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

[Damien Le Moal]

+ Miklos

On 2019/09/10 13:41, Kirill A. Shutemov wrote:
> On Tue, Sep 10, 2019 at 12:05:33PM +0000, Damien Le Moal wrote:
>> On 2019/09/10 11:00, Kirill A. Shutemov wrote:
>>> On Mon, Sep 09, 2019 at 11:28:04AM -0500, 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.
>>>>
>>>> Signed-off-by: Mike Christie <mchristi@xxxxxxxxxx>
>>>> ---
>>>>  Documentation/filesystems/proc.txt |  6 ++++
>>>>  fs/proc/base.c                     | 53 ++++++++++++++++++++++++++++++
>>>>  2 files changed, 59 insertions(+)
>>>>
>>>> diff --git a/Documentation/filesystems/proc.txt b/Documentation/filesystems/proc.txt
>>>> index 99ca040e3f90..b5456a61a013 100644
>>>> --- a/Documentation/filesystems/proc.txt
>>>> +++ b/Documentation/filesystems/proc.txt
>>>> @@ -46,6 +46,7 @@ Table of Contents
>>>>    3.10  /proc/<pid>/timerslack_ns - Task timerslack value
>>>>    3.11	/proc/<pid>/patch_state - Livepatch patch operation state
>>>>    3.12	/proc/<pid>/arch_status - Task architecture specific information
>>>> +  3.13  /proc/<pid>/memalloc - Control task's memory reclaim behavior
>>>>  
>>>>    4	Configuring procfs
>>>>    4.1	Mount options
>>>> @@ -1980,6 +1981,11 @@ Example
>>>>   $ cat /proc/6753/arch_status
>>>>   AVX512_elapsed_ms:      8
>>>>  
>>>> +3.13 /proc/<pid>/memalloc - Control task's memory reclaim behavior
>>>> +-----------------------------------------------------------------------
>>>> +A value of "noio" indicates that when a task allocates memory it will not
>>>> +reclaim memory that requires starting phisical IO.
>>>> +
>>>>  Description
>>>>  -----------
>>>>  
>>>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>>>> index ebea9501afb8..c4faa3464602 100644
>>>> --- a/fs/proc/base.c
>>>> +++ b/fs/proc/base.c
>>>> @@ -1223,6 +1223,57 @@ static const struct file_operations proc_oom_score_adj_operations = {
>>>>  	.llseek		= default_llseek,
>>>>  };
>>>>  
>>>> +static ssize_t memalloc_read(struct file *file, char __user *buf, size_t count,
>>>> +			     loff_t *ppos)
>>>> +{
>>>> +	struct task_struct *task;
>>>> +	ssize_t rc = 0;
>>>> +
>>>> +	task = get_proc_task(file_inode(file));
>>>> +	if (!task)
>>>> +		return -ESRCH;
>>>> +
>>>> +	if (task->flags & PF_MEMALLOC_NOIO)
>>>> +		rc = simple_read_from_buffer(buf, count, ppos, "noio", 4);
>>>> +	put_task_struct(task);
>>>> +	return rc;
>>>> +}
>>>> +
>>>> +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))
>>>> +		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;
>>>> +	}
>>>
>>> Really? Without any privilege check? So any random user can tap into
>>> __GFP_NOIO allocations?
>>
>> OK. It probably should have a test on capable(CAP_SYS_ADMIN) or similar. Since
>> these storage daemons are generally run as root anyway, that would still work
>> for most setup I think.
>>
>>>
>>> NAK.
>>>
>>> I don't think that it's great idea in general to expose this low-level
>>> machinery to userspace. But it's better to get comment from people move
>>> familiar with reclaim path.
>>
>> Any setup with stacked file systems and one of the IO path component being a
>> user level process can benefit from this. See the problem described in this
>> patch I pushed for (unsuccessfully as it was a heavy handed solution):
>> https://www.spinics.net/lists/linux-fsdevel/msg148912.html
>>
>> As the discussion in this thread shows, there is no existing simple solution to
>> deal with this reclaim recursion problem. And automatic detection is too hard,
>> if at all possible. With the proper access rights added, this user accessible
>> interface does look very sensible to me.
> 
> Looking into the thread, have you find out if there's anything on FUSE
> side that helps it to avoid deadlocks? Or FUSE just relies on luck with
> this?

I did not see anything relevant. The nofs allocations seem to all be in the
writpage/writepages methods for the client side, to prepare requests to send to
the fuse daemon serving them. I think that that is equivalent to a regular FS
(e.g. XFS) using NOFS allocations during writeback on top of the emulated device
served by a user level daemon (e.g. tcmu-runner in the problem case I reported).
So it does look like a fuse daemon actually serving the request may still
trigger a reclaim into the fuse FS. I wonder if such problem ever was reported
or if there are some clever tricks I am missing.

Miklos,

Could you comment on this ? Is there a mechanism in fuse preventing the
userspace fuse daemon memory triggering a reclaim into the fuse FS being processed ?

Best regards.

-- 
Damien Le Moal
Western Digital Research