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    SubjectRe: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?
    > > Just using cp to read the file is enough to cause problems but I included
    > > a very basic program below that produces the BUG_ON checks. Is this a known
    > > issue or am I using the interface incorrectly?
    >
    > I'd say you're using it correctly but you've found a hitherto unknown bug.
    > On i386 highmem machines with CONFIG_HIGHPTE (at least) pte_offset_map()
    > takes kmap_atomic(), so pagemap_pte_range() can't do copy_to_user() as it
    > presently does.
    >
    > Drat.
    >
    > Still, that shouldn't really disrupt the testing which you're doing. You
    > could disable CONFIG_HIGHPTE to shut it up.
    >

    Yes, that did the trick. Using pagemap, it was trivial to show that the
    2.6.24-rc5-mm1 kernel was placing pages in reverse physical order like
    the following output shows

    b: 32763 v: 753091 p: 65559 . 65558 contig: 1
    b: 32764 v: 753092 p: 65558 . 65557 contig: 1
    b: 32765 v: 753093 p: 65557 . 65556 contig: 1
    b: 32766 v: 753094 p: 65556 . 65555 contig: 1
    b: 32767 v: 753095 p: 65555 . 65555 contig: 1

    p: is the PFN of the page v: is the page offset within an anonymous
    mapping and b: is the number of non-contiguous blocks in the anonymous
    mapping. With the patch applied, it looks more like;

    b: 1232 v: 752964 p: 58944 ................ 87328 contig: 15
    b: 1233 v: 752980 p: 87328 ................ 91200 contig: 15
    b: 1234 v: 752996 p: 91200 ................ 40272 contig: 15
    b: 1235 v: 753012 p: 40272 ................ 85664 contig: 15
    b: 1236 v: 753028 p: 85664 ................ 87312 contig: 15

    so mappings are using contiguous pages again. This was the final test
    program I used in case it's of any interest.

    Thanks

    /*
    * showcontiguous.c
    *
    * Use the /proc/pid/pagemap interface to give an indication of how contiguous
    * physical memory is in an anonymous virtual memory mapping
    */
    #include <stdio.h>
    #include <sys/mman.h>
    #include <stdlib.h>
    #include <unistd.h>
    #include <linux/types.h>
    #include <sys/types.h>
    #include <sys/stat.h>
    #include <fcntl.h>

    #define MAPSIZE (128*1048576)
    #define PM_ENTRY_BYTES sizeof(__u64)

    int main(int argc, char **argv)
    {
    int pagemap_fd;
    unsigned long *anonmapping;
    __u64 pagemap_entry = 0ULL;

    unsigned long vpfn, ppfn, ppfn_last;
    int block_number = 0;
    int contig_count = 1;
    size_t mmap_offset;
    int pagesize = getpagesize();

    if (sizeof(pagemap_entry) < PM_ENTRY_BYTES) {
    printf("ERROR: Failed assumption on size of pagemap_entry\n");
    exit(EXIT_FAILURE);
    }

    /* Open the pagemap interface */
    pagemap_fd = open("/proc/self/pagemap", O_RDONLY);
    if (pagemap_fd == -1) {
    perror("fopen");
    exit(EXIT_FAILURE);
    }

    /* Create an anonymous mapping */
    anonmapping = mmap(NULL, MAPSIZE,
    PROT_READ|PROT_WRITE,
    MAP_PRIVATE|MAP_ANONYMOUS|MAP_POPULATE,
    -1, 0);
    if (anonmapping == MAP_FAILED) {
    perror("mmap");
    exit(1);
    }

    /* Work out the VPN the mapping is at and seek to it in pagemap */
    vpfn = ((unsigned long)anonmapping) / pagesize;
    mmap_offset = lseek(pagemap_fd, vpfn * PM_ENTRY_BYTES, SEEK_SET);
    if (mmap_offset == -1) {
    perror("fseek");
    exit(EXIT_FAILURE);
    }
    ppfn_last = 0;

    /* Read the PFN of each page in the mapping */
    for (mmap_offset = 0; mmap_offset < MAPSIZE; mmap_offset += pagesize) {
    vpfn = ((unsigned long)anonmapping + mmap_offset) / pagesize;

    if (read(pagemap_fd, &pagemap_entry, PM_ENTRY_BYTES) == 0) {
    perror("fread");
    exit(EXIT_FAILURE);
    }

    ppfn = (unsigned long)pagemap_entry;
    if (ppfn == ppfn_last + 1) {
    printf(".");
    contig_count++;
    } else {
    printf(" %lu contig: %d\nb: %6d v: %8lu p: %8lu .",
    ppfn, contig_count,
    block_number, vpfn, ppfn);
    contig_count = 1;
    block_number++;
    }
    ppfn_last = ppfn;
    }
    printf(" %lu config: %d\n", ppfn, contig_count);

    close(pagemap_fd);
    munmap(anonmapping, MAPSIZE);
    exit(EXIT_SUCCESS);
    }
    --
    Mel Gorman
    Part-time Phd Student Linux Technology Center
    University of Limerick IBM Dublin Software Lab


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