Subject: Re: [PATCH 3/5] mm/mremap: use pmd_addr_end to calculate next in move_page_tables()

From: Wei Yang <[email-masked]>
Date: Thu, 30 Jan 2020 09:30:00 +0800

On Wed, Jan 29, 2020 at 11:24:41PM +0000, Russell King - ARM Linux admin wrote:
>On Thu, Jan 30, 2020 at 05:57:45AM +0800, Wei Yang wrote:
>> On Wed, Jan 29, 2020 at 09:47:38AM +0000, Russell King - ARM Linux admin wrote:
>> >On Sun, Jan 26, 2020 at 05:47:57PM +0300, Dmitry Osipenko wrote:
>> >> 18.01.2020 02:22, Wei Yang пишет:
>> >> > Use the general helper instead of do it by hand.
>> >> > 
>> >> > Signed-off-by: Wei Yang <[email-masked]>
>> >> > ---
>> >> >  mm/mremap.c | 7 ++-----
>> >> >  1 file changed, 2 insertions(+), 5 deletions(-)
>> >> > 
>> >> > diff --git a/mm/mremap.c b/mm/mremap.c
>> >> > index c2af8ba4ba43..a258914f3ee1 100644
>> >> > --- a/mm/mremap.c
>> >> > +++ b/mm/mremap.c
>> >> > @@ -253,11 +253,8 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>> >> >  
>> >> >  	for (; old_addr < old_end; old_addr += extent, new_addr += extent) {
>> >> >  		cond_resched();
>> >> > -		next = (old_addr + PMD_SIZE) & PMD_MASK;
>> >> > -		/* even if next overflowed, extent below will be ok */
>> >> > +		next = pmd_addr_end(old_addr, old_end);
>> >> >  		extent = next - old_addr;
>> >> > -		if (extent > old_end - old_addr)
>> >> > -			extent = old_end - old_addr;
>> >> >  		old_pmd = get_old_pmd(vma->vm_mm, old_addr);
>> >> >  		if (!old_pmd)
>> >> >  			continue;
>> >> > @@ -301,7 +298,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma,
>> >> >  
>> >> >  		if (pte_alloc(new_vma->vm_mm, new_pmd))
>> >> >  			break;
>> >> > -		next = (new_addr + PMD_SIZE) & PMD_MASK;
>> >> > +		next = pmd_addr_end(new_addr, new_addr + len);
>> >> >  		if (extent > next - new_addr)
>> >> >  			extent = next - new_addr;
>> >> >  		move_ptes(vma, old_pmd, old_addr, old_addr + extent, new_vma,
>> >> > 
>> >> 
>> >> Hello Wei,
>> >> 
>> >> Starting with next-20200122, I'm seeing the following in KMSG on NVIDIA
>> >> Tegra (ARM32):
>> >> 
>> >>   BUG: Bad rss-counter state mm:(ptrval) type:MM_ANONPAGES val:190
>> >> 
>> >> and eventually kernel hangs.
>> >> 
>> >> Git's bisection points to this patch and reverting it helps. Please fix,
>> >> thanks in advance.
>> >
>> >The above is definitely wrong - pXX_addr_end() are designed to be used
>> >with an address index within the pXX table table and the address index
>> >of either the last entry in the same pXX table or the beginning of the
>> >_next_ pXX table.  Arbitary end address indicies are not allowed.
>> >
>> 
>> #define pmd_addr_end(addr, end)						\
>> ({	unsigned long __boundary = ((addr) + PMD_SIZE) & PMD_MASK;	\
>> 	(__boundary - 1 < (end) - 1)? __boundary: (end);		\
>> })
>> 
>> If my understanding is correct, the definition here align the addr to next PMD
>> boundary or end.
>> 
>> I don't see the possibility to across another PMD. Do I miss something?
>
>Look at the definition of p*_addr_end() that are used when page tables
>are rolled up.
>

Sorry, I don't get your point.

What's the meaning of "roll up" here?

Would you mind giving me an example? I see pmd_addr_end() is not used in many
places in core kernel. By glancing those usages, all the places use it like
pmd_addr_end(addr, end). Seems no specially handing on the end address.

Or you mean the case when pmd_addr_end() is defined to return "end" directly? 

>> >When page tables are "rolled up" when levels don't exist, it is common
>> >practice for these macros to just return their end address index.
>> >Hence, if they are used with arbitary end address indicies, then the
>> >iteration will fail.
>> >
>> >The only way to do this is:
>> >
>> >	next = pmd_addr_end(old_addr,
>> >			pud_addr_end(old_addr,
>> >				p4d_addr_end(old_addr,
>> >					pgd_addr_end(old_addr, old_end))));
>> >
>> >which gives pmd_addr_end() (and each of the intermediate pXX_addr_end())
>> >the correct end argument.  However, that's a more complex and verbose,
>> >and likely less efficient than the current code.
>> >
>> >I'd suggest that there's nothing to "fix" in the v5.5 code wrt this,
>> >and trying to "clean it up" will just result in less efficient or
>> >broken code.
>> >
>> >-- 
>> >RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
>> >FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
>> >According to speedtest.net: 11.9Mbps down 500kbps up
>> 
>> -- 
>> Wei Yang
>> Help you, Help me
>> 
>
>-- 
>RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
>FTTC broadband for 0.8mile line in suburbia: sync at 12.1Mbps down 622kbps up
>According to speedtest.net: 11.9Mbps down 500kbps up

-- 
Wei Yang
Help you, Help me

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