On Thu, Nov 2, 2017 at 4:05 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: > On Thu, Nov 02, 2017 at 02:47:07PM +0200, Amir Goldstein wrote: >> 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: > > Ok, I think I am beginning to understand it. Here is my understanding. > Please correct me if something is not right. > > So we basically have 4 core requirements. > > A. contstant st_dev/st_ino over copy up. > B. Persistent st_ino > C. unique st_dev/st_ino > D. Diff works fine even after copy up. > > As of today, for non-samefs case, ovl_getattr() reports st_dev/st_ino of > *real* file. This meets requirement B and C and D but not requirement A. > > To meet requirement A, one could make use of ORIGIN xattr and report > st_dev/st_ino of lower (even after file got copied up). This will > meet requirement A, B and C but not D. > > So to make all 4 work for non-samefs case, we don't report real st_dev > of lower and instead create a pseudo dev and report that. IOW, for > non-same fs case, report pseudo_st_dev/real_st_ino of lower. And > that should meet all the 4 core requirements. > Yes. Almost accurate. Requirement B is not met for directory inodes in non-samefs case. it wasn't met before this change and it is not met by this change. > And this patch series implements this. > > If this description is correct, I feel some of this should be used > in changelog somewhere to make it easier to understand the rationale > behind the change. > > I'll see where I can plug this description. BTW, found some bugs in V6 patch set. Stay tuned for V7. Thanks for review! 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
