Re: [PATCH v2 3/3] 9p: Enable symlink caching in page cache

[Remi Pommarel <repk@triplefau.lt>]

On Thu, Feb 12, 2026 at 04:35:24PM +0100, Christian Schoenebeck wrote:
> On Wednesday, 21 January 2026 20:56:10 CET Remi Pommarel wrote:
> > Currently, when cache=loose is enabled, file reads are cached in the
> > page cache, but symlink reads are not. This patch allows the results
> > of p9_client_readlink() to be stored in the page cache, eliminating
> > the need for repeated 9P transactions on subsequent symlink accesses.
> > 
> > This change improves performance for workloads that involve frequent
> > symlink resolution.
> > 
> > Signed-off-by: Remi Pommarel <repk@triplefau.lt>
> > ---
> >  fs/9p/vfs_addr.c       | 24 ++++++++++++--
> >  fs/9p/vfs_inode.c      |  6 ++--
> >  fs/9p/vfs_inode_dotl.c | 73 +++++++++++++++++++++++++++++++++++++-----
> >  3 files changed, 90 insertions(+), 13 deletions(-)
> > 
> > diff --git a/fs/9p/vfs_addr.c b/fs/9p/vfs_addr.c
> > index 862164181bac..ee672abbb02c 100644
> > --- a/fs/9p/vfs_addr.c
> > +++ b/fs/9p/vfs_addr.c
> > @@ -70,10 +70,19 @@ static void v9fs_issue_read(struct netfs_io_subrequest
> > *subreq) {
> >  	struct netfs_io_request *rreq = subreq->rreq;
> >  	struct p9_fid *fid = rreq->netfs_priv;
> > +	char *target;
> >  	unsigned long long pos = subreq->start + subreq->transferred;
> > -	int total, err;
> > -
> > -	total = p9_client_read(fid, pos, &subreq->io_iter, &err);
> > +	int total, err, len, n;
> > +
> > +	if (S_ISLNK(rreq->inode->i_mode)) {
> > +		err = p9_client_readlink(fid, &target);
> 
> Treadlink request requires 9p2000.L. So this would break with legacy protocol
> versions 9p2000 and 9p2000.u I guess:
> 
> https://wiki.qemu.org/Documentation/9p#9p_Protocol

I'm having trouble seeing how v9fs_issue_read() could be called for
S_ISLNK inodes under 9p2000 or 9p2000.u.

As I understand it, v9fs_issue_read() is only invoked through the page
cache operations via the inode’s a_ops. This seems to me that only
regular files and (now that I added a page_get_link() in
v9fs_symlink_inode_operations_dotl) symlinks using 9p2000.L can call
v9fs_issue_read(). But not symlinks using 9p2000 or 9p2000.u as I
haven't modified v9fs_symlink_inode_operations. But I may have missed
something here ?

Maybe what is misleading is that this performance optimization is only
applicable for 9p2000.L which I should possibly mention in the commit
message.

