On 2023/02/14 23:37, Lianbo Jiang wrote:
> The "kmem -i" option may output the bogus statistics of CACHED, which
> might be observed when some extreme situations occur in kernel, such as
> OOM, disk IO errors, etc.
>
> The following result of calculation may be a negative value, refer to
> the dump_kmeminfo():
>    page_cache_size = nr_file_pages - swapper_space_nrpages - buffer_pages;
>
> As a result, the negative value will be converted to unsigned long
> integer, eventually it overflows and gets a big integer.
>
>    crash> kmem -i
>                     PAGES        TOTAL      PERCENTAGE
>        TOTAL MEM  255314511     973.9 GB         ----
>             FREE   533574         2 GB    0% of TOTAL MEM
>             USED  254780937     971.9 GB   99% of TOTAL MEM
>           SHARED     1713       6.7 MB    0% of TOTAL MEM
>          BUFFERS      374       1.5 MB    0% of TOTAL MEM
>           CACHED     -114  70368744177664 GB  72251060080% of TOTAL MEM
>                      ^^^^  ^^^^^^^^^^^^^^     ^^^^^^^^^^^^

I think that this is not very bad output :) because we can notice
the negative value.  Maybe it's due to future kernel changes.

So if you set it to zero, what about emitting an info message?

+       if (page_cache_size < 0) {
+               error(INFO, "page_cache_size went negative (%ld), setting to 0\n",
+                       page_cache_size);
+               page_cache_size = 0;
+       }

For example,

crash> kmem -i
                  PAGES        TOTAL      PERCENTAGE
     TOTAL MEM  16252500        62 GB         ----
          FREE   246162     961.6 MB    1% of TOTAL MEM
          USED  16006338      61.1 GB   98% of TOTAL MEM
        SHARED   329258       1.3 GB    2% of TOTAL MEM
       BUFFERS     1436       5.6 MB    0% of TOTAL MEM
kmem: page_cache_size went negative (-114), setting to 0
        CACHED        0            0    0% of TOTAL MEM
          SLAB   311514       1.2 GB    1% of TOTAL MEM

Thanks,
Kazu

>           ...
>
> Let's normalize it to zero to fix such corner cases.
>
> Reported-by: Buland Kumar Singh <bsingh@redhat.com>
> Signed-off-by: Lianbo Jiang <lijiang@redhat.com>
> ---
>   memory.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/memory.c b/memory.c
> index e0742c1bd3a4..860a3a978a4d 100644
> --- a/memory.c
> +++ b/memory.c
> @@ -8615,6 +8615,8 @@ dump_kmeminfo(void)
>               page_cache_size = 0;
>   
>   
> +     if (page_cache_size < 0)
> +             page_cache_size = 0;
>           pct = (page_cache_size * 100)/totalram_pages;
>           fprintf(fp, "%13s  %7ld  %11s  %3ld%% of TOTAL MEM\n",
>               "CACHED", page_cache_size,