From 7e97b8daf63487c20f78487bd4045f39b0d97cf4 Mon Sep 17 00:00:00 2001 From: David Sterba Date: Fri, 7 Sep 2012 05:56:55 -0600 Subject: [PATCH] btrfs: allow setting NOCOW for a zero sized file via ioctl Hi, the patch si simple, but it has user visible impact and I'm not quite sure how to resolve it. In short, $subj says it, chattr -C supports it and we want to use it. The conditions that acutally allow to change the NOCOW flag are clear. What if I try to set the flag on a file that is not empty? Options: 1) whole ioctl will fail, EINVAL 2.1) ioctl will succeed, the NOCOW flag will be silently removed, but the file will stay COW-ed and checksummed 2.2) ioctl will succeed, flag will not be removed and a syslog message will warn that the COW flag has not been changed 2.2.1) dtto, no syslog message Man page of chattr states that "If it is set on a file which already has data blocks, it is undefined when the blocks assigned to the file will be fully stable." Yes, it's undefined and with current implementation it'll never happen. So from this end, the user cannot expect anything. I'm trying to find a reasonable behaviour, so that a command like 'chattr -R -aijS +C' to tweak a broad set of flags in a deep directory does not fail unnecessarily and does not pollute the log. My personal preference is 2.2.1, but my dev's oppinion is skewed, not counting the fact that I know the code and otherwise would look there before consulting the documentation. The patch implements 2.2.1. david -------------8<------------------- From: David Sterba It's safe to turn off checksums for a zero sized file. http://thread.gmane.org/gmane.comp.file-systems.btrfs/18030 "We cannot switch on NODATASUM for a file that already has extents that are checksummed. The invariant here is that either all the extents or none are checksummed. Theoretically it's possible to add/remove all checksums from a given file, but it's a potentially longtime operation, the file has to be in some intermediate state where the checksums partially exist but have to be ignored (for the csum->nocsum) until the file is fully converted, this brings more special cases to extent handling, it has to survive power failure and remain consistent, and probably needs to be restarted after next mount." Signed-off-by: David Sterba --- fs/btrfs/ioctl.c | 31 +++++++++++++++++++++++++++---- 1 file changed, 27 insertions(+), 4 deletions(-) diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c index 38e3d6ab89d3..4d7f4bbf4c96 100644 --- a/fs/btrfs/ioctl.c +++ b/fs/btrfs/ioctl.c @@ -181,6 +181,7 @@ static int btrfs_ioctl_setflags(struct file *file, void __user *arg) int ret; u64 ip_oldflags; unsigned int i_oldflags; + umode_t mode; if (btrfs_root_readonly(root)) return -EROFS; @@ -203,6 +204,7 @@ static int btrfs_ioctl_setflags(struct file *file, void __user *arg) ip_oldflags = ip->flags; i_oldflags = inode->i_flags; + mode = inode->i_mode; flags = btrfs_mask_flags(inode->i_mode, flags); oldflags = btrfs_flags_to_ioctl(ip->flags); @@ -237,10 +239,31 @@ static int btrfs_ioctl_setflags(struct file *file, void __user *arg) ip->flags |= BTRFS_INODE_DIRSYNC; else ip->flags &= ~BTRFS_INODE_DIRSYNC; - if (flags & FS_NOCOW_FL) - ip->flags |= BTRFS_INODE_NODATACOW; - else - ip->flags &= ~BTRFS_INODE_NODATACOW; + if (flags & FS_NOCOW_FL) { + if (S_ISREG(mode)) { + /* + * It's safe to turn csums off here, no extents exist. + * Otherwise we want the flag to reflect the real COW + * status of the file and will not set it. + */ + if (inode->i_size == 0) + ip->flags |= BTRFS_INODE_NODATACOW + | BTRFS_INODE_NODATASUM; + } else { + ip->flags |= BTRFS_INODE_NODATACOW; + } + } else { + /* + * Revert back under same assuptions as above + */ + if (S_ISREG(mode)) { + if (inode->i_size == 0) + ip->flags &= ~(BTRFS_INODE_NODATACOW + | BTRFS_INODE_NODATASUM); + } else { + ip->flags &= ~BTRFS_INODE_NODATACOW; + } + } /* * The COMPRESS flag can only be changed by users, while the NOCOMPRESS -- 2.20.1