Re: [PATCH][v2] fork_init: fix division by zero

Yuri Tikhonov
2008-12-18T07:58:39Z

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