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Subject[PATCH 7/9] VFS/namei: handle LOOKUP_RCU in page_follow_link_light.
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@suse.de>
---
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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