[PATCH 7/9] VFS/namei: handle LOOKUP_RCU in page_follow_link_light.

[NeilBrown]

If the symlink has already be been read-in, then
page_follow_link_light can succeed in RCU-walk mode.
page_getlink_rcu() is added to support this.

With this many filesystems can follow links in RCU-walk
mode when everything is cached.  This  includes ext?fs and
others.

If the page is a HighMem page we do *not* try to kmap_atomic,
but simply give up - only page_address() is used.
This is because we need to be able to sleep while holding
the address of the page, particularly over calls to do_last()
which can be quite slow and in particular takes a mutex.

If this were a problem, then copying into a GFP_ATOMIC allocation
might be a workable solution.

Signed-off-by: NeilBrown <neilb@xxxxxxx>
---
 fs/namei.c |   30 +++++++++++++++++++++++++++---
 1 file changed, 27 insertions(+), 3 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 11e6b2068c96..48571b2eaa18 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -4468,6 +4468,28 @@ static char *page_getlink(struct dentry * dentry, struct page **ppage)
 	return kaddr;
 }
 
+/* get the link contents from pagecache under RCU */
+static char *page_getlink_rcu(struct dentry * dentry, struct page **ppage)
+{
+	char *kaddr;
+	struct page *page;
+	struct address_space *mapping = dentry->d_inode->i_mapping;
+	page = find_get_page(mapping, 0);
+	if (page &&
+	    (!PageUptodate(page) || PageHighMem(page))) {
+		put_page(page);
+		page = NULL;
+	}
+	if (!page) {
+		*ppage = ERR_PTR(-ECHILD);
+		return NULL;
+	}
+	*ppage = page;
+	kaddr = page_address(page);
+	nd_terminate_link(kaddr, dentry->d_inode->i_size, PAGE_SIZE - 1);
+	return kaddr;
+}
+
 int page_readlink(struct dentry *dentry, char __user *buffer, int buflen)
 {
 	struct page *page = NULL;
@@ -4484,8 +4506,9 @@ void *page_follow_link_light(struct dentry *dentry, struct nameidata *nd)
 {
 	struct page *page = NULL;
 	if (nd->flags & LOOKUP_RCU)
-		return ERR_PTR(-ECHILD);
-	nd_set_link(nd, page_getlink(dentry, &page));
+		nd_set_link(nd, page_getlink_rcu(dentry, &page));
+	else
+		nd_set_link(nd, page_getlink(dentry, &page));
 	return page;
 }
 EXPORT_SYMBOL(page_follow_link_light);
@@ -4495,7 +4518,8 @@ void page_put_link(struct dentry *dentry, struct nameidata *nd, void *cookie)
 	struct page *page = cookie;
 
 	if (page) {
-		kunmap(page);
+		if (!(nd->flags & LOOKUP_LINK_RCU))
+			kunmap(page);
 		page_cache_release(page);
 	}
 }


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