On Wed, 27 Jul 2016, NeilBrown wrote:

On Tue, Jul 26 2016, Mikulas Patocka wrote:

quoted

On Sat, 23 Jul 2016, NeilBrown wrote:

quoted

"dirtying ... from the reclaim context" ??? What does that mean?
According to
  Commit: 26eecbf3543b ("[PATCH] vm: pageout throttling")
From the history tree, the purpose of throttle_vm_writeout() is to
limit the amount of memory that is concurrently under I/O.
That seems strange to me because I thought it was the responsibility of
each backing device to impose a limit - a maximum queue size of some
sort.

Device mapper doesn't impose any limit for in-flight bios.

I would suggest that it probably should. At least it should
"set_wb_congested()" when the number of in-flight bios reaches some
arbitrary threshold.

If we set the device mapper device as congested, it can again trigger that 
mempool alloc throttling bug.

I.e. suppose that we swap to a dm-crypt device. The dm-crypt device 
becomes clogged and sets its state as congested. The underlying block 
device is not congested.

The mempool_alloc function in the dm-crypt workqueue sets the 
PF_LESS_THROTTLE flag, and tries to allocate memory, but according to 
Michal's patches, processes with PF_LESS_THROTTLE may still get throttled.

So if we set the dm-crypt device as congested, it can incorrectly throttle 
the dm-crypt workqueue that does allocations of temporary pages and 
encryption.

I think that approach with PF_LESS_THROTTLE in mempool_alloc is incorrect 
and that mempool allocations should never be throttled.

quoted

I've made some patches that limit in-flight bios for device mapper in
the past, but there were not integrated into upstream.

I second the motion to resurrect these.

I uploaded those patches here:

http://people.redhat.com/~mpatocka/patches/kernel/dm-limit-outstanding-bios/

Mikulas

Thanks,
NeilBrown

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