Re: [PATCH] mm: fix oom work when memory is under pressure

[Michal Hocko <mhocko@suse.cz>]

[Let's CC Hugh]

On Wed 14-09-16 15:13:50, zhong jiang wrote:
[...]
>   hi, Michal
> 
>   Recently, I hit the same issue when run a OOM case of the LTP and ksm enable.
>  
> [  601.937145] Call trace:
> [  601.939600] [<ffffffc000086a88>] __switch_to+0x74/0x8c
> [  601.944760] [<ffffffc000a1bae0>] __schedule+0x23c/0x7bc
> [  601.950007] [<ffffffc000a1c09c>] schedule+0x3c/0x94
> [  601.954907] [<ffffffc000a1eb84>] rwsem_down_write_failed+0x214/0x350
> [  601.961289] [<ffffffc000a1e32c>] down_write+0x64/0x80
> [  601.966363] [<ffffffc00021f794>] __ksm_exit+0x90/0x19c
> [  601.971523] [<ffffffc0000be650>] mmput+0x118/0x11c
> [  601.976335] [<ffffffc0000c3ec4>] do_exit+0x2dc/0xa74
> [  601.981321] [<ffffffc0000c46f8>] do_group_exit+0x4c/0xe4
> [  601.986656] [<ffffffc0000d0f34>] get_signal+0x444/0x5e0
> [  601.991904] [<ffffffc000089fcc>] do_signal+0x1d8/0x450
> [  601.997065] [<ffffffc00008a35c>] do_notify_resume+0x70/0x78

So this is a hung task triggering because the exiting task cannot get
the mmap sem for write because the ksmd holds it for read while
allocating memory which just takes ages to complete, right?

> 
> The root case is that ksmd hold the read lock. and the lock is not released.
>  scan_get_next_rmap_item
>          down_read
>                    get_next_rmap_item
>                              alloc_rmap_item     #ksmd will loop permanently.
> 
> How do you see this kind of situation ? or  let the issue alone.

I am not familiar with the ksmd code so it is hard for me to judge but
one thing to do would be __GFP_NORETRY which would force a bail out from
the allocation rather than looping for ever. A quick look tells me that
the allocation failure here is quite easy to handle. There might be
others...

-- 
Michal Hocko
SUSE Labs

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