference anymore, but this needs to be verified.
Cc: Christian Koenig christian.koenig@amd.com Cc: David Airlie airlied@linux.ie Cc: Daniel Vetter daniel@ffwll.ch Cc: Andrew Morton akpm@linux-foundation.org Cc: Jason Gunthorpe jgg@nvidia.com Cc: linux-mm@kvack.org Cc: dri-devel@lists.freedesktop.org Cc: linux-kernel@vger.kernel.org Signed-off-by: Thomas Hellström (Intel) thomas_os@shipmail.org
drivers/gpu/drm/ttm/ttm_bo_vm.c | 22 ++++++++-------------- include/linux/mm.h | 5 +++++ mm/internal.h | 5 ----- 3 files changed, 13 insertions(+), 19 deletions(-)
diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c index 1c34983480e5..708c6fb9be81 100644 --- a/drivers/gpu/drm/ttm/ttm_bo_vm.c +++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c @@ -372,12 +372,7 @@ vm_fault_t ttm_bo_vm_fault_reserved(struct vm_fault *vmf, * at arbitrary times while the data is mmap'ed. * See vmf_insert_mixed_prot() for a discussion. */
if (vma->vm_flags & VM_MIXEDMAP)ret = vmf_insert_mixed_prot(vma, address,__pfn_to_pfn_t(pfn, PFN_DEV),prot);elseret = vmf_insert_pfn_prot(vma, address, pfn, prot);
ret = vmf_insert_pfn_prot(vma, address, pfn, prot);/* Never error on prefaulted PTEs */ if (unlikely((ret & VM_FAULT_ERROR))) {
@@ -555,18 +550,14 @@ static void ttm_bo_mmap_vma_setup(struct ttm_buffer_object *bo, struct vm_area_s * Note: We're transferring the bo reference to * vma->vm_private_data here. */
vma->vm_private_data = bo;
/*
* We'd like to use VM_PFNMAP on shared mappings, where* (vma->vm_flags & VM_SHARED) != 0, for performance reasons,* but for some reason VM_PFNMAP + x86 PAT + write-combine is very* bad for performance. Until that has been sorted out, use* VM_MIXEDMAP on all mappings. See freedesktop.org bug #75719
* PFNMAP forces us to block COW mappings in mmap(),* and with MIXEDMAP we would incorrectly allow fast gup* on TTM memory on architectures that don't have pte_special.
- vma->vm_flags |= VM_MIXEDMAP;
- vma->vm_flags |= VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
- vma->vm_flags |= VM_PFNMAP | VM_IO | VM_DONTEXPAND | VM_DONTDUMP;
}
int ttm_bo_mmap(struct file *filp, struct vm_area_struct *vma, @@ -579,6 +570,9 @@ int ttm_bo_mmap(struct file *filp, struct vm_area_struct *vma, if (unlikely(vma->vm_pgoff < DRM_FILE_PAGE_OFFSET_START)) return -EINVAL;
- if (unlikely(is_cow_mapping(vma->vm_flags)))
return -EINVAL;- bo = ttm_bo_vm_lookup(bdev, vma->vm_pgoff, vma_pages(vma)); if (unlikely(!bo)) return -EINVAL;
diff --git a/include/linux/mm.h b/include/linux/mm.h index 77e64e3eac80..c6ebf7f9ddbb 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -686,6 +686,11 @@ static inline bool vma_is_accessible(struct vm_area_struct *vma) return vma->vm_flags & VM_ACCESS_FLAGS; }
+static inline bool is_cow_mapping(vm_flags_t flags)
Bit a bikeshed, but I wonder whether the public interface shouldn't be vma_is_cow_mapping. Or whether this shouldn't be rejected somewhere else, since at least in drivers/gpu we have tons of cases that don't check for this and get it all kinds of wrong I think.
remap_pfn_range handles this for many cases, but by far not for all.
Anyway patch itself lgtm:
Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch
I'll try and find some -mm folks to look at this too. -Daniel
+{
- return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
+}
#ifdef CONFIG_SHMEM /*
- The vma_is_shmem is not inline because it is used only by slow
diff --git a/mm/internal.h b/mm/internal.h index 9902648f2206..1432feec62df 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -296,11 +296,6 @@ static inline unsigned int buddy_order(struct page *page) */ #define buddy_order_unsafe(page) READ_ONCE(page_private(page))
-static inline bool is_cow_mapping(vm_flags_t flags) -{
- return (flags & (VM_SHARED | VM_MAYWRITE)) == VM_MAYWRITE;
-}
/*
- These three helpers classifies VMAs for virtual memory accounting.
*/
2.30.2