On Tue 22-10-19 11:13:20, Mike Christie wrote: > On 10/22/2019 06:24 AM, Michal Hocko wrote: > > On Mon 21-10-19 16:41:37, Mike Christie wrote: > >> There are several storage drivers like dm-multipath, iscsi, tcmu-runner, > >> amd 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. > > > > Which code paths are we talking about here? Any ioctl or is this a > > general syscall path? Can we mark the process in a more generic way? > > It depends on the daemon. The common one for example are iscsi and nbd > need network related calls like sendmsg, recvmsg, socket, etc. > tcmu-runner could need the network ones and also read and write when it > does IO to a FS or device. dm-multipath needs the sg io ioctls. OK, so there is not a clear kernel entry point that could be explicitly annotated. This would imply a per task context. This is an important information. And I am wondering how those usecases ever worked in the first place. This is not a minor detail. > > E.g. we have PF_LESS_THROTTLE (used by nfsd). It doesn't affect the > > reclaim recursion but it shows a pattern that doesn't really exhibit > > too many internals. Maybe we need PF_IO_FLUSHER or similar? > > I am not familiar with PF_IO_FLUSHER. If it prevents the recursion > problem then please send me details and I will look into it for the next > posting. PF_IO_FLUSHER doesn't exist. I just wanted to point out that similarly to PF_LESS_THROTTLE it should be a more high level per task flag rather than something as low level as a direct control of gfp allocation context. PF_LESS_THROTTLE simply tells that the task is a part of the reclaim process and therefore it shouldn't be a subject of a normal throttling - whatever that means. PF_IO_FLUSHER would mean that the user context is a part of the IO path and therefore there are certain reclaim recursion restrictions. > >> This patch allows the userspace deamon to set the PF_MEMALLOC* flags > >> with prctl during their initialization so later allocations cannot > >> calling back into them. > > > > TBH I am not really happy to export these to the userspace. They are > > an internal implementation detail and the userspace shouldn't really > > They care in these cases, because block/fs drivers must be able to make > forward progress during writes. To meet this guarantee kernel block > drivers use mempools and memalloc/GFP flags. > > For these userspace components of the block/fs drivers they already do > things normal daemons do not to meet that guarantee like mlock their > memory, disable oom killer, and preallocate resources they have control > over. They have no control over reclaim like the kernel drivers do so > its easy for us to deadlock when memory gets low. OK, fair enough. How much of a control do they really need though. Is a single PF_IO_FLUSHER as explained above (essentially imply GPF_NOIO context) sufficient? -- Michal Hocko SUSE Labs
