Messages in this thread | | | Date | Tue, 16 Nov 2021 11:06:18 -0800 | Subject | Re: [PATCH RFC v2 2/2] mm: Rework swap handling of zap_pte_range | From | John Hubbard <> |
| |
On 11/16/21 05:11, Matthew Wilcox wrote: > On Tue, Nov 16, 2021 at 12:51:13AM -0800, John Hubbard wrote: >> On 11/15/21 05:57, Matthew Wilcox wrote: >>> On Mon, Nov 15, 2021 at 09:49:51PM +0800, Peter Xu wrote: >>>> Clean the code up by merging the device private/exclusive swap entry handling >>>> with the rest, then we merge the pte clear operation too. >>>> >>>> struct* page is defined in multiple places in the function, move it upward. >>> >>> Is that actually a good thing? There was a time when declaring >> >> Yes. It is a very good thing. Having multiple cases of shadowed variables >> (in this case I'm using programming language terminology, or what I >> remember it as, anyway) provides lots of opportunities to create >> hard-to-spot bugs. > > I think you're misremembering. These are shadowed variables:
OK, I remembered correctly, but read the diffs a little too quickly, and...
> > int a; > > int b(void) > { > int a; > if (c) { > int a; > } > } > > This is not: > > int b(void) > {
...missed that there is no longer a "int a" at the top level. But it does still present a small land mine, in that just adding a top level "int a" creates all these shadowed variables (not necessarily bugs, yet, I know).
It's less of an issue here, then I first thought. Generally, it's probably best to either use "int a" throughout, or differently named variables at lower levels...or make smaller functions. Because if a variable name is reused a lot in the same function then there is likely a relationship of sorts between the instances, and it's worth deciding what that is.
> if (c) { > int a; > } else { > int a; > } > } > > I really wish we could turn on -Wshadow, but we get compilation warnings > from header files right now. Or we did last time I checked. >
...and as you say, it would be nice if the programmer could just let the compiler figure out if there is a real problem. The elaborate rituals to stay out of harm's way are not as good as a tool that checks. :)
thanks, -- John Hubbard NVIDIA
| |