mm: convert pXd_devmap checks to vma_is_dax

Patch series "mm: Remove pXX_devmap page table bit and pfn_t type", v3.

All users of dax now require a ZONE_DEVICE page which is properly
refcounted.  This means there is no longer any need for the PFN_DEV,
PFN_MAP and PFN_SPECIAL flags.  Furthermore the PFN_SG_CHAIN and
PFN_SG_LAST flags never appear to have been used.  It is therefore
possible to remove the pfn_t type and replace any usage with raw pfns.

The remaining users of PFN_DEV have simply passed this to
vmf_insert_mixed() to create pte_devmap() mappings.  It is unclear why
this was the case but presumably to ensure vm_normal_page() does not
return these pages.  These users can be trivially converted to raw pfns
and creating a pXX_special() mapping to ensure vm_normal_page() still
doesn't return these pages.

Now that there are no users of PFN_DEV we can remove the devmap page table
bit and all associated functions and macros, freeing up a software page
table bit.


This patch (of 14):

Currently dax is the only user of pmd and pud mapped ZONE_DEVICE pages. 
Therefore page walkers that want to exclude DAX pages can check pmd_devmap
or pud_devmap.  However soon dax will no longer set PFN_DEV, meaning dax
pages are mapped as normal pages.

Ensure page walkers that currently use pXd_devmap to skip DAX pages
continue to do so by adding explicit checks of the VMA instead.

Remove vma_is_dax() check from mm/userfaultfd.c as validate_move_areas()
will already skip DAX VMA's on account of them not being anonymous.

The check in userfaultfd_must_wait() is also redundant as
vma_can_userfault() should have already filtered out dax vma's.

For HMM the pud_devmap check seems unnecessary as there is no reason we
shouldn't be able to handle any leaf PUD here so remove it also.

Link: https://lkml.kernel.org/r/cover.176965585864cb8d2cf41464b44dcc0471e643a0.1750323463.git-series.apopple@nvidia.com
Link: https://lkml.kernel.org/r/f0611f6f475f48fcdf34c65084a359aefef4cccc.1750323463.git-series.apopple@nvidia.com
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Cc: Balbir Singh <balbirs@nvidia.com>
Cc: Björn Töpel <bjorn@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Chunyan Zhang <zhang.lyra@gmail.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Deepak Gupta <debug@rivosinc.com>
Cc: Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc: Inki Dae <m.szyprowski@samsung.com>
Cc: John Groves <john@groves.net>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Björn Töpel <bjorn@rivosinc.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/userfaultfd.c

@@ -304,7 +304,7 @@ static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
 		goto out;
 
 	ret = false;
-	if (!pmd_present(_pmd) || pmd_devmap(_pmd))
+	if (!pmd_present(_pmd))
 		goto out;
 
 	if (pmd_trans_huge(_pmd)) {

mm/hmm.c

@@ -441,7 +441,7 @@ static int hmm_vma_walk_pud(pud_t *pudp, unsigned long start, unsigned long end,
 		return hmm_vma_walk_hole(start, end, -1, walk);
 	}
 
-	if (pud_leaf(pud) && pud_devmap(pud)) {
+	if (pud_leaf(pud)) {
 		unsigned long i, npages, pfn;
 		unsigned int required_fault;
 		unsigned long *hmm_pfns;

mm/userfaultfd.c

@@ -1818,12 +1818,6 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, unsigned long dst_start,
 
 		ptl = pmd_trans_huge_lock(src_pmd, src_vma);
 		if (ptl) {
-			if (pmd_devmap(*src_pmd)) {
-				spin_unlock(ptl);
-				err = -ENOENT;
-				break;
-			}
-
 			/* Check if we can move the pmd without splitting it. */
 			if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
 			    !pmd_none(dst_pmdval)) {