On Fri, 2024-02-16 at 17:11 -0800, Rick Edgecombe wrote: > > Do we care to check if alignment itself would satisfy start_gap > > requirement? > > Ugh, I think actually the alignment stuff clobbers the guard gap in > the > search up scenario. I'm also seeing some weird results as I throw > test > values into the existing logic, but very likely I just need to look > at > this not late on a Friday. Thanks for pointing it out. Ok, playing around with the address adjustment math in a separate test program, I think it is all ok functionally. But there are two gotchas: 1. The existing math for search up assumes that the requested length is bigger than the alignment mask. If the length is smaller, non- cannonical addresses can result (more than ->high_limit). I don't think any callers can call with this combination so it's fine functionally. 2. The newly added code can only hit the scenario you highlight if the start gap is more than the alignment size. If alignment mask is more than the start gap, the alignment will only shift the address more than the adjustment made for the start gap. So if it skips the start gap adjustment in the case of alignment adding the necessary gap it won't change the result and just add a branch. Similarly, if the start gap fulfills the alignment, there is no adjustment during the alignment step. I think maybe I'll add a comment covering both gotchas and leave the logic as is, unless there are any objections. Or maybe a VM_WARN_ON, hmm.
