Hi,
On 3/23/21 12:34 PM, Daniel Vetter wrote:
On Sun, Mar 21, 2021 at 07:45:28PM +0100, Thomas Hellström (Intel) wrote:
TTM sets up huge page-table-entries both to system- and device memory,
and we don't want gup to assume there are always valid backing struct
pages for these. For PTEs this is handled by setting the pte_special bit,
but for the huge PUDs and PMDs, we have neither pmd_special nor
pud_special. Normally, huge TTM entries are identified by looking at
vma_is_special_huge(), but fast gup can't do that, so as an alternative
define _devmap entries for which there are no backing dev_pagemap as
special, update documentation and make huge TTM entries _devmap, after
verifying that there is no backing dev_pagemap.
One other alternative would be to block TTM huge page-table-entries
completely, and while currently only vmwgfx use them, they would be
beneficial to other graphis drivers moving forward as well.
Cc: Christian Koenig <christian.koenig@xxxxxxx>
Cc: David Airlie <airlied@xxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Jason Gunthorpe <jgg@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Thomas Hellström (Intel) <thomas_os@xxxxxxxxxxxx>
---
drivers/gpu/drm/ttm/ttm_bo_vm.c | 17 ++++++++++++++++-
mm/gup.c | 21 +++++++++++----------
mm/memremap.c | 5 +++++
3 files changed, 32 insertions(+), 11 deletions(-)
diff --git a/drivers/gpu/drm/ttm/ttm_bo_vm.c b/drivers/gpu/drm/ttm/ttm_bo_vm.c
index 6dc96cf66744..1c34983480e5 100644
--- a/drivers/gpu/drm/ttm/ttm_bo_vm.c
+++ b/drivers/gpu/drm/ttm/ttm_bo_vm.c
@@ -195,6 +195,7 @@ static vm_fault_t ttm_bo_vm_insert_huge(struct vm_fault *vmf,
pfn_t pfnt;
struct ttm_tt *ttm = bo->ttm;
bool write = vmf->flags & FAULT_FLAG_WRITE;
+ struct dev_pagemap *pagemap;
/* Fault should not cross bo boundary. */
page_offset &= ~(fault_page_size - 1);
@@ -210,6 +211,20 @@ static vm_fault_t ttm_bo_vm_insert_huge(struct vm_fault *vmf,
if ((pfn & (fault_page_size - 1)) != 0)
goto out_fallback;
+ /*
+ * Huge entries must be special, that is marking them as devmap
+ * with no backing device map range. If there is a backing
+ * range, Don't insert a huge entry.
+ * If this check turns out to be too much of a performance hit,
+ * we can instead have drivers indicate whether they may have
+ * backing device map ranges and if not, skip this lookup.
+ */
I think we can do this statically:
- if it's system memory we know there's no devmap for it, and we do the
trick to block gup_fast
Yes, that should work.
- if it's iomem, we know gup_fast wont work anyway if don't set PFN_DEV,
so might as well not do that
I think gup_fast will unfortunately mistake a huge iomem page for an
ordinary page and try to access a non-existant struct page for it,
unless we do the devmap trick.
And the lookup would then be for the rare case where a driver would have
already registered a dev_pagemap for an iomem area which may also be
mapped through TTM (like the patch from Felix a couple of weeks ago). If
a driver can promise not to do that, then we can safely remove the lookup.
/Thomas
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