Re: Memory hotplug softlock issue

[Michal Hocko]

On Mon 19-11-18 13:40:33, Michal Hocko wrote:
> On Mon 19-11-18 18:52:02, Baoquan He wrote:
> [...]
> 
> There are few stacks directly in the offline path but those should be
> OK.
> The real culprit seems to be the swap in code
> 
> > [  +1.734416] CPU: 255 PID: 5558 Comm: stress Tainted: G             L    4.20.0-rc2+ #7
> > [  +0.007927] Hardware name:  9008/IT91SMUB, BIOS BLXSV512 03/22/2018
> > [  +0.006297] Call Trace:
> > [  +0.002537]  dump_stack+0x46/0x60
> > [  +0.003386]  __migration_entry_wait.cold.65+0x5/0x14
> > [  +0.005043]  do_swap_page+0x84e/0x960
> > [  +0.003727]  ? arch_tlb_finish_mmu+0x29/0xc0
> > [  +0.006412]  __handle_mm_fault+0x933/0x1330
> > [  +0.004265]  handle_mm_fault+0xc4/0x250
> > [  +0.003915]  __do_page_fault+0x2b7/0x510
> > [  +0.003990]  do_page_fault+0x2c/0x110
> > [  +0.003729]  ? page_fault+0x8/0x30
> > [  +0.003462]  page_fault+0x1e/0x30
> 
> There are many traces to this path. We are 
> 	/*
> 	 * Once page cache replacement of page migration started, page_count
> 	 * *must* be zero. And, we don't want to call wait_on_page_locked()
> 	 * against a page without get_page().
> 	 * So, we use get_page_unless_zero(), here. Even failed, page fault
> 	 * will occur again.
> 	 */
> 	if (!get_page_unless_zero(page))
> 		goto out;
> 	pte_unmap_unlock(ptep, ptl);
> 	wait_on_page_locked(page);
> 
> taking a reference to the page under the migration. I have to think
> about this much more but I suspec this is just calling for a problem.
> 
> Cc migration experts. For you background information. We are seeing
> memory offline not being able to converge because few heavily used pages
> fail to migrate away - e.g. http://lkml.kernel.org/r/20181116012433.GU2653@MiWiFi-R3L-srv
> A debugging page to dump stack for these pages http://lkml.kernel.org/r/20181116091409.GD14706@xxxxxxxxxxxxxx
> shows that references are taken from the swap in code (above). How are
> we supposed to converge when the swapin code waits for the migration to
> finish with the reference count elevated?

Just to clarify. This is not only about swapin obviously. Any caller of
__migration_entry_wait is affected the same way AFAICS.
-- 
Michal Hocko
SUSE Labs