On 11.09.2019 18:56 Mike Christie wrote: > 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? No, sounds the same as your case. As an example think of vdfuse (or qemu-nbd locally). You'd have something like ext4(a) <- loop <- fuse file system <- vdfuse <- disk.vdi container file <- ext4(b) <- block device If vdfuse threads cause writeback to ext4(a), you'd get the issue we have. Setting PF_LESS_THROTTLE and/or PF_MEMALLOC_NOIO mostly avoids this problem, but with only PF_LESS_THROTTLE there are still corner cases (I think if ext4(b) slows down suddenly) where it wedges itself and the side effect of setting PF_MEMALLOC_NOIO are being discussed... The best solution would be, I guess, to have a way for vdfuse to set something, such that write-back to ext4(a) isn't allowed from those threads, but write-back to ext4(b) (and all other block devices) is. But I only have a rough idea of how write-back works, so this is really only a guess.
