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    SubjectRe: [PATCH v10 13/69] mm/mmap: use maple tree for unmapped_area{_topdown}
    Date
    Hi Liam,

    Liam Howlett <liam.howlett@oracle.com> writes:

    > From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>
    >
    > The maple tree code was added to find the unmapped area in a previous
    > commit and was checked against what the rbtree returned, but the actual
    > result was never used. Start using the maple tree implementation and
    > remove the rbtree code.
    >
    > Add kernel documentation comment for these functions.
    >
    > Link: https://lkml.kernel.org/r/20220504010716.661115-15-Liam.Howlett@oracle.com
    > Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
    > Cc: Catalin Marinas <catalin.marinas@arm.com>
    > Cc: David Howells <dhowells@redhat.com>
    > Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
    > Cc: SeongJae Park <sj@kernel.org>
    > Cc: Vlastimil Babka <vbabka@suse.cz>
    > Cc: Will Deacon <will@kernel.org>
    > Cc: Davidlohr Bueso <dave@stgolabs.net>
    > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    > ---
    > mm/mmap.c | 253 +++++++-----------------------------------------------
    > 1 file changed, 32 insertions(+), 221 deletions(-)

    With next-20220718 git bisect pointed me to this commit because the
    following code fails in compat mode (31 bit addressing) on s390:

    #include <unistd.h>
    #include <stdlib.h>
    #include <sys/mman.h>
    #include <stdio.h>

    #define __2GB__ 0x80000000
    #define VMA_SIZE 0x10000000
    #define VMA_COUNT (__2GB__ / VMA_SIZE)

    /*
    * Repeated mapping below 2GB until all memory is exhausted
    */
    int main(void)
    {
    void *p;
    unsigned int i;

    for (i = 0; i < VMA_COUNT; i++) {
    p = mmap(NULL, VMA_SIZE, PROT_READ,MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
    printf("%d: %08x - %08x\n", i, (int)p, (int)p + VMA_SIZE);
    if (p == MAP_FAILED)
    break;
    if (p > (void *)__2GB__) {
    return 1;
    }
    }
    if ((i == 0) || (i >= VMA_COUNT))
    return 2;
    return 0;
    }

    With linux/master:
    # ./mmap_compat_anon
    0: 67a00000 - 77a00000
    1: 57a00000 - 67a00000
    2: 47a00000 - 57a00000
    3: 37a00000 - 47a00000
    4: 27a00000 - 37a00000
    5: 17a00000 - 27a00000
    6: 07a00000 - 17a00000
    7: ffffffff - 0fffffff

    With next-20220718 (and older, we're seeing this for a while, but i
    didn't notice due to vacation and other priorities):

    # ./mmap_compat_anon
    0: 67600000 - 77600000
    1: 57600000 - 67600000
    2: 47600000 - 57600000
    3: 37600000 - 47600000
    4: 27600000 - 37600000
    5: 17600000 - 27600000
    6: 07600000 - 17600000
    7: 40000000 - 50000000

    So the first output is correct - mmap succeeds until there's no space
    left, where it returns MAP_FAILED. In the lower (failing) one, mmap
    instead returns an address that is already in use.

    Regards
    Sven

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