On Thu, Nov 2, 2017 at 2:27 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: > On Wed, Nov 01, 2017 at 06:41:51PM +0200, Amir Goldstein wrote: > > [..] >> >> I can't remember if there was a reason for not allocating anonymous bdev >> >> for upper >> > >> > That's a good point. >> > >> >> or if it just because we did not need it to guaranty uniqueness >> >> of st_dev/st_ino *among* overlay inodes >> > >> > Even for lower, st_dev will be unique for different lower on non same-fs, >> > right. IOW, when it come to uniqueness of st_dev/st_ino pair, among >> > overlay inodes, lower and upper should have same requirements. >> > >> >> while guarantying constant >> >> st_dev/st_ino across copy up. >> > >> > Hmm..., I did not get this point. Over copy up, atleast st_ino will change >> > for non-samefs case. >> > >> > I will spend more time on patch. >> > >> >> Urgh! It took me a while to remember the reason why system wide uniqueness >> is important for lower but less for upper. >> An upper object has the same content as the "real" object and they have the >> same st_ino/st_dev so its ok that diff will skip comparing them. >> A copy-up object does not have the same content as the lower "real" object, >> so if it has the same st_ino/st_dev as real object, diff will skip compare and >> we have a problem. > > I am not sure I understand this. So you are doing a diff between a file > on overlayfs and same file accessed outside overlayfs? > Yes. > If a file is on lower, then it has not been modified and diff skipping > it makes perfect sense? Yes. But! With constant st_ino/st_dev across copy up (the next patch) the overlay object still has the lower inode st_dev/st_inode also *after* copy up and modification. Now if you diff overlay file and lower file diff will say they are equal, but in fact they have a different content. This is how I phrased this in latest patch set per your request: ----------------------- [V6 4/9] ovl: return anonymous st_dev for lower inodes For non-samefs setup, to make sure that st_dev/st_ino pair is unique across the system, we return a unique anonymous st_dev for stat(2) of lower layer inode. We need to make this change before fixing constant st_dev/st_ino across copy up for non-samefs. Otherwise, we can end up with two objects in the system, the real lower inode and the overlay inode which have same st_dev/st_ino value, but different content. Hope this is clear. Amir. -- To unsubscribe from this list: send the line "unsubscribe linux-unionfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
