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    SubjectRe: [RFC PATCH 1/1] mm/percpu.c: fix potential memory leakage for pcpu_embed_first_chunk()
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    On 2016/9/30 0:44, Tejun Heo wrote:
    > Hello,
    >
    > On Fri, Sep 30, 2016 at 12:03:20AM +0800, zijun_hu wrote:
    >> From: zijun_hu <zijun_hu@htc.com>
    >>
    >> it will cause memory leakage for pcpu_embed_first_chunk() to go to
    >> label @out_free if the chunk spans over 3/4 VMALLOC area. all memory
    >> are allocated and recorded into array @areas for each CPU group, but
    >> the memory allocated aren't be freed before returning after going to
    >> label @out_free
    >>
    >> in order to fix this bug, we check chunk spanned area immediately
    >> after completing memory allocation for all CPU group, we go to label
    >> @out_free_areas other than @out_free to free all memory allocated if
    >> the checking is failed.
    >>
    >> Signed-off-by: zijun_hu <zijun_hu@htc.com>
    > ...
    >> @@ -2000,6 +2001,21 @@ int __init pcpu_embed_first_chunk(size_t reserved_size, size_t dyn_size,
    >> areas[group] = ptr;
    >>
    >> base = min(ptr, base);
    >> + if (ptr > areas[j])
    >> + j = group;
    >> + }
    >> + max_distance = areas[j] - base;
    >> + max_distance += ai->unit_size * ai->groups[j].nr_units;
    >> +
    >> + /* warn if maximum distance is further than 75% of vmalloc space */
    >> + if (max_distance > VMALLOC_TOTAL * 3 / 4) {
    >> + pr_warn("max_distance=0x%lx too large for vmalloc space 0x%lx\n",
    >> + max_distance, VMALLOC_TOTAL);
    >> +#ifdef CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK
    >> + /* and fail if we have fallback */
    >> + rc = -EINVAL;
    >> + goto out_free_areas;
    >> +#endif
    >
    > Isn't it way simpler to make the error path jump to out_free_areas?
    > There's another similar case after pcpu_setup_first_chunk() failure
    > too. Also, can you please explain how you tested the changes?
    >
    > Thanks.
    >
    1) the simpler way don't work because it maybe free many memory block twice

    let us take a CPU group as a example, after we allocate All memory
    needed by a CPU group, we maybe free a unit memory block which
    don't map to a available CPU, we maybe free a part of unit memory which
    we don't used too, you can refer to following code segments for detailed
    info.
    for (group = 0; group < ai->nr_groups; group++) {
    struct pcpu_group_info *gi = &ai->groups[group];
    void *ptr = areas[group];

    for (i = 0; i < gi->nr_units; i++, ptr += ai->unit_size) {
    if (gi->cpu_map[i] == NR_CPUS) {
    /* unused unit, free whole */
    free_fn(ptr, ai->unit_size);
    continue;
    }
    /* copy and return the unused part */
    memcpy(ptr, __per_cpu_load, ai->static_size);
    free_fn(ptr + size_sum, ai->unit_size - size_sum);
    }
    }

    2) as we seen, pcpu_setup_first_chunk() doesn't cause a failure, it return 0
    always or panic by BUG_ON(), even if it fails, we can conclude the allocated
    memory based on information recorded by it, such as pcpu_base_addr and many of
    static variable, we can complete the free operations; but we can't if we
    fail in the case pointed by this patch

    3) my test way is simple, i force "if (max_distance > VMALLOC_TOTAL * 3 / 4)"
    to if (1) and print which memory i allocate before the jumping, then print which memory
    i free after the jumping and before returning, then check whether i free the memory i
    allocate in this function, the result is okay

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