[CCing Tetsuo again - please make sure you CC everybody who did respond
in earlier versions of the patch]
I am sorry to insist here but this doesn't address the previous review
feedback. Let me try to show you what I would find much better. I do not
insist on this precise wording of course but I do insist on mentioning
the current state and making clear why GFP_NORETRY is really ok.
On Tue 20-09-16 13:50:13, zhongjiang wrote:
> From: zhong jiang <zhongjiang@xxxxxxxxxx>
>
> I hit the following issue when run a OOM case of the LTP and
> ksm enable.
"
I hit the following hung task when running an OOM LTP test case with 4.1
kernel.
"
>
> Call trace:
> [<ffffffc000086a88>] __switch_to+0x74/0x8c
> [<ffffffc000a1bae0>] __schedule+0x23c/0x7bc
> [<ffffffc000a1c09c>] schedule+0x3c/0x94
> [<ffffffc000a1eb84>] rwsem_down_write_failed+0x214/0x350
> [<ffffffc000a1e32c>] down_write+0x64/0x80
> [<ffffffc00021f794>] __ksm_exit+0x90/0x19c
> [<ffffffc0000be650>] mmput+0x118/0x11c
> [<ffffffc0000c3ec4>] do_exit+0x2dc/0xa74
> [<ffffffc0000c46f8>] do_group_exit+0x4c/0xe4
> [<ffffffc0000d0f34>] get_signal+0x444/0x5e0
> [<ffffffc000089fcc>] do_signal+0x1d8/0x450
> [<ffffffc00008a35c>] do_notify_resume+0x70/0x78
>
> it will leads to a hung task because the exiting task cannot get the
> mmap sem for write. but the root cause is that the ksmd holds it for
> read while allocateing memory which just takes ages to complete.
> and ksmd will loop in the following path.
"
The oom victim cannot terminate because it needs to take mmap_sem for
write while the lock is held by ksmd for read which loops in the page
allocator
ksm_do_scan
scan_get_next_rmap_item
down_read
get_next_rmap_item
alloc_rmap_item #ksmd will loop permanently.
There is not way forward because the oom victim cannot release any
memory in 4.1 based kernel. Since 4.6 we have the oom reaper which would
solve this problem because it would release the memory asynchronously.
Nevertheless we can relax alloc_rmap_item requirements and use
__GFP_NORETRY because the allocation failure is acceptable as
ksm_do_scan would just retry later after the lock got dropped.
Such a patch would be also easy to backport to older stable kernels
which do not have oom_reaper.
While we are at it add GFP_NOWARN as the admin doesn't have to be
alarmed by the allocation failure.
>
> CC: <stable@xxxxxxxxxxxxxxx>
> Suggested-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Suggested-by: Michal Hocko <mhocko@xxxxxxx>
> Signed-off-by: zhong jiang <zhongjiang@xxxxxxxxxx>
> ---
> mm/ksm.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index 73d43ba..5048083 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -283,7 +283,8 @@ static inline struct rmap_item *alloc_rmap_item(void)
> {
> struct rmap_item *rmap_item;
>
> - rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL);
> + rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL |
> + __GFP_NORETRY | __GFP_NOWARN);
> if (rmap_item)
> ksm_rmap_items++;
> return rmap_item;
> --
> 1.8.3.1
--
Michal Hocko
SUSE Labs
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