[RFC][PATCH] Cross Memory Attach v2

[Christopher Yeoh]

The basic idea behind cross memory attach is to allow MPI programs doing
intra-node communication to do a single copy of the message rather than
a double copy of the message via shared memory.

The following patch attempts to achieve this by allowing a
destination process, given an address and size from a source process, to
copy memory directly from the source process into its own address space
via a system call. There is also a symmetrical ability to copy from 
the current process's address space into a destination process's
address space.

This is an updated version of the patch I posted a while ago. Changes since
the last version:

- Implementation is a bit more complicated now, but the syscall interface 
  has been modified to provide an iovec one for 
  both the source and destination. This allows for scattered read 
  -> scattered write.

- Rename of syscalls to process_vm_readv/process_vm_writev

- Use of /proc/pid/mem has been considered, but there are issues with 
  using it:
  - Does not allow for specifying iovecs for both src and dest, assuming
    preadv or pwritev was implemented either the area read from or written 
    to would need to be contiguous. 
  - Currently mem_read allows only processes who are currently ptrace'ing
    the target and are still able to ptrace the target to read from the
    target. This check could possibly be moved to the open call, but its not
    clear exactly what race this restriction is stopping (reason appears to
    have been lost)
  - Having to send the fd of /proc/self/mem via SCM_RIGHTS on unix domain
    socket is a bit ugly from a userspace point of view, especially when
    you may have hundreds if not (eventually) thousands of processes that
    all need to do this with each other
  - Doesn't allow for some future use of the interface we would like to consider
    adding in the future (see below)
  - Interestingly reading from /proc/pid/mem currently actually involves two copies! (But
    this could be fixed pretty easily)

- Fixed bug around using mm without correct locking

- call set_page_dirty_lock only on pages we may have written to

- Replaces custom security check with call to __ptrace_may_access

As mentioned previously use of vmsplice instead was considered, 
but has problems. Since you need the reader and writer working co-operatively
 if the pipe is not drained then you block. Which requires some wrapping to do non blocking
on the send side or polling on the receive. In all to all communication
it requires ordering otherwise you can deadlock. And in the example of
many MPI tasks writing to one MPI task vmsplice serialises the
copying.

There are some cases of MPI collectives where even a single copy interface does
not get us the performance gain we could. For example in an MPI_Reduce rather than
copy the data from the source we would like to instead use it directly in a mathops
(say the reduce is doing a sum) as this would save us doing a copy. 
We don't need to keep a copy of the data from the
source. I haven't implemented this, but I think this interface could in the future 
do all this through the use of the flags - eg could specify the math operation and type
and the kernel rather than just copying the data would apply the specified operation
between the source and destination and store it in the destination. 

Chris
-- 
cyeoh@xxxxxxxxxxx

Signed-off-by: Chris Yeoh <cyeoh@xxxxxxxxxxx>
diff --git a/arch/powerpc/include/asm/systbl.h b/arch/powerpc/include/asm/systbl.h
index aa0f1eb..f55dbc7 100644
--- a/arch/powerpc/include/asm/systbl.h
+++ b/arch/powerpc/include/asm/systbl.h
@@ -348,3 +348,5 @@ COMPAT_SYS_SPU(sendmsg)
 COMPAT_SYS_SPU(recvmsg)
 COMPAT_SYS_SPU(recvmmsg)
 SYSCALL_SPU(accept4)
+SYSCALL(process_vm_readv)
+SYSCALL(process_vm_writev)
diff --git a/arch/powerpc/include/asm/unistd.h b/arch/powerpc/include/asm/unistd.h
index 6151937..9ce27ec 100644
--- a/arch/powerpc/include/asm/unistd.h
+++ b/arch/powerpc/include/asm/unistd.h
@@ -367,10 +367,12 @@
 #define __NR_recvmsg		342
 #define __NR_recvmmsg		343
 #define __NR_accept4		344
+#define __NR_process_vm_readv	345
+#define __NR_process_vm_writev	346
 
 #ifdef __KERNEL__
 
-#define __NR_syscalls		345
+#define __NR_syscalls		347
 
 #define __NR__exit __NR_exit
 #define NR_syscalls	__NR_syscalls
diff --git a/arch/powerpc/kernel/sys_ppc32.c b/arch/powerpc/kernel/sys_ppc32.c
index b1b6043..7f6ed34 100644
--- a/arch/powerpc/kernel/sys_ppc32.c
+++ b/arch/powerpc/kernel/sys_ppc32.c
@@ -624,3 +624,4 @@ asmlinkage long compat_sys_fanotify_mark(int fanotify_fd, unsigned int flags,
 	u64 mask = ((u64)mask_hi << 32) | mask_lo;
 	return sys_fanotify_mark(fanotify_fd, flags, mask, dfd, pathname);
 }
+
diff --git a/arch/x86/include/asm/unistd_32.h b/arch/x86/include/asm/unistd_32.h
index b766a5e..1446daa 100644
--- a/arch/x86/include/asm/unistd_32.h
+++ b/arch/x86/include/asm/unistd_32.h
@@ -346,10 +346,12 @@
 #define __NR_fanotify_init	338
 #define __NR_fanotify_mark	339
 #define __NR_prlimit64		340
+#define __NR_process_vm_readv	341
+#define __NR_process_vm_writev	342
 
