On Mon, Nov 12, 2018 at 04:46:46AM +0100, Ingo Molnar wrote: > > * Peter Zijlstra <pet...@infradead.org> wrote: > > > On Sun, Nov 11, 2018 at 08:53:07PM +0000, Nadav Amit wrote: > > > > > >> + /* > > > >> + * The lock is not really needed, but this allows to avoid > > > >> open-coding. > > > >> + */ > > > >> + ptep = get_locked_pte(poking_mm, poking_addr, &ptl); > > > >> + > > > >> + /* > > > >> + * If we failed to allocate a PTE, fail. This should *never* > > > >> happen, > > > >> + * since we preallocate the PTE. > > > >> + */ > > > >> + if (WARN_ON_ONCE(!ptep)) > > > >> + goto out; > > > > > > > > Since we hard rely on init getting that right; can't we simply get rid > > > > of this? > > > > > > This is a repeated complaint of yours, which I do not feel comfortable > > > with. > > > One day someone will run some static analysis tool and start finding that > > > all these checks are missing. > > > > > > The question is why do you care about them. > > > > Mostly because they should not be happening, ever. > > Since get_locked_pte() might in principle return NULL, it's an entirely > routine pattern to check the return for NULL. This will save reviewer > time in the future.
The reviewer can read a comment. > > > If it is because they affect the > > > generated code and make it less efficient, I can fully understand and > > > perhaps > > > we should have something like PARANOID_WARN_ON_ONCE() which compiles into > > > nothing > > > unless a certain debug option is set. > > > > > > If it is about the way the source code looks - I guess it doesn’t sore my > > > eyes as hard as some other stuff, and I cannot do much about it (other > > > than > > > removing it as you asked). > > > > And yes on the above two points. It adds both runtime overhead (albeit > > trivially small) and code complexity. > > It's trivially small cycle level overhead in something that will be > burdened by two TLB flushes anyway is is utterly slow. The code complexity not so much. > > > >> +out: > > > >> + if (memcmp(addr, opcode, len)) > > > >> + r = -EFAULT; > > > > > > > > How could this ever fail? And how can we reliably recover from that? > > > > > > This code has been there before (with slightly uglier code). Before this > > > patch, a BUG_ON() was used here. However, I noticed that kgdb actually > > > checks that text_poke() succeeded after calling it and gracefully fail. > > > However, this was useless, since text_poke() would panic before kgdb gets > > > the chance to do anything (see patch 7). > > > > Yes, I know it was there before, and I did see kgdb do it too. But aside > > from that out-label case, which we also should never hit, how can we > > realistically ever fail that memcmp()? > > > > If we fail here, something is _seriously_ buggered. > > So wouldn't it be better to just document and verify our assumptions of > this non-trivial code by using return values intelligently? The thing is, I don't think there is realistically anything the caller can do; our text is not what we expect it to be, that is a fairly fundamentally buggered situation to be in. I'm fine with validating it; I'm as paranoid as the next guy; but passing along that information seems pointless. At best we can try poking again, but that's not going to help much if it failed the first time around.
