Re: [PATCH v2 1/4] ceph: always clone dentry name when building MDS request

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On Wed, Apr 17, 2019 at 10:42 PM Yan, Zheng <ukernel@xxxxxxxxx> wrote:
>
> How about cloning name only when parent dir is not locked.
>
> Regards
> Yan, Zheng
>
> On Thu, Apr 18, 2019 at 2:43 AM Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> >
> > Ben reported tripping the BUG_ON in create_request_message during some
> > performance testing. Analysis of the vmcore showed that the length of
> > the r_dentry->d_name string changed after we allocated the buffer, but
> > before we encoded it.
> >
> > build_dentry_path returns pointers to d_name in the common case of
> > non-snapped dentries, but this optimization isn't safe. Instead, make a
> > copy of the string in this case. That is less efficient, but safe.
> >
> > Cc: stable@xxxxxxxxxxxxxxx
> > Reported-by: Ben England <bengland@xxxxxxxxxx>
> > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > Reviewed-by: "Yan, Zheng" <zyan@xxxxxxxxxx>
> > ---
> >  fs/ceph/mds_client.c | 40 ++++++++++++++++++++++++++++++++--------
> >  1 file changed, 32 insertions(+), 8 deletions(-)
> >
> > diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
> > index e09feadd2ae1..4cfefe118128 100644
> > --- a/fs/ceph/mds_client.c
> > +++ b/fs/ceph/mds_client.c
> > @@ -2159,9 +2159,35 @@ char *ceph_mdsc_build_path(struct dentry *dentry, int *plen, u64 *base,
> >         return path;
> >  }
> >
> > +/* Duplicate the dentry->d_name.name safely */
> > +static int clone_dentry_name(struct dentry *dentry, const char **ppath,
> > +                            int *ppathlen)
> > +{
> > +       u32 len;
> > +       char *name;
> > +retry:
> > +       len = READ_ONCE(dentry->d_name.len);
> > +       name = kmalloc(len + 1, GFP_NOFS);
> > +       if (!name)
> > +               return -ENOMEM;
> > +
> > +       spin_lock(&dentry->d_lock);
> > +       if (dentry->d_name.len != len) {
> > +               spin_unlock(&dentry->d_lock);
> > +               kfree(name);
> > +               goto retry;
> > +       }
> > +       memcpy(name, dentry->d_name.name, len);
> > +       spin_unlock(&dentry->d_lock);
> > +
> > +       name[len] = '\0';
> > +       *ppath = name;
> > +       *ppathlen = len;
> > +       return 0;
> > +}
> > +
> >  static int build_dentry_path(struct dentry *dentry, struct inode *dir,
> > -                            const char **ppath, int *ppathlen, u64 *pino,
> > -                            int *pfreepath)
> > +                            const char **ppath, int *ppathlen, u64 *pino)
> >  {
> >         char *path;
> >
> > @@ -2171,16 +2197,13 @@ static int build_dentry_path(struct dentry *dentry, struct inode *dir,
> >         if (dir && ceph_snap(dir) == CEPH_NOSNAP) {
> >                 *pino = ceph_ino(dir);
> >                 rcu_read_unlock();
> > -               *ppath = dentry->d_name.name;
> > -               *ppathlen = dentry->d_name.len;
> > -               return 0;
> > +               return clone_dentry_name(dentry, ppath, ppathlen);
> >         }
> >         rcu_read_unlock();
> >         path = ceph_mdsc_build_path(dentry, ppathlen, pino, 1);
> >         if (IS_ERR(path))
> >                 return PTR_ERR(path);
> >         *ppath = path;
> > -       *pfreepath = 1;
> >         return 0;
> >  }
> >
> > @@ -2222,8 +2245,9 @@ static int set_request_path_attr(struct inode *rinode, struct dentry *rdentry,
> >                 dout(" inode %p %llx.%llx\n", rinode, ceph_ino(rinode),
> >                      ceph_snap(rinode));
> >         } else if (rdentry) {
> > -               r = build_dentry_path(rdentry, rdiri, ppath, pathlen, ino,
> > -                                       freepath);
> > +               r = build_dentry_path(rdentry, rdiri, ppath, pathlen, ino);
> > +               if (!r)
> > +                       *freepath = 1;
> >                 dout(" dentry %p %llx/%.*s\n", rdentry, *ino, *pathlen,
> >                      *ppath);
> >         } else if (rpath || rino) {
> > --
> > 2.20.1
> >

Good idea. I'll respin this one.

-- 
Jeff Layton <jlayton@xxxxxxxxxx>



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