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 05:57:45 +0800

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?

>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

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