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From:   weizhenliang <weizhenliang@huawei.com>
To:     Sean Anderson <seanga2@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>
CC:     Changhee Han <ch0.han@lge.com>,
        Tang Bin <tangbin@cmss.chinamobile.com>,
        Zhang Shengju <zhangshengju@cmss.chinamobile.com>
Subject: Re: [PATCH v2 1/2] tools/vm/page_owner_sort.c: Sort by stacktrace
 before culling
Thread-Topic: [PATCH v2 1/2] tools/vm/page_owner_sort.c: Sort by stacktrace
 before culling
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Date:   Thu, 25 Nov 2021 09:46:41 +0000
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On 2021/11/25 3:37, Sean Anderson <seanga2@gmail.com> wrote:

> static int compare_num(const void *p1, const void *p2) @@ -121,6 
>+122,7 @@ static void add_list(char *buf, int len)  
>list[list_size].page_num = get_page_num(buf);  
>memcpy(list[list_size].txt, buf, len);  list[list_size].txt[len] = 0;
>+ list[list_size].stacktrace = strchr(list[list_size].txt, '\n');

When read_block gets an empty line, buf is "\n", then the stacktrace is NULL

> list_size++;
> if (list_size % 1000 == 0) {
> printf("loaded %d\r", list_size);
>@@ -199,7 +201,7 @@ int main(int argc, char **argv)
> 
> printf("sorting ....\n");
> 
>- qsort(list, list_size, sizeof(list[0]), compare_txt);
>+ qsort(list, list_size, sizeof(list[0]), compare_stacktrace);
> 
> list2 = malloc(sizeof(*list) * list_size);  if (!list2) { @@ -211,7 
>+213,7 @@ int main(int argc, char **argv)
> 
> for (i = count = 0; i < list_size; i++) {  if (count == 0 ||
>- strcmp(list2[count-1].txt, list[i].txt) != 0) {
>+ strcmp(list2[count-1].stacktrace, list[i].stacktrace) != 0) {

And when stacktrace is NULL, a segmentation fault will be triggered here.

> list2[count++] = list[i];
> } else {
> list2[count-1].num += list[i].num;

1. Maybe you can check whether the ret of read_block is 0 before add_list,
or whether the len of buf is 0 in add_list
2. Is it necessary to check stacktrace==NULL before use ?

Wei.