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SubjectRe: Memory hotplug softlock issue
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@dhcp22.suse.cz
> 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

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