Re: [PATCH] mm,ksm: fix endless looping in allocating memory when ksm enable

[Andrew Morton <akpm@linux-foundation.org>]

On Sun, 18 Sep 2016 10:26:10 +0800 zhongjiang <zhongjiang@huawei.com> wrote:

> I hit the following issue when run a OOM case of the LTP and
> ksm enable.
> 
> 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.
> 
>  scan_get_next_rmap_item
>           down_read
>                 get_next_rmap_item
>                         alloc_rmap_item   #ksmd will loop permanently.
> 
> we fix it by changing the GFP to allow the allocation sometimes fail, and
> we're not at all interested in hearing abot that.

It would be better if the changelog were to describe *why* this is
harmless.  I assume that if the allocation fails,
scan_get_next_rmap_item() will bale out and ksmd just gives up and
takes a sleep?

Also, did you instead consider changing scan_get_next_rmap_item() to
simply not hold mmap_sem for so long?  Scan a megabyte or so then drop
mmap_sem for a while, then scan some more?  The whole thing is driven by
ksm.scan_address so handling the races should be simple.


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