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SubjectRe: [RFC PATCH] Add proc interface to set PF_MEMALLOC flags
Date
+ 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@redhat.com>
>>>> ---
>>>> 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

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