[patch 1/8] mm/hmm: bypass devmap pte when all pfn requested flags are fulfilled

From: Li Zhijian <redacted>
Subject: mm/hmm: bypass devmap pte when all pfn requested flags are fulfilled

Previously, we noticed the one rpma example was failed[1] since
36f30e486d, where it will use ODP feature to do RDMA WRITE between fsdax
files.

After digging into the code, we found hmm_vma_handle_pte() will still
return EFAULT even though all the its requesting flags has been fulfilled.
That's because a DAX page will be marked as (_PAGE_SPECIAL | PAGE_DEVMAP)
by pte_mkdevmap().

[1]: https://github.com/pmem/rpma/issues/1142

Link: https://lkml.kernel.org/r/20210830094232.203029-1-lizhijian@cn.fujitsu.com
Fixes: 405506274922 ("mm/hmm: add missing call to hmm_pte_need_fault in HMM_PFN_SPECIAL handling")
Signed-off-by: Li Zhijian <redacted>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Cc: <redacted>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/hmm.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/mm/hmm.c~mm-hmm-bypass-devmap-pte-when-all-pfn-requested-flags-are-fulfilled
+++ a/mm/hmm.c

@@ -295,10 +295,13 @@ static int hmm_vma_handle_pte(struct mm_
 		goto fault;
 
 	/*
+	 * Bypass devmap pte such as DAX page when all pfn requested
+	 * flags(pfn_req_flags) are fulfilled.
 	 * Since each architecture defines a struct page for the zero page, just
 	 * fall through and treat it like a normal page.
 	 */
-	if (pte_special(pte) && !is_zero_pfn(pte_pfn(pte))) {
+	if (pte_special(pte) && !pte_devmap(pte) &&
+	    !is_zero_pfn(pte_pfn(pte))) {
 		if (hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, 0)) {
 			pte_unmap(ptep);
 			return -EFAULT;

_