| Message ID | 20161021091211.GI31239@veci.piliscsaba.szeredi.hu (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Fri, Oct 21, 2016 at 11:12:11AM +0200, Miklos Szeredi wrote: > On Thu, Oct 20, 2016 at 04:54:08PM -0400, Vivek Goyal wrote: > > On Thu, Oct 20, 2016 at 04:46:30PM -0400, Vivek Goyal wrote: > > > > [..] > > > > +static ssize_t ovl_read_iter(struct kiocb *iocb, struct iov_iter *to) > > > > +{ > > > > + struct file *file = iocb->ki_filp; > > > > + bool isupper = OVL_TYPE_UPPER(ovl_path_type(file->f_path.dentry)); > > > > + ssize_t ret = -EINVAL; > > > > + > > > > + if (likely(!isupper)) { > > > > + const struct file_operations *fop = ovl_real_fop(file); > > > > + > > > > + if (likely(fop->read_iter)) > > > > + ret = fop->read_iter(iocb, to); > > > > + } else { > > > > + struct file *upperfile = filp_clone_open(file); > > > > + > > > > > > IIUC, every read of lower file will call filp_clone_open(). Looking at the > > > code of filp_clone_open(), I am concerned about the overhead of this call. > > > Is it significant? Don't want to be paying too much of penalty for read > > > operation on lower files. That would be a common case for containers. > > > > > > > Looks like I read the code in reverse. So if I open a file read-only, > > and if it has not been copied up, I will simply call read_iter() on > > lower filesystem. But if file has been copied up, then I will call > > filp_clone_open() and pay the cost. And this will continue till this > > file is closed by caller. > > > > When file is opened again, by that time it is upper file and we will > > install real fop in file (instead of overlay fop). > > Right. > > The lockdep issue seems to be real, we can't take i_mutex and s_vfs_rename_mutex > while mmap_sem is locked. Fortunately copy up doesn't need mmap_sem, so we can > do it while unlocked and retry the mmap. > > Here's an incremental workaround patch. > > I don't like adding such workarounds to the VFS/MM but they are really cheap for > the non-overlay case and there doesn't appear to be an alternative in this case. This incremental patch does fix the locking warning issue I was seeing. Vivek -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
--- a/fs/overlayfs/inode.c +++ b/fs/overlayfs/inode.c @@ -419,21 +419,12 @@ static int ovl_mmap(struct file *file, s bool isupper = OVL_TYPE_UPPER(ovl_path_type(file->f_path.dentry)); int err; - /* - * Treat MAP_SHARED as hint about future writes to the file (through - * another file descriptor). Caller might not have had such an intent, - * but we hope MAP_PRIVATE will be used in most such cases. - * - * If we don't copy up now and the file is modified, it becomes really - * difficult to change the mapping to match that of the file's content - * later. - */ if (unlikely(isupper || vma->vm_flags & VM_MAYSHARE)) { - if (!isupper) { - err = ovl_copy_up(file->f_path.dentry); - if (err) - goto out; - } + /* + * File should have been copied up by now. See vm_mmap_pgoff(). + */ + if (WARN_ON(!isupper)) + return -EIO; file = filp_clone_open(file); err = PTR_ERR(file); --- a/mm/util.c +++ b/mm/util.c @@ -297,6 +297,28 @@ unsigned long vm_mmap_pgoff(struct file ret = security_mmap_file(file, prot, flag); if (!ret) { + /* + * Special treatment for overlayfs: + * + * Take MAP_SHARED/PROT_READ as hint about future writes to the + * file (through another file descriptor). Caller might not + * have had such an intent, but we hope MAP_PRIVATE will be used + * in most such cases. + * + * If we don't copy up now and the file is modified, it becomes + * really difficult to change the mapping to match that of the + * file's content later. + * + * Copy up needs to be done without mmap_sem since it takes vfs + * locks which would potentially deadlock under mmap_sem. + */ + if ((flag & MAP_SHARED) && !(prot & PROT_WRITE)) { + void *p = d_real(file->f_path.dentry, NULL, O_WRONLY); + + if (IS_ERR(p)) + return PTR_ERR(p); + } + if (down_write_killable(&mm->mmap_sem)) return -EINTR; ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,