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@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 | 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