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

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

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)?

>> +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") ?

>> +		return -EFAULT;
>> +	buffer[count] = '\0';
>> +
>> +	task = get_proc_task(file_inode(file));
>> +	if (!task)
>> +		return -ESRCH;
>> +
>> +	if (!strcmp(buffer, "noio")) {
>> +		task->flags |= PF_MEMALLOC_NOIO;
>> +	} else {
>> +		rc = -EINVAL;
>> +	}
>> +
>> +	put_task_struct(task);
>> +	return rc;
>> +}