linux-kernel - Re: [PATCH 1/1] arm64/hugetlb: clear PG_dcache

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Message-ID: <20160824103044.GA11504@e104818-lin.cambridge.arm.com>
Date:   Wed, 24 Aug 2016 11:30:44 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
Cc:     Steve Capper <Steve.Capper@....com>,
        David Woods <dwoods@...hip.com>,
        Hanjun Guo <guohanjun@...wei.com>,
        Will Deacon <will.deacon@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Xinwei Hu <huxinwei@...wei.com>, Zefan Li <lizefan@...wei.com>,
        "fangwei (I)" <fangwei1@...wei.com>,
        Tianhong Ding <dingtianhong@...wei.com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/1] arm64/hugetlb: clear PG_dcache_clean if the page is
 dirty when munmap

On Wed, Aug 24, 2016 at 05:00:50PM +0800, Leizhen (ThunderTown) wrote:
> 
> 
> 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.

Thanks. Just to be clear, does the test generate a file on on a hard
drive?

> 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

How many tests did you run for the "passed" case? With NFS it may
sometime take minutes before a failure (I use the "watch" command with a
slightly modified test to return non-zero in case of value mismatch).

While we indeed see failures on multiple filesystem types, I wonder
whether this test case is actually expected to work. If I modify the
test to pass O_TRUNC to open(), I can no longer see failures. So any
standard tool that copies/creates executable files (gcc, dpkg, cp, rsync
etc.) wouldn't encounter such issues since they truncate the original
file and old page cache pages would be removed.

Do you have a real use-case where a task mmap's an executable file,
modifies it in place and expects another task to see the new
instructions without user-space cache maintenance?

-- 
Catalin