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