[OpenWrt-Devel] uci commit bumping amount of dentry active slab objects

[Felix Fietkau]

On 2016-01-11 12:57, Rafał Miłecki wrote:
> I was looking at slabinfo and noticed something about uci. Executing
> "uci commit" increases amount of "dentry" active objects.
> 
> # echo 2 > /proc/sys/vm/drop_caches
> 
> # egrep "dentry|kmalloc-64" /proc/slabinfo
> dentry               559   2040    136   30    1 : tunables  120   60    8 : sla
> kmalloc-64          5357   9324     64   63    1 : tunables  120   60    8 : sla
> 
> # for N in `seq 1 1 1000`; do uci commit network; done
> 
> # egrep "dentry|kmalloc-64" /proc/slabinfo
> dentry              3540   3540    136   30    1 : tunables  120   60    8 : sla
> kmalloc-64          6457   9324     64   63    1 : tunables  120   60    8 : sla
> 
> # for N in `seq 1 1 1000`; do uci commit network; done
> 
> # egrep "dentry|kmalloc-64" /proc/slabinfo
> dentry              6570   6570    136   30    1 : tunables  120   60    8 : sla
> kmalloc-64          7587   9324     64   63    1 : tunables  120   60    8 : sla
> 
> This is not critical. As you know Linux kernel loves caching. When
> amount of free RAM goes below ~2 MiB I can see kernel freeing dentry
> objects and it doesn't OOM.
> 
> But I'm still wondering if everything is OK. Is this normal kernel
> keeps increasing pool of dentry objects as the result of "uci commit"
> command?
> 
> I expected "uci command" first to bump amount of active and total
> dentry objects. Then when uci exits I expected amount of active
> objects to go down. Kernel could reuse inactive objects during next
> "uci commit" call. In such scenario amount of active objects would
> vary, but amount of total object would be more-or-less stable.
> 
> Can we be missing some (f)close in UCI? I checked uci_file_commit and
> it looks alright. Or is this just how the filesystem works? I'm using
> 3.18.21 (OpenWrt 15.05 branch) on device with serial flash (so JFFS2).
User space programs are not required to close all their file descriptor,
such a close happens implicitly on exit.
I did an strace on uci commit without any delta, and I believe the
likely cause is cached negative dentries for temporary files.
On commit it checks a temp file before bailing out due to the lack of a
config delta.

- Felix
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