linux-kernel - Re: [PATCH][v2] fork_init: fix division by zero

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Message-Id: <200812181047.50332.yur@emcraft.com>
Date:	Thu, 18 Dec 2008 10:47:50 +0300
From:	Yuri Tikhonov <yur@...raft.com>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, wd@...x.de, dzu@...x.de,
	linux-kernel@...r.kernel.org, miltonm@....com,
	linuxppc-dev@...abs.org, viro@...iv.linux.org.uk,
	Geert.Uytterhoeven@...ycom.com, yanok@...raft.com
Subject: Re: [PATCH][v2] fork_init: fix division by zero

Hello Paul,

On Friday 12 December 2008 03:48, Paul Mackerras wrote:
> Andrew Morton writes:
> 
> > > +#if (8 * THREAD_SIZE) > PAGE_SIZE
> > >  	max_threads = mempages / (8 * THREAD_SIZE / PAGE_SIZE);
> > > +#else
> > > +	max_threads = mempages * (PAGE_SIZE / (8 * THREAD_SIZE));
> > > +#endif
> > 
> > The expression you've chosen here can be quite inacccurate, because
> > ((PAGE_SIZE / (8 * THREAD_SIZE)) is a small number.  The way to
> > preserve accuracy is
> 
> The assumption is that THREAD_SIZE is a power of 2, as is PAGE_SIZE.
> 
> I think Yuri should be increasing THREAD_SIZE for the larger page
> sizes he's implementing, because we have on-stack arrays whose size
> depends on the page size.  I suspect that having THREAD_SIZE less than
> 1/8 of PAGE_SIZE risks stack overflows, and the better fix is for Yuri
> to make sure THREAD_SIZE is at least 1/8 of PAGE_SIZE.  (In fact, more
> may be needed - someone should work out what fraction is actually
> needed.)

  Right, thanks for pointing this. I guess, I was just lucky since didn't run into
problems with stack overflows. So, I agree that we should increase the 
THREAD_SIZE in case of 256KB pages up to 1/8 of PAGE_SIZE, that is up 
to 32KB.

 There is one more warning from the common code when I use 256KB pages:

   CC      mm/shmem.o
mm/shmem.c: In function 'shmem_truncate_range':
mm/shmem.c:613: warning: division by zero
mm/shmem.c:619: warning: division by zero
mm/shmem.c:644: warning: division by zero
mm/shmem.c: In function 'shmem_unuse_inode':
mm/shmem.c:873: warning: division by zero

 The problem here is that ENTRIES_PER_PAGEPAGE becomes 0x1.0000.0000
when PAGE_SIZE is 256K.

 How about the following fix ?

diff --git a/mm/shmem.c b/mm/shmem.c
index 0ed0752..99d7c91 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -57,7 +57,7 @@
 #include <asm/pgtable.h>
 
 #define ENTRIES_PER_PAGE (PAGE_CACHE_SIZE/sizeof(unsigned long))
-#define ENTRIES_PER_PAGEPAGE (ENTRIES_PER_PAGE*ENTRIES_PER_PAGE)
+#define ENTRIES_PER_PAGEPAGE ((unsigned long long)ENTRIES_PER_PAGE*ENTRIES_PER_PAGE)
 #define BLOCKS_PER_PAGE  (PAGE_CACHE_SIZE/512)
 
 #define SHMEM_MAX_INDEX  (SHMEM_NR_DIRECT + (ENTRIES_PER_PAGEPAGE/2) * (ENTRIES_PER_PAGE+1))
@@ -95,7 +95,7 @@ static unsigned long shmem_default_max_inodes(void)
 }
 #endif
 
-static int shmem_getpage(struct inode *inode, unsigned long idx,
+static int shmem_getpage(struct inode *inode, unsigned long long idx,
 			 struct page **pagep, enum sgp_type sgp, int *type);
 
 static inline struct page *shmem_dir_alloc(gfp_t gfp_mask)
@@ -533,7 +533,7 @@ static void shmem_truncate_range(struct inode *inode, loff_t start, loff_t end)
 	int punch_hole;
 	spinlock_t *needs_lock;
 	spinlock_t *punch_lock;
-	unsigned long upper_limit;
+	unsigned long long upper_limit;
 
 	inode->i_ctime = inode->i_mtime = CURRENT_TIME;
 	idx = (start + PAGE_CACHE_SIZE - 1) >> PAGE_CACHE_SHIFT;
@@ -1175,7 +1175,7 @@ static inline struct mempolicy *shmem_get_sbmpol(struct shmem_sb_info *sbinfo)
  * vm. If we swap it in we mark it dirty since we also free the swap
  * entry since a page cannot live in both the swap and page cache
  */
-static int shmem_getpage(struct inode *inode, unsigned long idx,
+static int shmem_getpage(struct inode *inode, unsigned long long idx,
 			struct page **pagep, enum sgp_type sgp, int *type)
 {
 	struct address_space *mapping = inode->i_mapping;

Regards, Yuri
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