Re: [PATCH] mm: call pte_unmap() against a proper pte (Re: [PATCH7/9] swap_info: swap count continuations)

[hugh.dickins@xxxxxxxxxxxxx]

>----Original Message----
>From: nishimura@xxxxxxxxxxxxxxxxx
>Date: 16/10/2009 7:30 
>To: "Hugh Dickins"<hugh.dickins@xxxxxxxxxxxxx>
>Cc: "Andrew Morton"<akpm@xxxxxxxxxxxxxxxxxxxx>, "Nitin Gupta"<ngupta@vflare.
org>, "KAMEZAWA Hiroyuki"<kamezawa.hiroyu@xxxxxxxxxxxxxx>, <hongshin@gmail.
com>, <linux-kernel@xxxxxxxxxxxxxxx>, <linux-mm@xxxxxxxxx>, "Daisuke Nishimura"
<nishimura@xxxxxxxxxxxxxxxxx>
>Subj: [PATCH] mm: call pte_unmap() against a proper pte (Re: [PATCH 7/9] 
swap_info: swap count continuations)
>
>Hi.
>
>> @@ -645,6 +648,7 @@ static int copy_pte_range(struct mm_stru
>>  	spinlock_t *src_ptl, *dst_ptl;
>>  	int progress = 0;
>>  	int rss[2];
>> +	swp_entry_t entry = (swp_entry_t){0};
>>  
>>  again:
>>  	rss[1] = rss[0] = 0;
>> @@ -671,7 +675,10 @@ again:
>>  			progress++;
>>  			continue;
>>  		}
>> -		copy_one_pte(dst_mm, src_mm, dst_pte, src_pte, vma, addr, rss);
>> +		entry.val = copy_one_pte(dst_mm, src_mm, dst_pte, src_pte,
>> +							vma, addr, rss);
>> +		if (entry.val)
>> +			break;
>>  		progress += 8;
>>  	} while (dst_pte++, src_pte++, addr += PAGE_SIZE, addr != end);
>>  
>It isn't the fault of only this patch, but I think breaking the loop without 
incrementing
>dst_pte(and src_pte) would be bad behavior because we do unmap_pte(dst_pte - 
1) later.
>(current copy_pte_range() already does it though... and this is only 
problematic
>when we break the first loop, IIUC.)

Good catch, thanks a lot for finding that.  I believe this is entirely a fault 
in my 7/9, the existing code
takes care not to break before it has made some progress (in part because of 
this unmap issue).

>
>> @@ -681,6 +688,12 @@ again:
>>  	add_mm_rss(dst_mm, rss[0], rss[1]);
>>  	pte_unmap_unlock(dst_pte - 1, dst_ptl);
>>  	cond_resched();
>> +
>> +	if (entry.val) {
>> +		if (add_swap_count_continuation(entry, GFP_KERNEL) < 0)
>> +			return -ENOMEM;
>> +		progress = 0;
>> +	}
>>  	if (addr != end)
>>  		goto again;
>>  	return 0;
>
>I've searched other places where we break a similar loop and do pte_unmap(pte 
- 1).
>Current copy_pte_range() and apply_to_pte_range() has the same problem.

And thank you for taking the trouble to look further afield: yes, 
apply_to_pte_range()
was already wrong.

>
>How about a patch like this ?
>===
>From: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>
>
>There are some places where we do like:
>
>	pte = pte_map();
>	do {
>		(do break in some conditions)
>	} while (pte++, ...);
>	pte_unmap(pte - 1);
>
>But if the loop breaks at the first loop, pte_unmap() unmaps invalid pte.
>
>This patch is a fix for this problem.
>
>Signed-off-by: Daisuke Nishimura <nishimura@xxxxxxxxxxxxxxxxx>

Acked-by: Hugh Dickins <hugh.dickins@xxxxxxxxxxxxx>





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