Re: [PATCH 0/6] rds: rdma: Add ability to force GFP_NOIO

[Jason Gunthorpe <jgg@ziepe.ca>]

On Mon, May 13, 2024 at 02:53:40PM +0200, Håkon Bugge wrote:
> This series enables RDS and the RDMA stack to be used as a block I/O
> device. This to support a filesystem on top of a raw block device
> which uses RDS and the RDMA stack as the network transport layer.
> 
> Under intense memory pressure, we get memory reclaims. Assume the
> filesystem reclaims memory, goes to the raw block device, which calls
> into RDS, which calls the RDMA stack. Now, if regular GFP_KERNEL
> allocations in RDS or the RDMA stack require reclaims to be fulfilled,
> we end up in a circular dependency.
> 
> We break this circular dependency by:
> 
> 1. Force all allocations in RDS and the relevant RDMA stack to use
>    GFP_NOIO, by means of a parenthetic use of
>    memalloc_noio_{save,restore} on all relevant entry points.

I didn't see an obvious explanation why each of these changes was
necessary. I expected this:
 
> 2. Make sure work-queues inherits current->flags
>    wrt. PF_MEMALLOC_{NOIO,NOFS}, such that work executed on the
>    work-queue inherits the same flag(s).

To broadly capture everything and understood this was the general plan
from the MM side instead of direct annotation?

So, can you explain in each case why it needs an explicit change?

And further, is there any validation of this? There is some lockdep
tracking of reclaim, I feel like it should be more robustly hooked up
in RDMA if we expect this to really work..

Jason