On 09/10/2019 05:12 PM, Tetsuo Handa wrote: > On 2019/09/10 3:26, 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. > > Interesting patch. But can't we instead globally mask __GFP_NOFS / __GFP_NOIO > than playing games with per a thread masking (which suffers from inability to > propagate current thread's mask to other threads indirectly involved)? If I understood you, then that had been discussed in the past: https://www.spinics.net/lists/linux-fsdevel/msg149035.html We only need this for specific threads which implement part of a storage driver in userspace. > >>> +static ssize_t memalloc_write(struct file *file, const char __user *buf, >>> + size_t count, loff_t *ppos) >>> +{ >>> + struct task_struct *task; >>> + char buffer[5]; >>> + int rc = count; >>> + >>> + memset(buffer, 0, sizeof(buffer)); >>> + if (count != sizeof(buffer) - 1) >>> + return -EINVAL; >>> + >>> + if (copy_from_user(buffer, buf, count)) > > copy_from_user() / copy_to_user() might involve memory allocation > via page fault which has to be done under the mask? Moreover, since > just open()ing this file can involve memory allocation, do we forbid > open("/proc/thread-self/memalloc") ? I was having the daemons set the flag when they initialize.
