The patch titled Subject: mm: swap: get rid of deadloop in swapin readahead has been added to the -mm tree. Its filename is mm-swap-get-rid-of-deadloop-in-swapin-readahead.patch

This patch should soon appear at https://ozlabs.org/~akpm/mmots/broken-out/mm-swap-get-rid-of-deadloop-in-swa... and later at https://ozlabs.org/~akpm/mmotm/broken-out/mm-swap-get-rid-of-deadloop-in-swa...

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------------------------------------------------------ From: Guo Ziliang guo.ziliang@zte.com.cn Subject: mm: swap: get rid of deadloop in swapin readahead

In our testing, a deadloop task was found. Through sysrq printing, same stack was found every time, as follows:

__swap_duplicate+0x58/0x1a0 swapcache_prepare+0x24/0x30 __read_swap_cache_async+0xac/0x220 read_swap_cache_async+0x58/0xa0 swapin_readahead+0x24c/0x628 do_swap_page+0x374/0x8a0 __handle_mm_fault+0x598/0xd60 handle_mm_fault+0x114/0x200 do_page_fault+0x148/0x4d0 do_translation_fault+0xb0/0xd4 do_mem_abort+0x50/0xb0

The reason for the deadloop is that swapcache_prepare() always returns EEXIST, indicating that SWAP_HAS_CACHE has not been cleared, so that it cannot jump out of the loop. We suspect that the task that clears the SWAP_HAS_CACHE flag never gets a chance to run. We try to lower the priority of the task stuck in a deadloop so that the task that clears the SWAP_HAS_CACHE flag will run. The results show that the system returns to normal after the priority is lowered.

In our testing, multiple real-time tasks are bound to the same core, and the task in the deadloop is the highest priority task of the core, so the deadloop task cannot be preempted.

Although cond_resched() is used by __read_swap_cache_async, it is an empty function in the preemptive system and cannot achieve the purpose of releasing the CPU. A high-priority task cannot release the CPU unless preempted by a higher-priority task. But when this task is already the highest priority task on this core, other tasks will not be able to be scheduled. So we think we should replace cond_resched() with schedule_timeout_uninterruptible(1), schedule_timeout_interruptible will call set_current_state first to set the task state, so the task will be removed from the running queue, so as to achieve the purpose of giving up the CPU and prevent it from running in kernel mode for too long.

(akpm: ugly hack becomes uglier. But it fixes the issue in a backportable-to-stable fashion while we hopefully work on something better)

Link: https://lkml.kernel.org/r/20220221111749.1928222-1-cgel.zte@gmail.com Signed-off-by: Guo Ziliang guo.ziliang@zte.com.cn Reported-by: Zeal Robot zealci@zte.com.cn Reviewed-by: Ran Xiaokai ran.xiaokai@zte.com.cn Reviewed-by: Jiang Xuexin jiang.xuexin@zte.com.cn Reviewed-by: Yang Yang yang.yang29@zte.com.cn Cc: Naoya Horiguchi naoya.horiguchi@nec.com Cc: Michal Hocko mhocko@kernel.org Cc: Minchan Kim minchan@kernel.org Cc: Johannes Weiner hannes@cmpxchg.org Cc: Roger Quadros rogerq@kernel.org Cc: Ziliang Guo guo.ziliang@zte.com.cn Cc: Hugh Dickins hughd@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/swap_state.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/mm/swap_state.c~mm-swap-get-rid-of-deadloop-in-swapin-readahead +++ a/mm/swap_state.c @@ -478,7 +478,7 @@ struct page *__read_swap_cache_async(swp * __read_swap_cache_async(), which has set SWAP_HAS_CACHE * in swap_map, but not yet added its page to swap cache. */ - cond_resched(); + schedule_timeout_uninterruptible(1); }

/* _

Patches currently in -mm which might be from guo.ziliang@zte.com.cn are

mm-swap-get-rid-of-deadloop-in-swapin-readahead.patch