On Thu, Nov 18, 2021 at 05:16:24PM +0000, Song Liu wrote: > > > > On Nov 17, 2021, at 11:54 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > On Wed, Nov 17, 2021 at 11:57:12PM +0000, Song Liu wrote: > > > >> I would agree that __text_poke() is a safer option. But in this case, we > >> will need the temporary hole to be 2MB in size. Also, we will probably > >> hold the temporary mapping for longer time (the whole JITing process). > >> Does this sound reasonable? > > > > No :-) > > > > Jit to a buffer, then copy the buffer into the 2M page using 4k aliases. > > IIRC each program is still smaller than a single page, right? So at no > > point do you need more than 2 pages mapped anyway. > > JITing to a separate buffer adds complexity to the JIT process, as we > need to redo some offsets before the copy to match the final location of > the program. I don't have much experience with the JIT engine, so I am > not very sure how much work it gonna be. You're going to have to do that anyway if you're going to write to the directmap while executing from the alias. > The BPF program could have up to 1000000 (BPF_COMPLEXITY_LIMIT_INSNS) > instructions (BPF instructions). So it could easily go beyond a few > pages. Mapping the 2MB page all together should make the logic simpler. Then copy it in smaller chunks I suppose.
