Re: [PATCH v1 RESEND 3/6] sparc/mm: don't unconditionally set HW writable bit when setting PTE dirty on 64bit

From: David Hildenbrand <david@redhat.com>
To: Sam Ravnborg <sam@ravnborg.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-kselftest@vger.kernel.org, sparclinux@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	"David S. Miller" <davem@davemloft.net>,
	Peter Xu <peterx@redhat.com>, Hugh Dickins <hughd@google.com>,
	Shuah Khan <shuah@kernel.org>, Yu Zhao <yuzhao@google.com>,
	Anshuman Khandual <anshuman.khandual@arm.com>
Subject: Re: [PATCH v1 RESEND 3/6] sparc/mm: don't unconditionally set HW writable bit when setting PTE dirty on 64bit
Date: Wed, 12 Apr 2023 11:48:15 +0200	[thread overview]
Message-ID: <de93ddc4-29ff-6113-a146-bc278dcce5f9@redhat.com> (raw)
In-Reply-To: <20230411193548.GA2094947@ravnborg.org>

[...]

>> Let's fix the remaining issues and prepare for reverting the workarounds
>> by setting the HW writable bit only if both, the SW dirty bit and the SW
>> writable bit are set.
>>
>> We have to move pte_dirty() and pte_dirty() up. The code patching
> One of the pte_dirty() should be replaced with pte_write().
> 

Indeed, thanks. I assume Andrew can change the latter to pte_write(). 
[unless I have to resend, of course]

> It would have been nice to separate moving and changes in two patches,
> but keeping it together works too.

I prefer to not have "move code within the same file around" in separate 
patches as long as it doesn't add too much noise. Here, I think it's 
acceptable.

> 
> 
>> mechanism and handling constants > 22bit is a bit special on sparc64.
>>
>> The ASM logic in pte_mkdirty() and pte_mkwrite() match the logic in
>> pte_mkold() to create the mask depending on the machine type. The ASM
>> logic in __pte_mkhwwrite() matches the logic in pte_present(), just
>> using an "or" instead of an "and" instruction.
>>
>> With this commit (sun4u in QEMU):
>> 	root@debian:~/linux/tools/testing/selftests/mm# ./mkdirty
>> 	# [INFO] detected THP size: 8192 KiB
>> 	TAP version 13
>> 	1..6
>> 	# [INFO] PTRACE write access
>> 	ok 1 SIGSEGV generated, page not modified
>> 	# [INFO] PTRACE write access to THP
>> 	ok 2 SIGSEGV generated, page not modified
>> 	# [INFO] Page migration
>> 	ok 3 SIGSEGV generated, page not modified
>> 	# [INFO] Page migration of THP
>> 	ok 4 SIGSEGV generated, page not modified
>> 	# [INFO] PTE-mapping a THP
>> 	ok 5 SIGSEGV generated, page not modified
>> 	# [INFO] UFFDIO_COPY
>> 	ok 6 SIGSEGV generated, page not modified
>> 	# Totals: pass:6 fail:0 xfail:0 xpass:0 skip:0 error:0
> Nice!
> 
>>
>> This handling seems to have been in place forever.
>>
>> [1] https://lkml.kernel.org/r/533a7c3d-3a48-b16b-b421-6e8386e0b142@redhat.com
>>
>> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>> Signed-off-by: David Hildenbrand <david@redhat.com>
> 
> I tried to follow your changes, but my knowledge of gcc assembler failed
> me. But based on the nice and detailed change log and the code I managed
> to understand:
> Acked-by: Sam Ravnborg <sam@ravnborg.org>

Thanks Sam!

-- 
Thanks,

David / dhildenb