> 
> > +		len = strnlen(target, PAGE_SIZE - 1);
> 
> Usually we are bound to PATH_MAX.
> 
> Target link path is coming from 9p server, which may run another OS and
> therefore target might be longer than PATH_MAX, in which case it would always
> yield in -ENAMETOOLONG when client requests that link target from cache.
> 
> But OTOH there is no real alternative for storing a link target in cache that
> can never be delivered to user. So I guess it is OK as-is.
> 
> Simply using PATH_MAX would make it worse, as it would potentially silently
> shorten the link from host.
> 
> > +		n = copy_to_iter(target, len, &subreq->io_iter);
> > +		if (n != len)
> > +			err = -EFAULT;
> > +		total = i_size_read(rreq->inode);
> > +	} else
> > +		total = p9_client_read(fid, pos, &subreq->io_iter, &err);
> > 
> >  	/* if we just extended the file size, any portion not in
> >  	 * cache won't be on server and is zeroes */
> > @@ -99,6 +108,7 @@ static void v9fs_issue_read(struct netfs_io_subrequest
> > *subreq) static int v9fs_init_request(struct netfs_io_request *rreq, struct
> > file *file) {
> >  	struct p9_fid *fid;
> > +	struct dentry *dentry;
> >  	bool writing = (rreq->origin == NETFS_READ_FOR_WRITE ||
> >  			rreq->origin == NETFS_WRITETHROUGH ||
> >  			rreq->origin == NETFS_UNBUFFERED_WRITE ||
> > @@ -115,6 +125,14 @@ static int v9fs_init_request(struct netfs_io_request
> > *rreq, struct file *file) if (!fid)
> >  			goto no_fid;
> >  		p9_fid_get(fid);
> > +	} else if (S_ISLNK(rreq->inode->i_mode)) {
> > +		dentry = d_find_alias(rreq->inode);
> > +		if (!dentry)
> > +			goto no_fid;
> > +		fid = v9fs_fid_lookup(dentry);
> > +		dput(dentry);
> > +		if (IS_ERR(fid))
> > +			goto no_fid;
> >  	} else {
> >  		fid = v9fs_fid_find_inode(rreq->inode, writing, INVALID_UID, true);
> >  		if (!fid)
> > diff --git a/fs/9p/vfs_inode.c b/fs/9p/vfs_inode.c
> > index a82a71be309b..e1b762f3e081 100644
> > --- a/fs/9p/vfs_inode.c
> > +++ b/fs/9p/vfs_inode.c
> > @@ -302,10 +302,12 @@ int v9fs_init_inode(struct v9fs_session_info *v9ses,
> >  			goto error;
> >  		}
> > 
> > -		if (v9fs_proto_dotl(v9ses))
> > +		if (v9fs_proto_dotl(v9ses)) {
> >  			inode->i_op = &v9fs_symlink_inode_operations_dotl;
> > -		else
> > +			inode_nohighmem(inode);
> 
> What is that for?

According to filesystems/porting.rst and commit 21fc61c73c39 ("don't put
symlink bodies in pagecache into highmem") all symlinks that need to use
page_follow_link_light() (which is now more or less page_get_link())
should not add highmem pages in pagecache or deadlocks could happen. The
inode_nohighmem() prevents that.

Also from __page_get_link()
  BUG_ON(mapping_gfp_mask(mapping) & __GFP_HIGHMEM);

A BUG_ON() is supposed to punish us if inode_nohighmem() is not used
here.

Of course this does not have any effect on 64bits platforms.

