[RFC PATCH 17/25] exchange page: Add exchange_page() syscall.

[Zi Yan <zi.yan@sent.com>]

From: Zi Yan <ziy@nvidia.com>

Users can use the syscall to exchange two lists of pages, similar
to move_pages() syscall.

Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 arch/x86/entry/syscalls/syscall_64.tbl |   1 +
 include/linux/syscalls.h               |   5 +
 mm/exchange.c                          | 346 +++++++++++++++++++++++++++++++++
 3 files changed, 352 insertions(+)

diff --git a/arch/x86/entry/syscalls/syscall_64.tbl b/arch/x86/entry/syscalls/syscall_64.tbl
index 92ee0b4..863a21e 100644
--- a/arch/x86/entry/syscalls/syscall_64.tbl
+++ b/arch/x86/entry/syscalls/syscall_64.tbl
@@ -343,6 +343,7 @@
 332	common	statx			__x64_sys_statx
 333	common	io_pgetevents		__x64_sys_io_pgetevents
 334	common	rseq			__x64_sys_rseq
+335	common	exchange_pages	__x64_sys_exchange_pages
 # don't use numbers 387 through 423, add new calls after the last
 # 'common' entry
 424	common	pidfd_send_signal	__x64_sys_pidfd_send_signal
diff --git a/include/linux/syscalls.h b/include/linux/syscalls.h
index e446806..2c1eb49 100644
--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -1203,6 +1203,11 @@ asmlinkage long sys_mmap_pgoff(unsigned long addr, unsigned long len,
 			unsigned long fd, unsigned long pgoff);
 asmlinkage long sys_old_mmap(struct mmap_arg_struct __user *arg);
 
+asmlinkage long sys_exchange_pages(pid_t pid, unsigned long nr_pages,
+				const void __user * __user *from_pages,
+				const void __user * __user *to_pages,
+				int __user *status,
+				int flags);
 
 /*
  * Not a real system call, but a placeholder for syscalls which are
diff --git a/mm/exchange.c b/mm/exchange.c
index 45c7013..48e344e 100644
--- a/mm/exchange.c
+++ b/mm/exchange.c
@@ -22,6 +22,7 @@
 #include <linux/buffer_head.h>
 #include <linux/fs.h> /* buffer_migrate_page  */
 #include <linux/backing-dev.h>
+#include <linux/sched/mm.h>
 
 
 #include "internal.h"
@@ -1212,3 +1213,348 @@ int exchange_pages_concur(struct list_head *exchange_list,
 
