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SubjectRe: [PATCH] Revert "mm/vunmap: add cond_resched() in vunmap_pmd_range"
Hi Andrew,

On Fri, Nov 06, 2020 at 05:59:33PM -0800, Andrew Morton wrote:
> On Thu, 5 Nov 2020 09:02:49 -0800 Minchan Kim <minchan@kernel.org> wrote:
>
> > This reverts commit e47110e90584a22e9980510b00d0dfad3a83354e.
> >
> > While I was doing zram testing, I found sometimes decompression failed
> > since the compression buffer was corrupted. With investigation,
> > I found below commit calls cond_resched unconditionally so it could
> > make a problem in atomic context if the task is reschedule.
> >
> > Revert the original commit for now.
> >
> > [ 55.109012] BUG: sleeping function called from invalid context at mm/vmalloc.c:108
> > [ 55.110774] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 946, name: memhog
> > [ 55.111973] 3 locks held by memhog/946:
> > [ 55.112807] #0: ffff9d01d4b193e8 (&mm->mmap_lock#2){++++}-{4:4}, at: __mm_populate+0x103/0x160
> > [ 55.114151] #1: ffffffffa3d53de0 (fs_reclaim){+.+.}-{0:0}, at: __alloc_pages_slowpath.constprop.0+0xa98/0x1160
> > [ 55.115848] #2: ffff9d01d56b8110 (&zspage->lock){.+.+}-{3:3}, at: zs_map_object+0x8e/0x1f0
> > [ 55.118947] CPU: 0 PID: 946 Comm: memhog Not tainted 5.9.3-00011-gc5bfc0287345-dirty #316
> > [ 55.121265] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1 04/01/2014
> > [ 55.122540] Call Trace:
> > [ 55.122974] dump_stack+0x8b/0xb8
> > [ 55.123588] ___might_sleep.cold+0xb6/0xc6
> > [ 55.124328] unmap_kernel_range_noflush+0x2eb/0x350
> > [ 55.125198] unmap_kernel_range+0x14/0x30
> > [ 55.125920] zs_unmap_object+0xd5/0xe0
> > [ 55.126604] zram_bvec_rw.isra.0+0x38c/0x8e0
> > [ 55.127462] zram_rw_page+0x90/0x101
> > [ 55.128199] bdev_write_page+0x92/0xe0
> > [ 55.128957] ? swap_slot_free_notify+0xb0/0xb0
> > [ 55.129841] __swap_writepage+0x94/0x4a0
> > [ 55.130636] ? do_raw_spin_unlock+0x4b/0xa0
> > [ 55.131462] ? _raw_spin_unlock+0x1f/0x30
> > [ 55.132261] ? page_swapcount+0x6c/0x90
> > [ 55.133038] pageout+0xe3/0x3a0
> > [ 55.133702] shrink_page_list+0xb94/0xd60
> > [ 55.134626] shrink_inactive_list+0x158/0x460
> >
> > ...
> >
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -102,8 +102,6 @@ static void vunmap_pmd_range(pud_t *pud, unsigned long addr, unsigned long end,
> > if (pmd_none_or_clear_bad(pmd))
> > continue;
> > vunmap_pte_range(pmd, addr, next, mask);
> > -
> > - cond_resched();
> > } while (pmd++, addr = next, addr != end);
> > }
>
> If this is triggering a warning then why isn't the might_sleep() in
> remove_vm_area() also triggering?

I don't understand what specific callpath you are talking but if
it's clearly called in atomic context, the reason would be config
combination I met.

CONFIG_PREEMPT_VOLUNTARY + no CONFIG_DEBUG_ATOMIC_SLEEP

It makes preempt_count logic void so might_sleep warning will not work.

>
> Sigh. I also cannot remember why these vfree() functions have to be so
> awkward. The mutex_trylock(&vmap_purge_lock) isn't permitted in
> interrupt context because mutex_trylock() is stupid, but what was the
> issue with non-interrupt atomic code?
>

Seems like a latency issue.

commit f9e09977671b
Author: Christoph Hellwig <hch@lst.de>
Date: Mon Dec 12 16:44:23 2016 -0800

mm: turn vmap_purge_lock into a mutex

The purge_lock spinlock causes high latencies with non RT kernel,

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