linux-kernel - [PATCH] mm/hmm: bypass devmap pte when all pfn requested flags are fulfilled

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Date:   Fri, 27 Aug 2021 22:45:00 +0800
From:   Li Zhijian <lizhijian@...fujitsu.com>
To:     <linux-mm@...ck.org>, <linux-rdma@...r.kernel.org>,
        <akpm@...ux-foundation.org>, <jglisse@...hat.com>, <jgg@...pe.ca>
CC:     <yishaih@...dia.com>, <linux-kernel@...r.kernel.org>,
        Li Zhijian <lizhijian@...fujitsu.com>, <stable@...r.kernel.org>
Subject: [PATCH] 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

CC: stable@...r.kernel.org
Signed-off-by: Li Zhijian <lizhijian@...fujitsu.com>
---
 mm/hmm.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/hmm.c b/mm/hmm.c
index fad6be2bf072..4766bdefb6c3 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -294,6 +294,12 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 	if (required_fault)
 		goto fault;
 
+	/*
+	 * just bypass devmap pte such as DAX page when all pfn requested
+	 * flags(pfn_req_flags) are fulfilled.
+	 */
+	if (pte_devmap(pte))
+		goto out;
 	/*
 	 * Since each architecture defines a struct page for the zero page, just
 	 * fall through and treat it like a normal page.
@@ -307,6 +313,7 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 		return 0;
 	}
 
+out:
 	*hmm_pfn = pte_pfn(pte) | cpu_flags;
 	return 0;
 
-- 
2.31.1