 	return nr_failed?-EFAULT:0;
 }
+
+static int store_status(int __user *status, int start, int value, int nr)
+{
+	while (nr-- > 0) {
+		if (put_user(value, status + start))
+			return -EFAULT;
+		start++;
+	}
+
+	return 0;
+}
+
+static int do_exchange_page_list(struct mm_struct *mm,
+		struct list_head *from_pagelist, struct list_head *to_pagelist,
+		bool migrate_mt, bool migrate_concur)
+{
+	int err;
+	struct exchange_page_info *one_pair;
+	LIST_HEAD(exchange_page_list);
+
+	while (!list_empty(from_pagelist)) {
+		struct page *from_page, *to_page;
+
+		from_page = list_first_entry_or_null(from_pagelist, struct page, lru);
+		to_page = list_first_entry_or_null(to_pagelist, struct page, lru);
+
+		if (!from_page || !to_page)
+			break;
+
+		one_pair = kzalloc(sizeof(struct exchange_page_info), GFP_ATOMIC);
+		if (!one_pair) {
+			err = -ENOMEM;
+			break;
+		}
+
+		list_del(&from_page->lru);
+		list_del(&to_page->lru);
+
+		one_pair->from_page = from_page;
+		one_pair->to_page = to_page;
+
+		list_add_tail(&one_pair->list, &exchange_page_list);
+	}
+
+	if (migrate_concur)
+		err = exchange_pages_concur(&exchange_page_list,
+			MIGRATE_SYNC | (migrate_mt ? MIGRATE_MT : MIGRATE_SINGLETHREAD),
+			MR_SYSCALL);
+	else
+		err = exchange_pages(&exchange_page_list,
+			MIGRATE_SYNC | (migrate_mt ? MIGRATE_MT : MIGRATE_SINGLETHREAD),
+			MR_SYSCALL);
+
+	while (!list_empty(&exchange_page_list)) {
+		struct exchange_page_info *one_pair =
+			list_first_entry(&exchange_page_list,
+							 struct exchange_page_info, list);
+
+		list_del(&one_pair->list);
+		kfree(one_pair);
+	}
+
+	if (!list_empty(from_pagelist))
+		putback_movable_pages(from_pagelist);
+
+	if (!list_empty(to_pagelist))
+		putback_movable_pages(to_pagelist);
+
+	return err;
+}
+
+static int add_page_for_exchange(struct mm_struct *mm,
+		unsigned long from_addr, unsigned long to_addr,
+		struct list_head *from_pagelist, struct list_head *to_pagelist,
+		bool migrate_all)
+{
+	struct vm_area_struct *from_vma, *to_vma;
+	struct page *from_page, *to_page;
+	LIST_HEAD(err_page_list);
+	unsigned int follflags;
+	int err;
+
+	err = -EFAULT;
+	from_vma = find_vma(mm, from_addr);
+	if (!from_vma || from_addr < from_vma->vm_start ||
+		!vma_migratable(from_vma))
+		goto set_from_status;
+
+	/* FOLL_DUMP to ignore special (like zero) pages */
+	follflags = FOLL_GET | FOLL_DUMP;
+	from_page = follow_page(from_vma, from_addr, follflags);
+
+	err = PTR_ERR(from_page);
+	if (IS_ERR(from_page))
+		goto set_from_status;
+
+	err = -ENOENT;
+	if (!from_page)
+		goto set_from_status;
+
+	err = -EACCES;
+	if (page_mapcount(from_page) > 1 && !migrate_all)
+		goto put_and_set_from_page;
+
+	if (PageHuge(from_page)) {
+		if (PageHead(from_page))
+			if (isolate_huge_page(from_page, &err_page_list)) {
+				err = 0;
+			}
+		goto put_and_set_from_page;
+	} else if (PageTransCompound(from_page)) {
+		if (PageTail(from_page)) {
+			err = -EACCES;
+			goto put_and_set_from_page;
+		}
+	}
+
+	err = isolate_lru_page(from_page);
+	if (!err)
+		mod_node_page_state(page_pgdat(from_page), NR_ISOLATED_ANON +
+					page_is_file_cache(from_page), hpage_nr_pages(from_page));
+put_and_set_from_page:
+	/*
+	 * Either remove the duplicate refcount from
+	 * isolate_lru_page() or drop the page ref if it was
+	 * not isolated.
+	 *
+	 * Since FOLL_GET calls get_page(), and isolate_lru_page()
+	 * also calls get_page()
+	 */
+	put_page(from_page);
+set_from_status:
+	if (err)
+		goto out;
+
+	/* to pages  */
+	err = -EFAULT;
+	to_vma = find_vma(mm, to_addr);
+	if (!to_vma ||
+		to_addr < to_vma->vm_start ||
+		!vma_migratable(to_vma))
+		goto set_to_status;
+
+	/* FOLL_DUMP to ignore special (like zero) pages */
+	to_page = follow_page(to_vma, to_addr, follflags);
+
+	err = PTR_ERR(to_page);
+	if (IS_ERR(to_page))
+		goto set_to_status;
+
+	err = -ENOENT;
+	if (!to_page)
+		goto set_to_status;
+
+	err = -EACCES;
+	if (page_mapcount(to_page) > 1 &&
+			!migrate_all)
+		goto put_and_set_to_page;
+
+	if (PageHuge(to_page)) {
+		if (PageHead(to_page))
+			if (isolate_huge_page(to_page, &err_page_list)) {
+				err = 0;
+			}
+		goto put_and_set_to_page;
+	} else if (PageTransCompound(to_page)) {
+		if (PageTail(to_page)) {
+			err = -EACCES;
+			goto put_and_set_to_page;
+		}
+	}
+
+	err = isolate_lru_page(to_page);
+	if (!err)