 #ifdef __KERNEL__
 
-#define NR_syscalls 341
+#define NR_syscalls 343
 
 #define __ARCH_WANT_IPC_PARSE_VERSION
 #define __ARCH_WANT_OLD_READDIR
diff --git a/arch/x86/kernel/syscall_table_32.S b/arch/x86/kernel/syscall_table_32.S
index b35786d..f1ed82c 100644
--- a/arch/x86/kernel/syscall_table_32.S
+++ b/arch/x86/kernel/syscall_table_32.S
@@ -340,3 +340,5 @@ ENTRY(sys_call_table)
 	.long sys_fanotify_init
 	.long sys_fanotify_mark
 	.long sys_prlimit64		/* 340 */
+	.long sys_process_vm_readv
+	.long sys_process_vm_writev
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index e6319d1..a8b4f32 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -831,5 +831,17 @@ asmlinkage long sys_mmap_pgoff(unsigned long addr, unsigned long len,
 			unsigned long prot, unsigned long flags,
 			unsigned long fd, unsigned long pgoff);
 asmlinkage long sys_old_mmap(struct mmap_arg_struct __user *arg);
+asmlinkage long sys_process_vm_readv(pid_t pid,
+				     const struct iovec __user *lvec,
+				     unsigned long liovcnt,
+				     const struct iovec __user *rvec,
+				     unsigned long riovcnt,
+				     unsigned long flags);
+asmlinkage long sys_process_vm_writev(pid_t pid,
+				      const struct iovec __user *lvec,
+				      unsigned long liovcnt,
+				      const struct iovec __user *rvec,
+				      unsigned long riovcnt,
+				      unsigned long flags);
 
 #endif
diff --git a/mm/memory.c b/mm/memory.c
index 98b58fe..956afbd 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -57,6 +57,8 @@
 #include <linux/swapops.h>
 #include <linux/elf.h>
 #include <linux/gfp.h>
+#include <linux/syscalls.h>
+#include <linux/ptrace.h>
 
 #include <asm/io.h>
 #include <asm/pgalloc.h>
@@ -3566,6 +3568,334 @@ void print_vma_addr(char *prefix, unsigned long ip)
 	up_read(&current->mm->mmap_sem);
 }
 
