[PATCH v2] mm/hmm: bypass devmap pte when all pfn requested flags are fulfilled

[PATCH v2] mm/hmm: bypass devmap pte when all pfn requested flags are fulfilled

[Li Zhijian]

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@vger.kernel.org
Fixes: 405506274922 ("mm/hmm: add missing call to hmm_pte_need_fault in HMM_PFN_SPECIAL handling")
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
---
 mm/hmm.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index fad6be2bf072..d324fb1a5352 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -295,10 +295,13 @@ static int hmm_vma_handle_pte(struct mm_walk *walk, unsigned long addr,
 		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_devmap(pte) && pte_special(pte) &&
+	    !is_zero_pfn(pte_pfn(pte))) {
 		if (hmm_pte_need_fault(hmm_vma_walk, pfn_req_flags, 0)) {
 			pte_unmap(ptep);
 			return -EFAULT;
-- 
2.31.1

Re: [PATCH v2] mm/hmm: bypass devmap pte when all pfn requested flags are fulfilled

[Christoph Hellwig]

On Sat, Aug 28, 2021 at 09:04:41AM +0800, Li Zhijian wrote:
> +	if (!pte_devmap(pte) && pte_special(pte) &&
> +	    !is_zero_pfn(pte_pfn(pte))) {

Maybe this is a little too superficial and nitpicky, but I find the
ordering of the checks a little strange.  Why not do the pte_special
first and then the exlusions from it later?

Re: [PATCH v2] mm/hmm: bypass devmap pte when all pfn requested flags are fulfilled

[lizhijian]

On 30/08/2021 15:32, Christoph Hellwig wrote:
> On Sat, Aug 28, 2021 at 09:04:41AM +0800, Li Zhijian wrote:
>> +	if (!pte_devmap(pte) && pte_special(pte) &&
>> +	    !is_zero_pfn(pte_pfn(pte))) {
> Maybe this is a little too superficial and nitpicky, but I find the
> ordering of the checks a little strange.  Why not do the pte_special
> first and then the exlusions from it later?
>
It sounds good to me, let's update it

Thanks
Zhijian