> 
> > +		} else {
> >  			inode->i_op = &v9fs_symlink_inode_operations;
> > +		}
> > 
> >  		break;
> >  	case S_IFDIR:
> > diff --git a/fs/9p/vfs_inode_dotl.c b/fs/9p/vfs_inode_dotl.c
> > index 6312b3590f74..486b11dbada3 100644
> > --- a/fs/9p/vfs_inode_dotl.c
> > +++ b/fs/9p/vfs_inode_dotl.c
> > @@ -686,9 +686,13 @@ v9fs_vfs_symlink_dotl(struct mnt_idmap *idmap, struct
> > inode *dir, int err;
> >  	kgid_t gid;
> >  	const unsigned char *name;
> > +	umode_t mode;
> > +	struct v9fs_session_info *v9ses;
> >  	struct p9_qid qid;
> >  	struct p9_fid *dfid;
> >  	struct p9_fid *fid = NULL;
> > +	struct inode *inode;
> > +	struct posix_acl *dacl = NULL, *pacl = NULL;
> > 
> >  	name = dentry->d_name.name;
> >  	p9_debug(P9_DEBUG_VFS, "%lu,%s,%s\n", dir->i_ino, name, symname);
> > @@ -702,6 +706,15 @@ v9fs_vfs_symlink_dotl(struct mnt_idmap *idmap, struct
> > inode *dir,
> > 
> >  	gid = v9fs_get_fsgid_for_create(dir);
> > 
> > +	/* Update mode based on ACL value */
> > +	err = v9fs_acl_mode(dir, &mode, &dacl, &pacl);
> > +	if (err) {
> > +		p9_debug(P9_DEBUG_VFS,
> > +			 "Failed to get acl values in symlink %d\n",
> > +			 err);
> > +		goto error;
> > +	}
> > +
> >  	/* Server doesn't alter fid on TSYMLINK. Hence no need to clone it. */
> >  	err = p9_client_symlink(dfid, name, symname, gid, &qid);
> > 
> > @@ -712,8 +725,30 @@ v9fs_vfs_symlink_dotl(struct mnt_idmap *idmap, struct
> > inode *dir,
> > 
> >  	v9fs_invalidate_inode_attr(dir);
> > 
> > +	/* instantiate inode and assign the unopened fid to the dentry */
> > +	fid = p9_client_walk(dfid, 1, &name, 1);
> > +	if (IS_ERR(fid)) {
> > +		err = PTR_ERR(fid);
> > +		p9_debug(P9_DEBUG_VFS, "p9_client_walk failed %d\n",
> > +			 err);
> > +		goto error;
> > +	}
> > +
> > +	v9ses = v9fs_inode2v9ses(dir);
> > +	inode = v9fs_get_new_inode_from_fid(v9ses, fid, dir->i_sb);
> > +	if (IS_ERR(inode)) {
> > +		err = PTR_ERR(inode);
> > +		p9_debug(P9_DEBUG_VFS, "inode creation failed %d\n",
> > +			 err);
> > +		goto error;
> > +	}
> > +	v9fs_set_create_acl(inode, fid, dacl, pacl);
> > +	v9fs_fid_add(dentry, &fid);
> > +	d_instantiate(dentry, inode);
> > +	err = 0;
> >  error:
> >  	p9_fid_put(fid);
> > +	v9fs_put_acl(dacl, pacl);
> >  	p9_fid_put(dfid);
> >  	return err;
> >  }
> > @@ -853,24 +888,23 @@ v9fs_vfs_mknod_dotl(struct mnt_idmap *idmap, struct
> > inode *dir, }
> > 
> >  /**
> > - * v9fs_vfs_get_link_dotl - follow a symlink path
> > + * v9fs_vfs_get_link_nocache_dotl - Resolve a symlink directly.
> > + *
> > + * To be used when symlink caching is not enabled.
> > + *
> >   * @dentry: dentry for symlink
> >   * @inode: inode for symlink
> >   * @done: destructor for return value
> >   */
> > -
> >  static const char *
> > -v9fs_vfs_get_link_dotl(struct dentry *dentry,
> > -		       struct inode *inode,
> > -		       struct delayed_call *done)
> > +v9fs_vfs_get_link_nocache_dotl(struct dentry *dentry,
> > +			       struct inode *inode,
> > +			       struct delayed_call *done)
> >  {
> >  	struct p9_fid *fid;
> >  	char *target;
> >  	int retval;
> > 
> > -	if (!dentry)
> > -		return ERR_PTR(-ECHILD);
> > -
> >  	p9_debug(P9_DEBUG_VFS, "%pd\n", dentry);
> > 
> >  	fid = v9fs_fid_lookup(dentry);
> > @@ -884,6 +918,29 @@ v9fs_vfs_get_link_dotl(struct dentry *dentry,
> >  	return target;
> >  }
> > 
> > +/**
> > + * v9fs_vfs_get_link_dotl - follow a symlink path
> > + * @dentry: dentry for symlink
> > + * @inode: inode for symlink
> > + * @done: destructor for return value
> > + */
> > +static const char *
> > +v9fs_vfs_get_link_dotl(struct dentry *dentry,
> > +		       struct inode *inode,
> > +		       struct delayed_call *done)
> > +{
> > +	struct v9fs_session_info *v9ses;
> > +
> > +	if (!dentry)
> > +		return ERR_PTR(-ECHILD);
> > +
> > +	v9ses = v9fs_inode2v9ses(inode);
> > +	if (v9ses->cache & (CACHE_META|CACHE_LOOSE))
> > +		return page_get_link(dentry, inode, done);
> > +
> > +	return v9fs_vfs_get_link_nocache_dotl(dentry, inode, done);
> > +}
> > +
> >  int v9fs_refresh_inode_dotl(struct p9_fid *fid, struct inode *inode)
> >  {
> >  	struct p9_stat_dotl *st;
> 

Thanks for the review,

-- 
Remi