linux-kernel - [PATCH] mm: call pte_unmap() against a proper pte (Re: [PATCH 7/9] swap

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Message-Id: <20091016153056.6a1f4fe1.nishimura@mxp.nes.nec.co.jp>
Date:	Fri, 16 Oct 2009 15:30:56 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	Hugh Dickins <hugh.dickins@...cali.co.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	hongshin@...il.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Daisuke Nishimura <nishimura@....nes.nec.co.jp>
Subject: [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.)

> @@ -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.

How about a patch like this ?
===
From: Daisuke Nishimura <nishimura@....nes.nec.co.jp>

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@....nes.nec.co.jp>
---
 mm/memory.c |   11 +++++++----
 1 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 72a2494..492de38 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -641,6 +641,7 @@ static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
 		pmd_t *dst_pmd, pmd_t *src_pmd, struct vm_area_struct *vma,
 		unsigned long addr, unsigned long end)
 {
+	pte_t *orig_src_pte, *orig_dst_pte;
 	pte_t *src_pte, *dst_pte;
 	spinlock_t *src_ptl, *dst_ptl;
 	int progress = 0;
@@ -654,6 +655,8 @@ again:
 	src_pte = pte_offset_map_nested(src_pmd, addr);
 	src_ptl = pte_lockptr(src_mm, src_pmd);
 	spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
+	orig_src_pte = src_pte;
+	orig_dst_pte = dst_pte;
 	arch_enter_lazy_mmu_mode();
 
 	do {
@@ -677,9 +680,9 @@ again:
 
 	arch_leave_lazy_mmu_mode();
 	spin_unlock(src_ptl);
-	pte_unmap_nested(src_pte - 1);
+	pte_unmap_nested(orig_src_pte);
 	add_mm_rss(dst_mm, rss[0], rss[1]);
-	pte_unmap_unlock(dst_pte - 1, dst_ptl);
+	pte_unmap_unlock(orig_dst_pte, dst_ptl);
 	cond_resched();
 	if (addr != end)
 		goto again;
@@ -1822,10 +1825,10 @@ static int apply_to_pte_range(struct mm_struct *mm, pmd_t *pmd,
 	token = pmd_pgtable(*pmd);
 
 	do {
-		err = fn(pte, token, addr, data);
+		err = fn(pte++, token, addr, data);
 		if (err)
 			break;
-	} while (pte++, addr += PAGE_SIZE, addr != end);
+	} while (addr += PAGE_SIZE, addr != end);
 
 	arch_leave_lazy_mmu_mode();
 
--
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