Re: weird memory access problem running on dom0

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 22/10/13 16:41, Alice Wan wrote:
> hi all,
>
>     recently we met an unbelievable weird memory problem running on
> dom0, test case is very simple, code is as following:
>
> #define BUF_SIZE        4096
> #define IO_PATTERN      0xab
>
> int main(int argc, char *argv[])
> {
>         void *buf;
>         char cmp_buf[BUF_SIZE];
>         int err = 0;
>
>         buf = malloc(BUF_SIZE);
>         if (!buf) {
>                 fprintf(stderr, "error %s during %s\n",
>                         strerror(-err),
>                         "malloc");
>                 return 1;
>         }
>         memset(buf, IO_PATTERN, BUF_SIZE);
>         memset(cmp_buf, IO_PATTERN, BUF_SIZE);
>
>         if (memcmp(buf, cmp_buf, BUF_SIZE)) {
>                 unsigned long long *ubuf = (unsigned long long *)buf;
>                 int i;
>
>                 for (i = 0; i < BUF_SIZE / sizeof(unsigned long long);
> i++)
>                         printf("%d: 0x%llx\n", i, ubuf[i]);
>                         
>                 return 2;
>         }
>
>         return 0;
> }
>
>     memcmp failure occurs while the case is running on 500 machines
> with Xen, each for billion times. 
>     error log has two results, one is 0x0, it shows buf is zero,  the
> other one is 0xabababa...ababa,  it shows cmp_buf isn't 0xabab..ab
>
>     both of error log shows either buf or cmp_buf is all incorrect.
>
>     However, this case pass when we run on native linux kernel(2.6.32)
> without Xen.
>
>     we suspect maybe it's relevent to pvops behavior of dom0.
>
>     we're not sure whether it's a bug fixed in newer version of kernel
> and xen, so we have tried diffrent version of Xen and dom0 including
> Xen4.0.1+kernel2.6.32/3.0/3.11 and Xen4.2 + kernel2.6.32,
> unfortunately, all of these failed.
>
>     we found PAT behaves differenly between linux and xen, so we try
> to add nopat into command line of kernel 3.11, and it also failed.
>
>     now we're blocked, realy need some help.
>
>     any advice will be appreciated
>
>     thanks in advance
>
>
>
> regards,
> wanjia

Picking randomly at some ideas:

Do you have ballooning enabled?

At the time of a failure, is there anything interesting in the Linux or
Xen dmesg?

Are you running a debug version of Linux or Xen?

~Andrew

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