On Sat, Mar 17, 2018 at 09:56:19AM +0200, Amir Goldstein wrote: > On Sat, Mar 17, 2018 at 7:40 AM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > > On Fri, Mar 16, 2018 at 11:24 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > >> On Fri, Mar 16, 2018 at 04:05:22PM +0200, Amir Goldstein wrote: > >>> Hi guys, > >>> > >>> I am trying to get a lower bound for unused inode number MSB on > >>> a mounted xfs super block, so I can publish it on struct super_block. > >> > >> Sorry, what? > >> > >> The inode number is owned by the filesystem - nobody should be > >> touching it or making assumptions they can screw with it in any way. > >> > > Let me clarify with the simplest example: > > With overlay of 2 layers, lower and upper on 2 different xfs fs > assuming that stat(2) from xfs will not be using the 63 MSB: > > On stat(2) of an overlay upper inode we want to return: > st_dev = <overlay anon bdev> > st_ino = <real upper st_ino> > > On stat(2) of an overlay lower inode we want to return: > st_dev = <overlay anon bdev> > st_ino = <real lower st_ino> | 1 << 63 > > Now for ext4 this is always safe to do and we find that automatically > due to the fact that ext4 uses the default encode_fh generic 32bit > inode encoding. > > For xfs this should also be safe, but we don't want to whitelist xfs > by name/magic, so we want xfs to publish the max amount of bits > exposed to user with stat(2)/getdents(3). > > Recently, I became aware of an nfsd use case that also looks > at inode->i_ino, so we may want to also be able to assume > max_ino_bits also applies to inode->i_ino, but if you tell us to > stay clear of inode->i_ino, then we can always use stat.st_ino. > > Thanks, > Amir. > On Sat, Mar 17, 2018 at 10:24:39AM +0200, Amir Goldstein wrote: > On Sat, Mar 17, 2018 at 10:04 AM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: > > On Sat, Mar 17, 2018 at 06:40:23AM +0100, Miklos Szeredi wrote: > [...] > >> I ask, because we've thought long and hard about what to do for > >> multiplexing inum space in overlayfs, and found no other sane options. > >> Ideas welcome, of course. > > > > Why do you need to "multiplex" the inum space? perhaps you'd do > > better to start with a description of why you want to play games > > with inode numbers, rather than just posting a patch to steal bits > > from other filesytem inode number spaces.... > > > > I think this patch perhaps explains best what we want to do: > https://marc.info/?l=linux-unionfs&m=151007386219743&w=2 > > I had already given a simple example in an earlier response. So, I'll quote that here: > > > On stat(2) of an overlay upper inode we want to return: > > > st_dev = <overlay anon bdev> > > > st_ino = <real upper st_ino> > > > > > > On stat(2) of an overlay lower inode we want to return: > > > st_dev = <overlay anon bdev> > > > st_ino = <real lower st_ino> | 1 << 63 This makes no sense to me - this implies the inode number changes on copy-up, and .... > As the the "why" question, we have several requirements for > overlay inode numbers: > 1. st_ino is persistent > 2. st_ino/st_dev pair is unique in the system > 3. st_ino is consistent with d_ino > 4. st_ino doesn't change on copy up > 5. st_dev is uniform across all overlay inodes .... this means requierment #4 isn't met, even on the same filesystem. IOWs, if overlay has already met #4 on the same filesystem, then there is a persistent mapping between lower and upper inodes (Req. #1) that maps the upper inode # to the lower inode #. That has to be overlay information, because the underlying filesystem doesn't store it. And because the lower inode/dev is unique, then req. 2 is met, too. FWIW, req 5 is badly worded - st_dev is uniform across all inodes in a single overlay filesystem, not all overlay inodes. > With upstream overlayfs we meet all requirements above for > the case of all underlying layers on the same fs, by using a real > underlying inode st_ino and the overlay st_dev. Yeah, that's what I thought. So why can't you do exactly the same thing for different underlying filesystems? You've already got a mapping between upper and lower inode numbers, why can't that map across different superblocks? Why do you need special "inode number bits" exposed to userspace to identify upper->lower inode mappings that overlay should already have a persistent mapping mechanism for? > With the 'xino' patch set [1], we can meet all requirements above > also for the case of underlying layers on different fs, by multiplpexing > the inum space, as long as we know about unused high ino bits. Your example makes no sense to me - I don't see how adding extra bits to the lower inode number allows you to meet requirement #4, not why presenting "st_ino = <real upper st_ino>" for inodes that have been copied up iis being done because that violates requirement #4.... > The ovl-xino branch already has the xfs patch (not yet posted) to publish > max_ino_bits. That has no explanation of why you need to screw with inode number bits, either. It's all mechanism, and there's zero explanation of what problem it solves. Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
