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Subject[RFC PATCH] Add proc interface to set PF_MEMALLOC flags
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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;
+ }
+
+ 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)
--
2.21.0
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