+		mod_node_page_state(page_pgdat(to_page), NR_ISOLATED_ANON +
+					page_is_file_cache(to_page), hpage_nr_pages(to_page));
+put_and_set_to_page:
+	/*
+	 * Either remove the duplicate refcount from
+	 * isolate_lru_page() or drop the page ref if it was
+	 * not isolated.
+	 *
+	 * Since FOLL_GET calls get_page(), and isolate_lru_page()
+	 * also calls get_page()
+	 */
+	put_page(to_page);
+set_to_status:
+	if (!err) {
+		if ((PageHuge(from_page) != PageHuge(to_page)) ||
+			(PageTransHuge(from_page) != PageTransHuge(to_page))) {
+			list_add(&from_page->lru, &err_page_list);
+			list_add(&to_page->lru, &err_page_list);
+		} else {
+			list_add_tail(&from_page->lru, from_pagelist);
+			list_add_tail(&to_page->lru, to_pagelist);
+		}
+	} else
+		list_add(&from_page->lru, &err_page_list);
+out:
+	if (!list_empty(&err_page_list))
+		putback_movable_pages(&err_page_list);
+	return err;
+}
+/*
+ * Migrate an array of page address onto an array of nodes and fill
+ * the corresponding array of status.
+ */
+static int do_pages_exchange(struct mm_struct *mm, nodemask_t task_nodes,
+			 unsigned long nr_pages,
+			 const void __user * __user *from_pages,
+			 const void __user * __user *to_pages,
+			 int __user *status, int flags)
+{
+	LIST_HEAD(from_pagelist);
+	LIST_HEAD(to_pagelist);
+	int start, i;
+	int err = 0, err1;
+
+	migrate_prep();
+
+	down_read(&mm->mmap_sem);
+	for (i = start = 0; i < nr_pages; i++) {
+		const void __user *from_p, *to_p;
+		unsigned long from_addr, to_addr;
+
+		err = -EFAULT;
+		if (get_user(from_p, from_pages + i))
+			goto out_flush;
+		if (get_user(to_p, to_pages + i))
+			goto out_flush;
+
+		from_addr = (unsigned long)from_p;
+		to_addr = (unsigned long)to_p;
+
+		err = -EACCES;
+		/*
+		 * Errors in the page lookup or isolation are not fatal and we simply
+		 * report them via status
+		 */
+		err = add_page_for_exchange(mm, from_addr, to_addr,
+				&from_pagelist, &to_pagelist,
+				flags & MPOL_MF_MOVE_ALL);
+
+		if (!err)
+			continue;
+
+		err = store_status(status, i, err, 1);
+		if (err)
+			goto out_flush;
+
+		err = do_exchange_page_list(mm, &from_pagelist, &to_pagelist,
+				flags & MPOL_MF_MOVE_MT,
+				flags & MPOL_MF_MOVE_CONCUR);
+		if (err)
+			goto out;
+		if (i > start) {
+			err = store_status(status, start, 0, i - start);
+			if (err)
+				goto out;
+		}
+		start = i;
+	}
+out_flush:
+	/* Make sure we do not overwrite the existing error */
+	err1 = do_exchange_page_list(mm, &from_pagelist, &to_pagelist,
+				flags & MPOL_MF_MOVE_MT,
+				flags & MPOL_MF_MOVE_CONCUR);
+	if (!err1)
+		err1 = store_status(status, start, 0, i - start);
+	if (!err)
+		err = err1;
+out:
+	up_read(&mm->mmap_sem);
+	return err;
+}
+
+SYSCALL_DEFINE6(exchange_pages, pid_t, pid, unsigned long, nr_pages,
+		const void __user * __user *, from_pages,
+		const void __user * __user *, to_pages,
+		int __user *, status, int, flags)
+{
+	const struct cred *cred = current_cred(), *tcred;
+	struct task_struct *task;
+	struct mm_struct *mm;
+	int err;
+	nodemask_t task_nodes;
+
+	/* Check flags */
+	if (flags & ~(MPOL_MF_MOVE|
+				  MPOL_MF_MOVE_ALL|
+				  MPOL_MF_MOVE_MT|
+				  MPOL_MF_MOVE_CONCUR))
+		return -EINVAL;
+
+	if ((flags & MPOL_MF_MOVE_ALL) && !capable(CAP_SYS_NICE))
+		return -EPERM;
+
+	/* Find the mm_struct */
+	rcu_read_lock();
+	task = pid ? find_task_by_vpid(pid) : current;
+	if (!task) {
+		rcu_read_unlock();
+		return -ESRCH;
+	}
+	get_task_struct(task);
+
+	/*
+	 * Check if this process has the right to modify the specified
+	 * process. The right exists if the process has administrative
+	 * capabilities, superuser privileges or the same
+	 * userid as the target process.
+	 */
+	tcred = __task_cred(task);
+	if (!uid_eq(cred->euid, tcred->suid) && !uid_eq(cred->euid, tcred->uid) &&
+	    !uid_eq(cred->uid,  tcred->suid) && !uid_eq(cred->uid,  tcred->uid) &&
+	    !capable(CAP_SYS_NICE)) {
+		rcu_read_unlock();
+		err = -EPERM;
+		goto out;
+	}
+	rcu_read_unlock();
+
+	err = security_task_movememory(task);
+	if (err)
+		goto out;
+
+	task_nodes = cpuset_mems_allowed(task);
+	mm = get_task_mm(task);
+	put_task_struct(task);
+
+	if (!mm)
+		return -EINVAL;
+
+	err = do_pages_exchange(mm, task_nodes, nr_pages, from_pages,
+				    to_pages, status, flags);
+
+	mmput(mm);
+
+	return err;
+
+out:
+	put_task_struct(task);
+
+	return err;
+}
-- 
2.7.4