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: >> https://www.spinics.net/lists/linux-fsdevel/msg148912.html). >> >> 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?
