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    SubjectRe: [RFC PATCH] Add proc interface to set PF_MEMALLOC flags
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    On 10.09.2019 10:35 Damien Le Moal wrote:
    > Mike,
    >
    > On 2019/09/09 19:26, 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.
    > Awesome. That probably will be the perfect solution for the problem we hit with
    > tcmu-runner a while back (please see this thread:
    > https://www.spinics.net/lists/linux-fsdevel/msg148912.html).
    >
    > I think we definitely need nofs as well for dealing with cases where the backend
    > storage for the user daemon is a file.
    >
    > I will give this patch a try as soon as possible (I am traveling currently).
    >
    > Best regards.
    I had issues with this as well, and work on this is appreciated! In my
    case it is a loop block device on a fuse file system.

    Setting PF_LESS_THROTTLE was the one that helped the most, though, so
    add an option for that as well? I set this via prctl() for the thread
    calling it (was easiest to add to).

    Sorry, I have no idea about the current rationale, but wouldn't it be
    better to have a way to mask a set of block devices/file systems not to
    write-back to in a thread. So in my case I'd specify that the fuse
    daemon threads cannot write-back to the file system and loop device
    running on top of the fuse file system, while all other block
    devices/file systems can be write-back to (causing less swapping/OOM
    issues).

    >
    >>> 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;
    >>> + }
    >>> +
    >>> + put_task_struct(task);
    >>> + return rc;
    >>> +}
    >>> +
    >>> +static const struct file_operations proc_memalloc_operations = {
    >>> + .read = memalloc_read,
    >>> + .write = memalloc_write,
    >>> + .llseek = default_llseek,
    >>> +};
    >>> +
    >>> #ifdef CONFIG_AUDIT
    >>> #define TMPBUFLEN 11
    >>> static ssize_t proc_loginuid_read(struct file * file, char __user * buf,
    >>> @@ -3097,6 +3148,7 @@ static const struct pid_entry tgid_base_stuff[] = {
    >>> #ifdef CONFIG_PROC_PID_ARCH_STATUS
    >>> ONE("arch_status", S_IRUGO, proc_pid_arch_status),
    >>> #endif
    >>> + REG("memalloc", S_IRUGO|S_IWUSR, proc_memalloc_operations),
    >>> };
    >>>
    >>> static int proc_tgid_base_readdir(struct file *file, struct dir_context *ctx)
    >>> @@ -3487,6 +3539,7 @@ static const struct pid_entry tid_base_stuff[] = {
    >>> #ifdef CONFIG_PROC_PID_ARCH_STATUS
    >>> ONE("arch_status", S_IRUGO, proc_pid_arch_status),
    >>> #endif
    >>> + REG("memalloc", S_IRUGO|S_IWUSR, proc_memalloc_operations),
    >>> };
    >>>
    >>> static int proc_tid_base_readdir(struct file *file, struct dir_context *ctx)
    >>>
    >>
    >

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