Re: [PATCH net v2 0/2] Revert the 'socket_alloc' life cycle change

[SeongJae Park]

CC-ing stable@xxxxxxxxxxxxxxx and adding some more explanations.

On Tue, 5 May 2020 10:10:33 +0200 SeongJae Park <sjpark@xxxxxxxxxx> wrote:

> From: SeongJae Park <sjpark@xxxxxxxxx>
> 
> The commit 6d7855c54e1e ("sockfs: switch to ->free_inode()") made the
> deallocation of 'socket_alloc' to be done asynchronously using RCU, as
> same to 'sock.wq'.  And the following commit 333f7909a857 ("coallocate
> socket_sq with socket itself") made those to have same life cycle.
> 
> The changes made the code much more simple, but also made 'socket_alloc'
> live longer than before.  For the reason, user programs intensively
> repeating allocations and deallocations of sockets could cause memory
> pressure on recent kernels.

I found this problem on a production virtual machine utilizing 4GB memory while
running lebench[1].  The 'poll big' test of lebench opens 1000 sockets, polls
and closes those.  This test is repeated 10,000 times.  Therefore it should
consume only 1000 'socket_alloc' objects at once.  As size of socket_alloc is
about 800 Bytes, it's only 800 KiB.  However, on the recent kernels, it could
consume up to 10,000,000 objects (about 8 GiB).  On the test machine, I
confirmed it consuming about 4GB of the system memory and results in OOM.

[1] https://github.com/LinuxPerfStudy/LEBench

> 
> To avoid the problem, this commit reverts the changes.

I also tried to make fixup rather than reverts, but I couldn't easily find
simple fixup.  As the commits 6d7855c54e1e and 333f7909a857 were for code
refactoring rather than performance optimization, I thought introducing complex
fixup for this problem would make no sense.  Meanwhile, the memory pressure
regression could affect real machines.  To this end, I decided to quickly
revert the commits first and consider better refactoring later.


Thanks,
SeongJae Park

> 
> SeongJae Park (2):
>   Revert "coallocate socket_wq with socket itself"
>   Revert "sockfs: switch to ->free_inode()"
> 
>  drivers/net/tap.c      |  5 +++--
>  drivers/net/tun.c      |  8 +++++---
>  include/linux/if_tap.h |  1 +
>  include/linux/net.h    |  4 ++--
>  include/net/sock.h     |  4 ++--
>  net/core/sock.c        |  2 +-
>  net/socket.c           | 23 ++++++++++++++++-------
>  7 files changed, 30 insertions(+), 17 deletions(-)
> 
> -- 
> 2.17.1