+
+/*
+ * process_vm_rw_pages - read/write pages from task specified
+ * @task: task to read/write from
+ * @process_pages: struct pages area that can store at least
+ *  nr_pages_to_copy struct page pointers
+ * @pa: address of page in task to start copying from/to
+ * @start_offset: offset in page to start copying from/to
+ * @len: number of bytes to copy
+ * @lvec: iovec array specifying where to copy to/from
+ * @lvec_cnt: number of elements in iovec array
+ * @lvec_current: index in iovec array we are up to
+ * @lvec_offset: offset in bytes from current iovec iov_base we are up to
+ * @vm_write: 0 means copy from, 1 means copy to
+ * @nr_pages_to_copy: number of pages to copy
+ */
+
+static int process_vm_rw_pages(struct task_struct *task,
+			       struct page **process_pages,
+			       unsigned long pa,
+			       unsigned long start_offset,
+			       unsigned long len,
+			       struct iovec *lvec,
+			       unsigned long lvec_cnt,
+			       unsigned long *lvec_current,
+			       size_t *lvec_offset,
+			       int vm_write,
+			       unsigned int nr_pages_to_copy)
+{
+	int pages_pinned;
+	void *target_kaddr;
+	int i = 0;
+	int j;
+	int ret;
+	unsigned long bytes_to_copy;
+	unsigned long bytes_copied = 0;
+	int rc = -EFAULT;
+
+	/* Get the pages we're interested in */
+	pages_pinned = get_user_pages(task, task->mm, pa,
+				      nr_pages_to_copy,
+				      vm_write, 0, process_pages, NULL);
+
+	if (pages_pinned != nr_pages_to_copy)
+		goto end;
+
+	/* Do the copy for each page */
+	for (i = 0; (i < nr_pages_to_copy) && (*lvec_current < lvec_cnt); i++) {
+		/* Make sure we have a non zero length iovec */
+		while (*lvec_current < lvec_cnt
+		       && lvec[*lvec_current].iov_len == 0)
+			(*lvec_current)++;
+		if (*lvec_current == lvec_cnt)
+			break;
+
+		/* Will copy smallest of:
+		   - bytes remaining in page
+		   - bytes remaining in destination iovec */
+		bytes_to_copy = min(PAGE_SIZE - start_offset,
+				    len - bytes_copied);
+		bytes_to_copy = min((size_t)bytes_to_copy,
+				    lvec[*lvec_current].iov_len - *lvec_offset);
+
+
+		target_kaddr = kmap(process_pages[i]) + start_offset;
+
+		if (vm_write)
+			ret = copy_from_user(target_kaddr,
+					     lvec[*lvec_current].iov_base
+					     + *lvec_offset,
+					     bytes_to_copy);
+		else
+			ret = copy_to_user(lvec[*lvec_current].iov_base
+					   + *lvec_offset,
+					   target_kaddr, bytes_to_copy);
+		kunmap(process_pages[i]);
+		if (ret) {
+			i++;
+			goto end;
+		}
+		bytes_copied += bytes_to_copy;
+		*lvec_offset += bytes_to_copy;
+		if (*lvec_offset == lvec[*lvec_current].iov_len) {
+			/* Need to copy remaining part of page into
+			   the next iovec if there are any bytes left in page */
+			(*lvec_current)++;
+			*lvec_offset = 0;
+			start_offset = (start_offset + bytes_to_copy)
+				% PAGE_SIZE;
+			if (start_offset)
+				i--;
+		} else {
+			if (start_offset)
+				start_offset = 0;
+		}
+	}
+
+	rc = bytes_copied;
+
+end:
+	for (j = 0; j < pages_pinned; j++) {
+		if (vm_write && j < i)
+			set_page_dirty_lock(process_pages[j]);
+		put_page(process_pages[j]);
+	}
+
+	return rc;
+}
+
+
+
+static int process_vm_rw(pid_t pid, unsigned long addr,
+			 unsigned long len,
+			 struct iovec *lvec,
+			 unsigned long lvec_cnt,
+			 unsigned long *lvec_current,
+			 size_t *lvec_offset,
+			 struct page **process_pages,
+			 struct mm_struct *mm,
+			 struct task_struct *task,
+			 unsigned long flags, int vm_write)
+{
+	unsigned long pa = addr & PAGE_MASK;
+	unsigned long start_offset = addr - pa;
+	int nr_pages;
+	unsigned long bytes_copied = 0;
+	int rc;
+	unsigned int nr_pages_copied = 0;
+	unsigned int nr_pages_to_copy;
+	unsigned int max_pages_per_loop = (PAGE_SIZE * 2)
+		/ sizeof(struct pages *);
+
+
+	/* Work out address and page range required */
+	if (len == 0)
+		return 0;
+	nr_pages = (addr + len - 1) / PAGE_SIZE - addr / PAGE_SIZE + 1;
+
+
+	down_read(&mm->mmap_sem);
+	while ((nr_pages_copied < nr_pages) && (*lvec_current < lvec_cnt)) {
+		nr_pages_to_copy = min(nr_pages - nr_pages_copied,
+				       max_pages_per_loop);
+
+		rc = process_vm_rw_pages(task, process_pages, pa,
+					 start_offset, len,
+					 lvec, lvec_cnt,
