[dm-devel] [PATCH] dm-crypt: limit the number of allocated pages

[Mikulas Patocka]

On Fri, 18 Aug 2017, John Stoffel wrote:

> Mikulas> The limit can't be changed. I expect that no one will run a system with 
> Mikulas> such a tight requirements that the memory is used up to the last 2%.
> 
> Mikulas> If the user is near hitting the memory limit, he can just add swap space 
> Mikulas> to solve the problem instead of tuning dm-crypt.
> 
> My real concern is that we build up too many outstanding BIOs so that
> they take along time to process.  I'm already seeing some reports
> about timeouts after 120 seconds due to stacking of dm-crypt with
> other stuff.
> 
> What I'm trying to say is that as memory grows, you need to slow down
> the growth of the amount of memory that dm-crypt can allocate.  It's
> more important to scale by the speed of the IO sub-system than the
> size of memory.
> 
> But hey, you really addresed my concern with smaller memory systems
> already.
> 
> John

This patch doesn't restrict the number of in-flight bios. Lowering the 
memory limit has no effect on the number of bios being allocated by some 
other part of the kernel and submitted for dm-crypt.

Some times ago I made a patch that limits the number of in-flight bios in 
device mapper - see this:
http://people.redhat.com/~mpatocka/patches/kernel/dm-limit-outstanding-bios/

Mikulas