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@xxxxxxxxxx>
> Signed-off-by: Lianbo Jiang <lijiang@xxxxxxxxxx>
> ---
> 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,
--
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