mm/shmem.c: fix division by zero

The following patch fixes division by zero, which we have in shmem_truncate_range() and shmem_unuse_inode(), if use big PAGE_SIZE values (e.g. 256KB on ppc44x).

With 256KB PAGE_SIZE the ENTRIES_PER_PAGEPAGE constant becomes too large (0x1.0000.0000), so this patch just changes the types from 'ulong' to 'ullong' where it's necessary.

Signed-off-by Yuri Tikhonov <[email protected]>

---
 mm/shmem.c |    8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

mm/shmem.c +3 -3

index 0ed0752..99d7c91 100644
@@ -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;

The following patch fixes division by zero, which we have in shmem_truncate_range() and shmem_unuse_inode(), if use big PAGE_SIZE values (e.g. 256KB on ppc44x).

With 256KB PAGE_SIZE the ENTRIES_PER_PAGEPAGE constant becomes too large (0x1.0000.0000), so this patch just changes the types from 'ulong' to 'ullong' where it's necessary.

Signed-off-by Yuri Tikhonov <[email protected]>

Quoted 10 lines · Yuri Tikhonov

=20
The following patch fixes division by zero, which we have in shmem_truncate_range() and shmem_unuse_inode(), if use big PAGE_SIZE values (e.g. 256KB on ppc44x).
=20
With 256KB PAGE_SIZE the ENTRIES_PER_PAGEPAGE constant becomes too large (0x1.0000.0000), so this patch just changes the types from 'ulong' to 'ullong' where it's necessary.
=20

Signed-off-by Yuri Tikhonov <[email protected]>

Sorry for the slow reply, but I'm afraid I don't like spattering around an increasing number of unsigned long longs to fix that division by zero on an unusual configuration: I doubt that's the right solution.

It's ridiculous for shmem.c to be trying to support a wider address range than the page cache itself can support, and it's wasteful for it to be using 256KB pages for its index blocks (not to mention its data blocks! but we'll agree to differ on that).

Maybe it should be doing a kmalloc(4096) instead of using alloc_pages(); though admittedly that's not a straightforward change, since we do make use of highmem and page->private. Maybe I should use this as stimulus to switch shmem over to storing its swap entries in the pagecache radix tree. Maybe we should simply disable its use of swap in such an extreme configuration.

But I need to understand more about your ppc44x target to make the right decision. What's very strange to me is this: since unsigned long long is the same size as unsigned long on 64-bit, this change appears to be for a 32-bit machine with 256KB pages. I wonder what market segment that is targeted at?