Re: [PATCH v4] page cache: Store only head pages in i_pages

[Chris Wilson]

Quoting Matthew Wilcox (2019-03-07 15:30:51)
> Transparent Huge Pages are currently stored in i_pages as pointers to
> consecutive subpages.  This patch changes that to storing consecutive
> pointers to the head page in preparation for storing huge pages more
> efficiently in i_pages.
> 
> Large parts of this are "inspired" by Kirill's patch
> https://lore.kernel.org/lkml/20170126115819.58875-2-kirill.shutemov@xxxxxxxxxxxxxxx/
> 
> Signed-off-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Acked-by: Jan Kara <jack@xxxxxxx>
> Reviewed-by: Kirill Shutemov <kirill@xxxxxxxxxxxxx>
> Reviewed-and-tested-by: Song Liu <songliubraving@xxxxxx>
> Tested-by: William Kucharski <william.kucharski@xxxxxxxxxx>
> Reviewed-by: William Kucharski <william.kucharski@xxxxxxxxxx>

I've bisected some new softlockups under THP mempressure to this patch.
They are all rcu stalls that look similar to:
[  242.645276] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
[  242.645293] rcu: 	Tasks blocked on level-0 rcu_node (CPUs 0-3): P828
[  242.645301] 	(detected by 1, t=5252 jiffies, g=55501, q=221)
[  242.645307] gem_syslatency  R  running task        0   828    815 0x00004000
[  242.645315] Call Trace:
[  242.645326]  ? __schedule+0x1a0/0x440
[  242.645332]  ? preempt_schedule_irq+0x27/0x50
[  242.645337]  ? apic_timer_interrupt+0xa/0x20
[  242.645342]  ? xas_load+0x3c/0x80
[  242.645347]  ? xas_load+0x8/0x80
[  242.645353]  ? find_get_entry+0x4f/0x130
[  242.645358]  ? pagecache_get_page+0x2b/0x210
[  242.645364]  ? lookup_swap_cache+0x42/0x100
[  242.645371]  ? do_swap_page+0x6f/0x600
[  242.645375]  ? unmap_region+0xc2/0xe0
[  242.645380]  ? __handle_mm_fault+0x7a9/0xfa0
[  242.645385]  ? handle_mm_fault+0xc2/0x1c0
[  242.645393]  ? __do_page_fault+0x198/0x410
[  242.645399]  ? page_fault+0x5/0x20
[  242.645404]  ? page_fault+0x1b/0x20

Any suggestions as to what information you might want?
-Chris