On Wed, 2020-12-02 at 10:07 -0500, Vivek Goyal wrote: > On Wed, Dec 02, 2020 at 01:27:20AM -0800, Sargun Dhillon wrote: > > Overlayfs's volatile option allows the user to bypass all forced sync calls > > to the upperdir filesystem. This comes at the cost of safety. We can never > > ensure that the user's data is intact, but we can make a best effort to > > expose whether or not the data is likely to be in a bad state. > > > > We decided[1] that the best way to handle this in the time being is that if > > an overlayfs's upperdir experiences an error after a volatile mount occurs, > > that error will be returned on fsync, fdatasync, sync, and syncfs. This is > > contradictory to the traditional behaviour of VFS which fails the call > > once, and only raises an error if a subsequent fsync error has occured, > > and been raised by the filesystem. > > > > One awkward aspect of the patch is that we have to manually set the > > superblock's errseq_t after the sync_fs callback as opposed to just > > returning an error from syncfs. This is because the call chain looks > > something like this: > > > > sys_syncfs -> > > sync_filesystem -> > > __sync_filesystem -> > > /* The return value is ignored here > > sb->s_op->sync_fs(sb) > > _sync_blockdev > > /* Where the VFS fetches the error to raise to userspace */ > > errseq_check_and_advance > > > > Because of this we call errseq_set every time the sync_fs callback occurs. > > > > [1]: https://lore.kernel.org/linux-fsdevel/36d820394c3e7cd1faa1b28a8135136d5001dadd.camel@xxxxxxxxxx/T/#u > > > > Signed-off-by: Sargun Dhillon <sargun@xxxxxxxxx> > > Suggested-by: Amir Goldstein <amir73il@xxxxxxxxx> > > Cc: linux-fsdevel@xxxxxxxxxxxxxxx > > Cc: linux-unionfs@xxxxxxxxxxxxxxx > > Cc: Jeff Layton <jlayton@xxxxxxxxxx> > > Cc: Miklos Szeredi <miklos@xxxxxxxxxx> > > Cc: Amir Goldstein <amir73il@xxxxxxxxx> > > Cc: Vivek Goyal <vgoyal@xxxxxxxxxx> > > --- > > Documentation/filesystems/overlayfs.rst | 8 ++++++++ > > fs/overlayfs/file.c | 5 +++-- > > fs/overlayfs/overlayfs.h | 1 + > > fs/overlayfs/ovl_entry.h | 2 ++ > > fs/overlayfs/readdir.c | 5 +++-- > > fs/overlayfs/super.c | 24 +++++++++++++++------- > > fs/overlayfs/util.c | 27 +++++++++++++++++++++++++ > > 7 files changed, 61 insertions(+), 11 deletions(-) > > > > diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst > > index 580ab9a0fe31..3af569cea6a7 100644 > > --- a/Documentation/filesystems/overlayfs.rst > > +++ b/Documentation/filesystems/overlayfs.rst > > @@ -575,6 +575,14 @@ without significant effort. > > The advantage of mounting with the "volatile" option is that all forms of > > sync calls to the upper filesystem are omitted. > > > > > > > > > > +In order to avoid a giving a false sense of safety, the syncfs (and fsync) > > +semantics of volatile mounts are slightly different than that of the rest of > > +VFS. If any error occurs on the upperdir's filesystem after a volatile mount > > +takes place, all sync functions will return the last error observed on the > > +upperdir filesystem. Once this condition is reached, the filesystem will not > > +recover, and every subsequent sync call will return an error, even if the > > +upperdir has not experience a new error since the last sync call. > > + > > When overlay is mounted with "volatile" option, the directory > > "$workdir/work/incompat/volatile" is created. During next mount, overlay > > checks for this directory and refuses to mount if present. This is a strong > > diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c > > index 802259f33c28..2479b297a966 100644 > > --- a/fs/overlayfs/file.c > > +++ b/fs/overlayfs/file.c > > @@ -445,8 +445,9 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync) > > const struct cred *old_cred; > > int ret; > > > > > > > > > > - if (!ovl_should_sync(OVL_FS(file_inode(file)->i_sb))) > > - return 0; > > + ret = ovl_check_sync(OVL_FS(file_inode(file)->i_sb)); > > + if (ret <= 0) > > + return ret; > > > > > > > > > > ret = ovl_real_fdget_meta(file, &real, !datasync); > > if (ret) > > diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h > > index f8880aa2ba0e..af79c3a2392e 100644 > > --- a/fs/overlayfs/overlayfs.h > > +++ b/fs/overlayfs/overlayfs.h > > @@ -322,6 +322,7 @@ int ovl_check_metacopy_xattr(struct ovl_fs *ofs, struct dentry *dentry); > > bool ovl_is_metacopy_dentry(struct dentry *dentry); > > char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry, > > int padding); > > +int ovl_check_sync(struct ovl_fs *ofs); > > > > > > > > > > static inline bool ovl_is_impuredir(struct super_block *sb, > > struct dentry *dentry) > > diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h > > index 1b5a2094df8e..9460a52abea3 100644 > > --- a/fs/overlayfs/ovl_entry.h > > +++ b/fs/overlayfs/ovl_entry.h > > @@ -79,6 +79,8 @@ struct ovl_fs { > > atomic_long_t last_ino; > > /* Whiteout dentry cache */ > > struct dentry *whiteout; > > + /* snapshot of upperdir's errseq_t at mount time for volatile mounts */ > > + errseq_t upper_errseq; > > }; > > > > > > > > > > static inline struct vfsmount *ovl_upper_mnt(struct ovl_fs *ofs) > > diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c > > index 01620ebae1bd..f7f1a29e290f 100644 > > --- a/fs/overlayfs/readdir.c > > +++ b/fs/overlayfs/readdir.c > > @@ -909,8 +909,9 @@ static int ovl_dir_fsync(struct file *file, loff_t start, loff_t end, > > struct file *realfile; > > int err; > > > > > > > > > > - if (!ovl_should_sync(OVL_FS(file->f_path.dentry->d_sb))) > > - return 0; > > + err = ovl_check_sync(OVL_FS(file->f_path.dentry->d_sb)); > > + if (err <= 0) > > + return err; > > > > > > > > > > realfile = ovl_dir_real_file(file, true); > > err = PTR_ERR_OR_ZERO(realfile); > > diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c > > index 290983bcfbb3..82a096a05bce 100644 > > --- a/fs/overlayfs/super.c > > +++ b/fs/overlayfs/super.c > > @@ -261,11 +261,18 @@ static int ovl_sync_fs(struct super_block *sb, int wait) > > struct super_block *upper_sb; > > int ret; > > > > > > > > > > - if (!ovl_upper_mnt(ofs)) > > - return 0; > > + ret = ovl_check_sync(ofs); > > + /* > > + * We have to always set the err, because the return value isn't > > + * checked, and instead VFS looks at the writeback errseq after > > + * this call. > > + */ > > + if (ret < 0) > > + errseq_set(&sb->s_wb_err, ret); > > I was wondering that why errseq_set() will result in returning error > all the time. Then realized that last syncfs() call must have set > ERRSEQ_SEEN flag and that will mean errseq_set() will increment > counter and that means this syncfs() will will return error too. Cool. > > > + > > + if (!ret) > > + return ret; > > > > > > > > > > - if (!ovl_should_sync(ofs)) > > - return 0; > > /* > > * Not called for sync(2) call or an emergency sync (SB_I_SKIP_SYNC). > > * All the super blocks will be iterated, including upper_sb. > > @@ -1927,6 +1934,8 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) > > sb->s_op = &ovl_super_operations; > > > > > > > > > > if (ofs->config.upperdir) { > > + struct super_block *upper_mnt_sb; > > + > > if (!ofs->config.workdir) { > > pr_err("missing 'workdir'\n"); > > goto out_err; > > @@ -1943,9 +1952,10 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) > > if (!ofs->workdir) > > sb->s_flags |= SB_RDONLY; > > > > > > > > > > - sb->s_stack_depth = ovl_upper_mnt(ofs)->mnt_sb->s_stack_depth; > > - sb->s_time_gran = ovl_upper_mnt(ofs)->mnt_sb->s_time_gran; > > - > > + upper_mnt_sb = ovl_upper_mnt(ofs)->mnt_sb; > > + sb->s_stack_depth = upper_mnt_sb->s_stack_depth; > > + sb->s_time_gran = upper_mnt_sb->s_time_gran; > > + ofs->upper_errseq = errseq_sample(&upper_mnt_sb->s_wb_err); > > I asked this question in last email as well. errseq_sample() will return > 0 if current error has not been seen yet. That means next time a sync > call comes for volatile mount, it will return an error. But that's > not what we want. When we mounted a volatile overlay, if there is an > existing error (seen/unseen), we don't care. We only care if there > is a new error after the volatile mount, right? > > I guess we will need another helper similar to errseq_smaple() which > just returns existing value of errseq. And then we will have to > do something about errseq_check() to not return an error if "since" > and "eseq" differ only by "seen" bit. > > Otherwise in current form, volatile mount will always return error > if upperdir has error and it has not been seen by anybody. > > How did you finally end up testing the error case. Want to simualate > error aritificially and test it. > If you don't want to see errors that occurred before you did the mount, then you probably can just resurrect and rename the original version of errseq_sample. Something like this, but with a different name: +errseq_t errseq_sample(errseq_t *eseq) +{ + errseq_t old = READ_ONCE(*eseq); + errseq_t new = old; + + /* + * For the common case of no errors ever having been set, we can skip + * marking the SEEN bit. Once an error has been set, the value will + * never go back to zero. + */ + if (old != 0) { + new |= ERRSEQ_SEEN; + if (old != new) + cmpxchg(eseq, old, new); + } + return new; +} > } > oe = ovl_get_lowerstack(sb, splitlower, numlower, ofs, layers); > err = PTR_ERR(oe); > diff --git a/fs/overlayfs/util.c b/fs/overlayfs/util.c > index 23f475627d07..9b460cd7b151 100644 > --- a/fs/overlayfs/util.c > +++ b/fs/overlayfs/util.c > @@ -950,3 +950,30 @@ char *ovl_get_redirect_xattr(struct ovl_fs *ofs, struct dentry *dentry, > kfree(buf); > return ERR_PTR(res); > } > + > +/* > + * ovl_check_sync provides sync checking, and safety for volatile mounts > + * > + * Returns 1 if sync required. > + * > + * Returns 0 if syncing can be skipped because mount is volatile, and no errors > + * have occurred on the upperdir since the mount. > + * > + * Returns -errno if it is a volatile mount, and the error that occurred since > + * the last mount. If the error code changes, it'll return the latest error > + * code. > + */ > + > +int ovl_check_sync(struct ovl_fs *ofs) > +{ > + struct vfsmount *mnt; > + > + if (ovl_should_sync(ofs)) > + return 1; > + > + mnt = ovl_upper_mnt(ofs); > + if (!mnt) > + return 0; > + > + return errseq_check(&mnt->mnt_sb->s_wb_err, ofs->upper_errseq); I guess we can do another patch later to set one global flag in overlayfs super block and use that flag to return errors on other paths like read/write etc. But that's for a separate patch later. Thanks Vivek -- Jeff Layton <jlayton@xxxxxxxxxx>
