Re: [patch 0/7] unencrypted atomic DMA pools with dynamic expansion

[David Rientjes]

On Fri, 17 Apr 2020, Christoph Hellwig wrote:

> So modulo a few comments that I can fix up myself this looks good.  Unless
> you want to resend for some reason I'm ready to pick this up once I open
> the dma-mapping tree after -rc2.
> 

Yes, please do, and thanks to both you and Thomas for the guidance and 
code reviews.

Once these patches take on their final form in your branch, how supportive 
would you be of stable backports going back to 4.19 LTS?

There have been several changes to this area over time, so there are 
varying levels of rework that need to be done for each stable kernel back 
to 4.19.  But I'd be happy to do that work if you are receptive to it.

For rationale, without these fixes, all SEV enabled guests warn of 
blocking in rcu read side critical sections when using drivers that 
allocate atomically though the DMA API that calls set_memory_decrypted().  
Users can see warnings such as these in the guest:

BUG: sleeping function called from invalid context at mm/vmalloc.c:1710
in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 3383, name: fio
2 locks held by fio/3383:
 #0: ffff93b6a8568348 (&sb->s_type->i_mutex_key#16){+.+.}, at: ext4_file_write_iter+0xa2/0x5d0
 #1: ffffffffa52a61a0 (rcu_read_lock){....}, at: hctx_lock+0x1a/0xe0
CPU: 0 PID: 3383 Comm: fio Tainted: G        W         5.5.10 #14
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 dump_stack+0x98/0xd5
 ___might_sleep+0x175/0x260
 __might_sleep+0x4a/0x80
 _vm_unmap_aliases+0x45/0x250
 vm_unmap_aliases+0x19/0x20
 __set_memory_enc_dec+0xa4/0x130
 set_memory_decrypted+0x10/0x20
 dma_direct_alloc_pages+0x148/0x150
 dma_direct_alloc+0xe/0x10
 dma_alloc_attrs+0x86/0xc0
 dma_pool_alloc+0x16f/0x2b0
 nvme_queue_rq+0x878/0xc30 [nvme]
 __blk_mq_try_issue_directly+0x135/0x200
 blk_mq_request_issue_directly+0x4f/0x80
 blk_mq_try_issue_list_directly+0x46/0xb0
 blk_mq_sched_insert_requests+0x19b/0x2b0
 blk_mq_flush_plug_list+0x22f/0x3b0
 blk_flush_plug_list+0xd1/0x100
 blk_finish_plug+0x2c/0x40
 iomap_dio_rw+0x427/0x490
 ext4_file_write_iter+0x181/0x5d0
 aio_write+0x109/0x1b0
 io_submit_one+0x7d0/0xfa0
 __x64_sys_io_submit+0xa2/0x280
 do_syscall_64+0x5f/0x250