Re: [RFC PATCH] Add proc interface to set PF_MEMALLOC flags

From: Mike Christie
Date: Wed Sep 11 2019 - 12:57:00 EST

On 09/11/2019 03:40 AM, Martin Raiber wrote:
> On 10.09.2019 10:35 Damien Le Moal wrote:
>> Mike,
>> On 2019/09/09 19:26, Mike Christie wrote:
>>> Forgot to cc linux-mm.
>>> On 09/09/2019 11:28 AM, Mike Christie wrote:
>>>> There are several storage drivers like dm-multipath, iscsi, and nbd that
>>>> have userspace components that can run in the IO path. For example,
>>>> iscsi and nbd's userspace deamons may need to recreate a socket and/or
>>>> send IO on it, and dm-multipath's daemon multipathd may need to send IO
>>>> to figure out the state of paths and re-set them up.
>>>> In the kernel these drivers have access to GFP_NOIO/GFP_NOFS and the
>>>> memalloc_*_save/restore functions to control the allocation behavior,
>>>> but for userspace we would end up hitting a allocation that ended up
>>>> writing data back to the same device we are trying to allocate for.
>>>> This patch allows the userspace deamon to set the PF_MEMALLOC* flags
>>>> through procfs. It currently only supports PF_MEMALLOC_NOIO, but
>>>> depending on what other drivers and userspace file systems need, for
>>>> the final version I can add the other flags for that file or do a file
>>>> per flag or just do a memalloc_noio file.
>> Awesome. That probably will be the perfect solution for the problem we hit with
>> tcmu-runner a while back (please see this thread:
>> I think we definitely need nofs as well for dealing with cases where the backend
>> storage for the user daemon is a file.
>> I will give this patch a try as soon as possible (I am traveling currently).
>> Best regards.
> I had issues with this as well, and work on this is appreciated! In my
> case it is a loop block device on a fuse file system.
> Setting PF_LESS_THROTTLE was the one that helped the most, though, so
> add an option for that as well? I set this via prctl() for the thread
> calling it (was easiest to add to).
> Sorry, I have no idea about the current rationale, but wouldn't it be
> better to have a way to mask a set of block devices/file systems not to
> write-back to in a thread. So in my case I'd specify that the fuse
> daemon threads cannot write-back to the file system and loop device
> running on top of the fuse file system, while all other block
> devices/file systems can be write-back to (causing less swapping/OOM
> issues).

I'm not sure I understood you.

The storage daemons I mentioned normally kick off N threads per M
devices. The threads handle duties like IO and error handling for those
devices. Those threads would set the flag, so those IO/error-handler
related operations do not end up writing back to them. So it works
similar to how storage drivers work in the kernel where iscsi_tcp has an
xmit thread and that does memalloc_noreclaim_save. Only the threads for
those specific devices being would set the flag.

In your case, it sounds like you have a thread/threads that would
operate on multiple devices and some need the behavior and some do not.
Is that right?