Re: [PATCH 1/1] arm64/hugetlb: clear PG_dcache_clean if the page is dirty when munmap

From: "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: Steve Capper <Steve.Capper@arm.com>,
	David Woods <dwoods@ezchip.com>,
	Tianhong Ding <dingtianhong@huawei.com>,
	Will Deacon <will.deacon@arm.com>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	Xinwei Hu <huxinwei@huawei.com>, Zefan Li <lizefan@huawei.com>,
	"fangwei (I)" <fangwei1@huawei.com>,
	"Hanjun Guo" <guohanjun@huawei.com>,
	linux-arm-kernel <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 1/1] arm64/hugetlb: clear PG_dcache_clean if the page is dirty when munmap
Date: Wed, 24 Aug 2016 17:00:50 +0800	[thread overview]
Message-ID: <57BD6242.1080801@huawei.com> (raw)
In-Reply-To: <20160823172852.GB16213@e104818-lin.cambridge.arm.com>


On 2016/8/24 1:28, Catalin Marinas wrote:
> On Mon, Aug 22, 2016 at 12:19:04PM +0800, Leizhen (ThunderTown) wrote:
>> On 2016/7/20 17:19, Catalin Marinas wrote:
>>> On Wed, Jul 20, 2016 at 10:46:27AM +0800, Leizhen (ThunderTown) wrote:
>>>>>>>> On 2016/7/8 21:54, Catalin Marinas wrote:
>>>>>>>>> ------------8<----------------
>>>>>>>>> diff --git a/arch/arm64/mm/flush.c b/arch/arm64/mm/flush.c
>>>>>>>>> index dbd12ea8ce68..c753fa804165 100644
>>>>>>>>> --- a/arch/arm64/mm/flush.c
>>>>>>>>> +++ b/arch/arm64/mm/flush.c
>>>>>>>>> @@ -75,7 +75,8 @@ void __sync_icache_dcache(pte_t pte, unsigned long addr)
>>>>>>>>>  	if (!page_mapping(page))
>>>>>>>>>  		return;
>>>>>>>>>  
>>>>>>>>> -	if (!test_and_set_bit(PG_dcache_clean, &page->flags))
>>>>>>>>> +	if (!test_and_set_bit(PG_dcache_clean, &page->flags) ||
>>>>>>>>> +	    PageDirty(page))
>>>>>>>>>  		sync_icache_aliases(page_address(page),
>>>>>>>>>  				    PAGE_SIZE << compound_order(page));
>>>>>>>>>  	else if (icache_is_aivivt())
>>>>>>>>> ----------------8<---------------------
>>>>
>>>> Do you plan to send this patch? My colleagues told me that if our
>>>> patches are quite different, it should be Signed-off-by you.
>>>
>>> The reason I'm not sending it is that I don't fully understand how it
>>> solves the problem for a shared file mmap(), not just hugetlbfs. As I
>>> said in an earlier email: after an msync() in user space we
>>> should flush the pages to disk via write_cache_pages(). This function
>> Hi Catalin:
>>    I'm so sorry for my fault. The previous small pages test result I actually ran on ramfs.
>> Today, I ran the case on harddisk fs, it worked well without this patch.
>>
>> Summarized as follows:
>> small pages on ramfs: need this patch
>> small pages on harddisk fs: no need this patch
>> hugetlbfs: need this patch
> 
> I would add:
> 
> small pages over nfs: fails with or without this patch
> 
> (tested on Juno, Cortex-A57; seems to be fixed if I remove the
> PG_dcache_clean test altogether but, well, we end up over-flushing)
> 
> I assume that when using a hard drive, it goes through the block I/O
> layer and we may have a flush_dcache_page() called when the kernel is
> about to read a page that has been mapped in user space. This would
> clear the PG_dcache_clean bit and subsequent __sync_icache_dcache()
> would perform cache maintenance.
> 
> Could you try on your system the test case without the msync() call? I'm
According to my test results: without msync, the test case may failed.

10-175-112-211:~ # ./tst_small_page_no_msync
Test is Failed: The result is 0x316b9, expect = 0x365a5
10-175-112-211:~ # ./tst_small_page_no_msync
Test is Failed: The result is 0x31023, expect = 0x31efa
10-175-112-211:~ # ./tst_small_page_no_msync
Test is Passed: The result is 0x31efa, expect = 0x31efa

10-175-112-211:~ # ./tst_small_page
Test is Passed: The result is 0x31eb7, expect = 0x31eb7
10-175-112-211:~ # ./tst_small_page
Test is Passed: The result is 0x3111f, expect = 0x3111f
10-175-112-211:~ # ./tst_small_page
Test is Passed: The result is 0x3111f, expect = 0x3111f

> not sure whether munmap() would trigger an immediate write-back, in
> which case we may see the issue even with the filesystem on a hard
> drive.
>