On 3/23/21 5:37 PM, Jason Gunthorpe wrote:
On Tue, Mar 23, 2021 at 05:34:51PM +0100, Thomas Hellström (Intel) wrote:
@@ -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.
Isn't the devmap PTE flag arch optional? Does this fall back to not
using huge pages on arches that don't support it?
Good point. No, currently it's only conditioned on transhuge page support.
Need to condition it on also devmap support.
Also, I feel like this code to install "pte_special" huge pages does
not belong in the drm subsystem..
I could add helpers in huge_memory.c:
vmf_insert_pfn_pmd_prot_special() and
vmf_insert_pfn_pud_prot_special()
/Thomas
Jason
