On 02/16, Chao Yu wrote:
> On 2017/2/16 9:16, Jaegeuk Kim wrote:
> > On 02/14, Jaegeuk Kim wrote:
> >> On 02/15, Chao Yu wrote:
> >>> On 2017/2/15 2:03, Jaegeuk Kim wrote:
> >>>> VFS uses f2fs_lookup() to decide f2fs_add_link() call during file creation.
> >>>> But, if there is a race condition, f2fs_add_link() can be called multiple
> >>>> times, resulting in multiple dentries with a same name. This patches fixes
> >>>> it by adding __f2fs_find_entry() in f2fs_add_link() path.
> >>>
> >>> As during ->lookup, i_mutex will be held all the time, so there is no race
> >>> condition in between different file creators?
> >>
> >> Hehe, yup.
> >> I dropped this patch.
> >
> > It turns out sdcardfs has a race condition between lookup and create, and it
> > calls vfs_create() twice. Workaround by f2fs would be needed for whole AOSP as
> > well. And I added to check current to avoid performance regression.
> >
> > Thanks,
> >
> >>From 2f433dbfa491550e666a3d08f5a59ac5bed42b0f Mon Sep 17 00:00:00 2001
> > From: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> > Date: Tue, 14 Feb 2017 09:54:37 -0800
> > Subject: [PATCH] f2fs: fix multiple f2fs_add_link() calls having same name
> >
> > It turns out a stakable filesystem like sdcardfs in AOSP can trigger multiple
> > vfs_create() to lower filesystem. In that case, f2fs will add multiple dentries
> > having same name which breaks filesystem consistency.
> >
> > Until upper layer fixes, let's work around by f2fs, which shows actually not
> > much performance regression.
> >
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> > ---
> > fs/f2fs/dir.c | 26 ++++++++++++++++++++++++--
> > fs/f2fs/f2fs.h | 1 +
> > 2 files changed, 25 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
> > index 827c5daef4fc..4f5b3bb514c6 100644
> > --- a/fs/f2fs/dir.c
> > +++ b/fs/f2fs/dir.c
> > @@ -207,9 +207,11 @@ static struct f2fs_dir_entry *find_in_level(struct inode *dir,
> > f2fs_put_page(dentry_page, 0);
> > }
> >
> > + /* This is to increase the speed of f2fs_create */
> > if (!de && room && F2FS_I(dir)->chash != namehash) {
> > F2FS_I(dir)->chash = namehash;
> > F2FS_I(dir)->clevel = level;
> > + F2FS_I(dir)->task = current;
> > }
>
> Hash collision can happen on different file names, here, assignment of .task
> should be more accurately to prevent racing between lookup and creat, so how about:
>
> if (!de && room) {
> .task = current;
> if (chash != namehash) {
> .chash = namehash;
> .clevel = level;
> }
> }
Makes sense.
Will apply that. ;)
Thanks,
>
> Thanks,
>
> >
> > return de;
> > @@ -643,14 +645,34 @@ int __f2fs_add_link(struct inode *dir, const struct qstr *name,
> > struct inode *inode, nid_t ino, umode_t mode)
> > {
> > struct fscrypt_name fname;
> > + struct page *page = NULL;
> > + struct f2fs_dir_entry *de = NULL;
> > int err;
> >
> > err = fscrypt_setup_filename(dir, name, 0, &fname);
> > if (err)
> > return err;
> >
> > - err = __f2fs_do_add_link(dir, &fname, inode, ino, mode);
> > -
> > + /*
> > + * An immature stakable filesystem shows a race condition between lookup
> > + * and create. If we have same task when doing lookup and create, it's
> > + * definitely fine as expected by VFS normally. Otherwise, let's just
> > + * verify on-disk dentry one more time, which guarantees filesystem
> > + * consistency more.
> > + */
> > + if (current != F2FS_I(dir)->task) {
> > + de = __f2fs_find_entry(dir, &fname, &page);
> > + F2FS_I(dir)->task = NULL;
> > + }
> > + if (de) {
> > + f2fs_dentry_kunmap(dir, page);
> > + f2fs_put_page(page, 0);
> > + err = -EEXIST;
> > + } else if (!IS_ERR(page)) {
> > + err = __f2fs_do_add_link(dir, &fname, inode, ino, mode);
> > + } else {
> > + err = PTR_ERR(page);
> > + }
> > fscrypt_free_filename(&fname);
> > return err;
> > }
> > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > index d263dade5e4c..cc22dc458896 100644
> > --- a/fs/f2fs/f2fs.h
> > +++ b/fs/f2fs/f2fs.h
> > @@ -454,6 +454,7 @@ struct f2fs_inode_info {
> > atomic_t dirty_pages; /* # of dirty pages */
> > f2fs_hash_t chash; /* hash value of given file name */
> > unsigned int clevel; /* maximum level of given file name */
> > + struct task_struct *task; /* lookup and create consistency */
> > nid_t i_xattr_nid; /* node id that contains xattrs */
> > loff_t last_disk_size; /* lastly written file size */
> >
> >