On Thu, Oct 31, 2024 at 03:17:18PM -1000, Linus Torvalds wrote: > On Thu, 31 Oct 2024 at 12:34, Linus Torvalds > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > So I'd rather start with just the cheap inode-only "ACL is clearly not > > there" check, and later if we find that the ACL_NOT_CACHED case is > > problematic do we look at that. > > Actually, if I switch the tests around so that I do the permission bit > check first, it becomes very natural to just check IS_POSIXACL() at > the end (where we're about to go to the slow case, which will be > touching i_sb anyway). > > Plus I can actually improve code generation by not shifting the mode > bits down into the low bits, but instead spreading the requested > permission bits around. > > The "spread bits around" becomes a simple constant multiplication with > just three bits set, and the compiler will actually generate much > better code (you can do it with two consecutive 'lea' instructions). > > The expression for this ends up looking a bit like line noise, so a > comment explaining each step is a good idea. > > IOW, here's a rewritten patch that does it that way around, and thus > deals with IS_POSIXACL() very naturally and seems to generate quite > good code for me. Acked-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
