Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes: > On Sat, Aug 15, 2015 at 7:12 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote: >> >> I think you are underestimating the frequency of .. traversals. Any build >> process that creates relative symlinks will be hitting it all the time, >> for one thing. > > I suspect you're over-estimating how expensive it is to just walk down > to the mount-point. It's just a few pointer traversals. > > Realistically, we probably do more than that for a *regular* path > component lookup, when we follow the hash chains. Following a d_parent > chain for ".." isn't that different. > > Just looking at the last patch Eric sent, that one looks _trivial_. It > didn't need *any* preparation or new rules. Compared to the mess with > marking things MNT_DIR_ESCAPED etc, I know which approach I'd prefer. > > But hey, if you think you can simplify it... I just don't think that > even totally ignoring the d_splice_alias() things, and totally > ignoring any locking around __d_move(), the whole "mark things > MNT_DIR_ESCAPED" is a lot more complex. It occurs to me that there is a fairly simple way we can emperically test to see how expensive calling is_subdir for every .. on a bind mount is in practice. - Take my last patch - run a benchmark outside of a bind mount (perhaps a kernel compile). - run the same benchmark inside of a bind mount. See if the performance differs. I am going to try to find time to do this, but I am travelling for the next couple of days. If someone who has a bit more time wants to try it and beats me to that would be great. I think having some emperical numbers would be nice in this part of the conversation. Eric -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
