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

[Haakon Bugge <haakon.bugge@oracle.com>]

Hi Jason,


> On 14 May 2024, at 01:03, Jason Gunthorpe <jgg@ziepe.ca> wrote:
> 
> 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).

When the modules initialize, it does not help to have 2., unless PF_MEMALLOC_NOIO is set in current->flags. That is most probably not set, e.g. considering modprobe. That is why we have these steps in all the five modules. During module initialization, work queues are allocated in all mentioned modules. Therefore, the module initialization functions need the paranthetic use of memalloc_noio_{save,restore}.

> 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?

I hope my comment above explains this.

> 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..

Oracle is about to launch a product using this series, so the techniques used have been thoroughly validated, allthough on an older kernel version.


Thxs, Håkon