+					 lvec_current, lvec_offset,
+					 vm_write, nr_pages_to_copy);
+		start_offset = 0;
+
+		if (rc == -EFAULT)
+			goto free_mem;
+		else {
+			bytes_copied += rc;
+			len -= rc;
+			nr_pages_copied += nr_pages_to_copy;
+			pa += nr_pages_to_copy * PAGE_SIZE;
+		}
+	}
+
+	rc = bytes_copied;
+
+free_mem:
+	up_read(&mm->mmap_sem);
+
+	return rc;
+}
+
+static int process_vm_rw_v(pid_t pid, const struct iovec __user *lvec,
+			   unsigned long liovcnt,
+			   const struct iovec __user *rvec,
+			   unsigned long riovcnt,
+			   unsigned long flags, int vm_write)
+{
+	struct task_struct *task;
+	struct page **process_pages = NULL;
+	struct mm_struct *mm;
+	int i;
+	int rc;
+	int bytes_copied;
+	struct iovec iovstack_l[UIO_FASTIOV];
+	struct iovec iovstack_r[UIO_FASTIOV];
+	struct iovec *iov_l = iovstack_l;
+	struct iovec *iov_r = iovstack_r;
+	unsigned int nr_pages = 0;
+	unsigned int nr_pages_iov;
+	unsigned long iov_l_curr_idx = 0;
+	size_t iov_l_curr_offset = 0;
+	int iov_len_total = 0;
+
+	/* Get process information */
+	rcu_read_lock();
+	task = find_task_by_vpid(pid);
+	if (task)
+		get_task_struct(task);
+	rcu_read_unlock();
+	if (!task)
+		return -ESRCH;
+
+	task_lock(task);
+	if (__ptrace_may_access(task, PTRACE_MODE_ATTACH)) {
+		task_unlock(task);
+		rc = -EPERM;
+		goto end;
+	}
+	mm = task->mm;
+
+	if (!mm) {
+		rc = -EINVAL;
+		goto end;
+	}
+
+	atomic_inc(&mm->mm_users);
+	task_unlock(task);
+
+
+	if ((liovcnt > UIO_MAXIOV) || (riovcnt > UIO_MAXIOV)) {
+		rc = -EINVAL;
+		goto release_mm;
+	}
+
+	if (liovcnt > UIO_FASTIOV)
+		iov_l = kmalloc(liovcnt*sizeof(struct iovec), GFP_KERNEL);
+
+	if (riovcnt > UIO_FASTIOV)
+		iov_r = kmalloc(riovcnt*sizeof(struct iovec), GFP_KERNEL);
+
+	if (iov_l == NULL || iov_r == NULL) {
+		rc = -ENOMEM;
+		goto free_iovecs;
+	}
+
+	rc = copy_from_user(iov_l, lvec, liovcnt*sizeof(*lvec));
+	if (rc) {
+		rc = -EFAULT;
+		goto free_iovecs;
+	}
+	rc = copy_from_user(iov_r, rvec, riovcnt*sizeof(*lvec));
+	if (rc) {
+		rc = -EFAULT;
+		goto free_iovecs;
+	}
+
+	/* Work out how many pages of struct pages we're going to need
+	   when eventually calling get_user_pages */
+	for (i = 0; i < riovcnt; i++) {
+		if (iov_r[i].iov_len > 0) {
+			nr_pages_iov = ((unsigned long)iov_r[i].iov_base
+					+ iov_r[i].iov_len) /
+				PAGE_SIZE - (unsigned long)iov_r[i].iov_base
+				/ PAGE_SIZE + 1;
+			nr_pages = max(nr_pages, nr_pages_iov);
+			iov_len_total += iov_r[i].iov_len;
+			if (iov_len_total < 0) {
+				rc = -EINVAL;
+				goto free_iovecs;
+			}
+		}
+	}
+
+	if (nr_pages == 0)
+		goto free_iovecs;
+
+	/* For reliability don't try to kmalloc more than 2 pages worth */
+	process_pages = kmalloc(min((size_t)PAGE_SIZE * 2,
+				    sizeof(struct pages *) * nr_pages),
+				GFP_KERNEL);
+
+	if (!process_pages) {
+		rc = -ENOMEM;
+		goto free_iovecs;
+	}
+
+	rc = 0;
+	for (i = 0; i < riovcnt && iov_l_curr_idx < liovcnt; i++) {
+		bytes_copied = process_vm_rw(pid,
+					     (unsigned long)iov_r[i].iov_base,
+					     iov_r[i].iov_len,
+					     iov_l, liovcnt,
+					     &iov_l_curr_idx,
+					     &iov_l_curr_offset,
+					     process_pages, mm,
+					     task, flags, vm_write);
+		if (bytes_copied < 0) {
+			rc = bytes_copied;
+			goto free_proc_pages;
+		} else {
+			rc += bytes_copied;
+		}
+	}
+
+
+free_proc_pages:
+	kfree(process_pages);
+
+free_iovecs:
+	if (riovcnt > UIO_FASTIOV)
+		kfree(iov_r);
+	if (liovcnt > UIO_FASTIOV)
+		kfree(iov_l);
+
+release_mm:
+	mmput(mm);
+
+end:
+	put_task_struct(task);
+	return rc;
+}
+
+
+SYSCALL_DEFINE6(process_vm_readv, pid_t, pid, const struct iovec __user *, lvec,
+		unsigned long, liovcnt, const struct iovec __user *, rvec,
+		unsigned long, riovcnt,	unsigned long, flags)
+{
+	return process_vm_rw_v(pid, lvec, liovcnt, rvec, riovcnt, flags, 0);
+}
+
+SYSCALL_DEFINE6(process_vm_writev, pid_t, pid,
+		const struct iovec __user *, lvec,
+		unsigned long, liovcnt, const struct iovec __user *, rvec,
+		unsigned long, riovcnt,	unsigned long, flags)
+{
+	return process_vm_rw_v(pid, lvec, liovcnt, rvec, riovcnt, flags, 1);
+}
+
+
+
 #ifdef CONFIG_PROVE_LOCKING
 void might_fault(void)
 {
--
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