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From: NeilBrown <neilb@suse.de>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH 10/13] VFS/namei: handle LOOKUP_RCU in page_follow_link_light.
Date: Mon, 16 Mar 2015 15:43:20 +1100	[thread overview]
Message-ID: <20150316044320.23648.90827.stgit@notabene.brown> (raw)
In-Reply-To: <20150316043602.23648.52734.stgit@notabene.brown>

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.

This selective calling of kmap requires us to know, in page_put_link,
whether or not kunmap() need to be called.  Pass this information in
the lsb of the cookie.

Signed-off-by: NeilBrown <neilb@suse.de>
---
 fs/namei.c |   35 ++++++++++++++++++++++++++++++-----
 1 file changed, 30 insertions(+), 5 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index c9c58cd1af2a..2602d31ecc99 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -4499,6 +4499,28 @@ 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;
@@ -4514,19 +4536,22 @@ EXPORT_SYMBOL(page_readlink);
 void *page_follow_link_light(struct dentry *dentry, int flags)
 {
 	struct page *page = NULL;
-	if (flags & LOOKUP_RCU)
-		return ERR_PTR(-ECHILD);
-	nd_set_link(page_getlink(dentry, &page));
+	if (flags & LOOKUP_RCU) {
+		nd_set_link(page_getlink_rcu(dentry, &page));
+		page = (void*)((unsigned long)page | 1);
+	} else
+		nd_set_link(page_getlink(dentry, &page));
 	return page;
 }
 EXPORT_SYMBOL(page_follow_link_light);
 
 void page_put_link(struct dentry *dentry, void *cookie)
 {
-	struct page *page = cookie;
+	struct page *page = (void*)((unsigned long)cookie & ~1UL) ;
 
 	if (page) {
-		kunmap(page);
+		if (page == cookie)
+			kunmap(page);
 		page_cache_release(page);
 	}
 }



  parent reply	other threads:[~2015-03-16  4:45 UTC|newest]

Thread overview: 27+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-16  4:43 [PATCH 00/13] Support follow_link in RCU-walk. - V2 NeilBrown
2015-03-16  4:43 ` [PATCH 01/13] VFS: replace {, total_}link_count in task_struct with pointer to nameidata NeilBrown
2015-03-16 19:46   ` Al Viro
2015-03-16  4:43 ` [PATCH 02/13] VFS: make all ->follow_link handlers aware for LOOKUP_RCU NeilBrown
2015-03-16  4:43 ` [PATCH 04/13] security/selinux: check for LOOKUP_RCU in _follow_link NeilBrown
2015-03-16 21:00   ` Al Viro
2015-03-20  4:39     ` NeilBrown
2015-03-20  5:12       ` Al Viro
2015-03-16  4:43 ` [PATCH 03/13] VFS: remove nameidata args from ->follow_link and ->put_link NeilBrown
2015-03-16 20:47   ` Al Viro
2015-03-16  4:43 ` [PATCH 05/13] VFS/namei: use terminate_walk when symlink lookup fails NeilBrown
2015-03-16  4:43 ` [PATCH 12/13] XFS: allow follow_link to often succeed in RCU-walk NeilBrown
2015-03-16 22:37   ` Al Viro
2015-03-16  4:43 ` NeilBrown [this message]
2015-03-16 22:50   ` [PATCH 10/13] VFS/namei: handle LOOKUP_RCU in page_follow_link_light Al Viro
2015-03-19 22:38     ` NeilBrown
2015-03-19 23:46       ` Al Viro
2015-03-16  4:43 ` [PATCH 11/13] xfs: use RCU to free 'struct xfs_mount' NeilBrown
2015-03-16  4:43 ` [PATCH 13/13] NFS: support LOOKUP_RCU in nfs_follow_link NeilBrown
2015-03-16  4:43 ` [PATCH 06/13] VFS/namei: new flag to support RCU symlinks: LOOKUP_LINK_RCU NeilBrown
2015-03-16 22:33   ` Al Viro
2015-03-17  0:59     ` Al Viro
2015-03-16  4:43 ` [PATCH 07/13] VFS/namei: abort RCU-walk on symlink if atime needs updating NeilBrown
2015-03-16  4:43 ` [PATCH 08/13] VFS/namei: enhance follow_link to support RCU-walk NeilBrown
2015-03-16  4:43 ` [PATCH 09/13] VFS/namei: enable RCU-walk when following symlinks NeilBrown
2015-03-16 22:44   ` Al Viro
2015-03-16 19:14 ` [PATCH 00/13] Support follow_link in RCU-walk. - V2 Al Viro

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