Re: [PATCH] Revert "mm/vunmap: add cond_resched() in vunmap_pmd_range"

[Minchan Kim]

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@xxxxxxxxxx> 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@xxxxxx>
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,