From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f195.google.com ([209.85.128.195]:41098 "EHLO mail-wr0-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753543AbeDLPI6 (ORCPT ); Thu, 12 Apr 2018 11:08:58 -0400 Received: by mail-wr0-f195.google.com with SMTP id s12so5460890wrc.8 for ; Thu, 12 Apr 2018 08:08:57 -0700 (PDT) From: Miklos Szeredi To: linux-unionfs@vger.kernel.org Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [RFC PATCH 22/35] ovl: copy-up on MAP_SHARED Date: Thu, 12 Apr 2018 17:08:13 +0200 Message-Id: <20180412150826.20988-23-mszeredi@redhat.com> In-Reply-To: <20180412150826.20988-1-mszeredi@redhat.com> References: <20180412150826.20988-1-mszeredi@redhat.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: A corner case of a corner case is when - file opened for O_RDONLY - which is then memory mapped SHARED - file opened for O_WRONLY - contents modified - contents read back though the shared mapping Unfortunately it looks very difficult to do anything about the established shared map after the file is copied up. Instead, when a read-only file is mapped shared, copy up the file before actually doing the map. This may result in unnecessary copy-ups (but so may copy-up on open(O_RDWR) for exampe). We can revisit this later if it turns out to be a performance problem in real life. Signed-off-by: Miklos Szeredi --- fs/overlayfs/Kconfig | 21 +++++++++++++++++++++ fs/overlayfs/file.c | 22 ++++++++++++++++++++++ fs/overlayfs/overlayfs.h | 7 +++++++ fs/overlayfs/ovl_entry.h | 1 + fs/overlayfs/super.c | 22 ++++++++++++++++++++++ 5 files changed, 73 insertions(+) diff --git a/fs/overlayfs/Kconfig b/fs/overlayfs/Kconfig index 17032631c5cf..991c0a5a0e00 100644 --- a/fs/overlayfs/Kconfig +++ b/fs/overlayfs/Kconfig @@ -103,3 +103,24 @@ config OVERLAY_FS_XINO_AUTO For more information, see Documentation/filesystems/overlayfs.txt If unsure, say N. + +config OVERLAY_FS_COPY_UP_SHARED + bool "Overlayfs: copy up when mapping a file shared" + default n + depends on OVERLAY_FS + help + If this option is enabled then on mapping a file with MAP_SHARED + overlayfs copies up the file in anticipation of it being modified (just + like we copy up the file on O_WRONLY and O_RDWR in anticipation of + modification). This does not interfere with shared library loading, as + that uses MAP_PRIVATE. But there might be use cases out there where + this impacts performance and disk usage. + + This just selects the default, the feature can also be enabled or + disabled in the running kernel or individually on each overlay mount. + + To get maximally standard compliant behavior, enable this option. + + To get a maximally backward compatible kernel, disable this option. + + If unsure, say N. diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c index 39b1b73334ad..23638d8ebab5 100644 --- a/fs/overlayfs/file.c +++ b/fs/overlayfs/file.c @@ -10,6 +10,7 @@ #include #include #include +#include #include #include "overlayfs.h" @@ -245,6 +246,26 @@ static int ovl_fsync(struct file *file, loff_t start, loff_t end, int datasync) return ret; } +static int ovl_pre_mmap(struct file *file, unsigned long prot, + unsigned long flag) +{ + int err = 0; + + /* + * Take 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 ((flag & MAP_SHARED) && ovl_copy_up_shared(file_inode(file)->i_sb)) + err = ovl_copy_up(file_dentry(file)); + + return err; +} + static int ovl_mmap(struct file *file, struct vm_area_struct *vma) { struct fd real; @@ -434,6 +455,7 @@ const struct file_operations ovl_file_operations = { .read_iter = ovl_read_iter, .write_iter = ovl_write_iter, .fsync = ovl_fsync, + .pre_mmap = ovl_pre_mmap, .mmap = ovl_mmap, .fallocate = ovl_fallocate, .unlocked_ioctl = ovl_ioctl, diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h index b29c1688f372..dad54bc8de7d 100644 --- a/fs/overlayfs/overlayfs.h +++ b/fs/overlayfs/overlayfs.h @@ -279,6 +279,13 @@ static inline unsigned int ovl_xino_bits(struct super_block *sb) return ofs->xino_bits; } +static inline bool ovl_copy_up_shared(struct super_block *sb) +{ + struct ovl_fs *ofs = sb->s_fs_info; + + return !(sb->s_flags & SB_RDONLY) && ofs->config.copy_up_shared; +} + /* namei.c */ int ovl_check_fh_len(struct ovl_fh *fh, int fh_len); diff --git a/fs/overlayfs/ovl_entry.h b/fs/overlayfs/ovl_entry.h index 41655a7d6894..3bea47c63fd9 100644 --- a/fs/overlayfs/ovl_entry.h +++ b/fs/overlayfs/ovl_entry.h @@ -18,6 +18,7 @@ struct ovl_config { const char *redirect_mode; bool index; bool nfs_export; + bool copy_up_shared; int xino; }; diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c index ad6a5baf226b..c3d8c7ea180f 100644 --- a/fs/overlayfs/super.c +++ b/fs/overlayfs/super.c @@ -56,6 +56,12 @@ module_param_named(xino_auto, ovl_xino_auto_def, bool, 0644); MODULE_PARM_DESC(ovl_xino_auto_def, "Auto enable xino feature"); +static bool ovl_copy_up_shared_def = + IS_ENABLED(CONFIG_OVERLAY_FS_COPY_UP_SHARED); +module_param_named(copy_up_shared, ovl_copy_up_shared_def, bool, 0644); +MODULE_PARM_DESC(ovl_copy_up_shared_def, + "Copy up when mapping a file shared"); + static void ovl_entry_stack_free(struct ovl_entry *oe) { unsigned int i; @@ -380,6 +386,9 @@ static int ovl_show_options(struct seq_file *m, struct dentry *dentry) "on" : "off"); if (ofs->config.xino != ovl_xino_def()) seq_printf(m, ",xino=%s", ovl_xino_str[ofs->config.xino]); + if (ofs->config.copy_up_shared != ovl_copy_up_shared_def) + seq_printf(m, ",copy_up_shared=%s", + ofs->config.copy_up_shared ? "on" : "off"); return 0; } @@ -417,6 +426,8 @@ enum { OPT_XINO_ON, OPT_XINO_OFF, OPT_XINO_AUTO, + OPT_COPY_UP_SHARED_ON, + OPT_COPY_UP_SHARED_OFF, OPT_ERR, }; @@ -433,6 +444,8 @@ static const match_table_t ovl_tokens = { {OPT_XINO_ON, "xino=on"}, {OPT_XINO_OFF, "xino=off"}, {OPT_XINO_AUTO, "xino=auto"}, + {OPT_COPY_UP_SHARED_ON, "copy_up_shared=on"}, + {OPT_COPY_UP_SHARED_OFF, "copy_up_shared=off"}, {OPT_ERR, NULL} }; @@ -559,6 +572,14 @@ static int ovl_parse_opt(char *opt, struct ovl_config *config) config->xino = OVL_XINO_AUTO; break; + case OPT_COPY_UP_SHARED_ON: + config->copy_up_shared = true; + break; + + case OPT_COPY_UP_SHARED_OFF: + config->copy_up_shared = false; + break; + default: pr_err("overlayfs: unrecognized mount option \"%s\" or missing value\n", p); return -EINVAL; @@ -1380,6 +1401,7 @@ static int ovl_fill_super(struct super_block *sb, void *data, int silent) ofs->config.index = ovl_index_def; ofs->config.nfs_export = ovl_nfs_export_def; ofs->config.xino = ovl_xino_def(); + ofs->config.copy_up_shared = ovl_copy_up_shared_def; err = ovl_parse_opt((char *) data, &ofs->config); if (err) goto out_err; -- 2.14.3