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    SubjectRe: [PATCH] mm/shmem.c: fix division by zero
    On Fri, 19 Dec 2008 08:44:57 +0300
    Yuri Tikhonov <yur@emcraft.com> wrote:

    >
    > 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 <yur@emcraft.com>
    > ---
    > mm/shmem.c | 8 ++++----
    > 1 files changed, 4 insertions(+), 4 deletions(-)
    >
    > 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;

    umm, ok, maybe. I guess that it would be more appropriate to use
    loff_t in here, and things like

    if (!diroff && !offset && upper_limit >= stage) {

    need a bit of thought (`stage' is still unsigned long).

    I'll queue the patch as-is pending review from Hugh. Thanks.


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