[Toybox] [PATCH] memeater: fixes.

[enh]

On Fri, Jan 26, 2024 at 3:41 PM Rob Landley <rob at landley.net> wrote:

> On 1/26/24 11:28, enh via Toybox wrote:
> > The ULONG_MAX turns into -1 here, which isn't what was intended.
>
> Ah. That one ONLY works on 32 bit. Oops.
>

(yeah, sorry, i realized after sending my patch that my LP64 bigotry was
showing again, and it might not be obvious _why_.)


> (I went down a research rathole trying to work out whether the
> spectre/meltdown
> mitigations that unmapped the kernel from normal userspace brought back
> Ingo's
> old 4G/4G mapping or not on 32 bit, didn't really get a useful answer,
> went with
> unsigned rather than try to read through the kernel code... and missed the
> obvious.)
>
> > I'll avoid the bikeshed of the least worst full fix and take half
> > the range which is plenty for my purposes.
>
> Neither Intel nor Arm support the full 64 bits (Intel did 5-level page
> tables to
> hit 57 bits, Arm increased page size to hit 52, and atomic limits are
> really
> starting to bite ala https://www.youtube.com/watch?v=2G4_RZo41Zw), so
> signed
> long is probably fine for the forseeable future. :)
>

the main users for this are working on 32-bit devices anyway, and
explicitly only devices that have less than 4GiB anyway (if i've understood
https://source.android.com/docs/compatibility/14/android-14-cdd#76_memory_and_storage
correctly).


> > Also fix the array indexing to not segfault.
>
> Sigh, I only ran "memeater 10k" on my laptop (in a 32 bit build). My
> musl-cross
> toolchain hasn't got ASAN wired up and hangs right after so never noticed.
> (Still embarassing...)
>
> Your fix is writing more than once per page. I'm adjusting stride already
> in the
> increment, the problem is size needs to be adjusted to not go off the end.
> (Moving adjustment to the index adds math to the loop instead of resolving
> it at
> compile time, but eh, keep that one. I fixed up your patch with vi, but
> actually
> tested the result this time. :)
>
> The _next_ problem is trying "memeater 1m" so it actually gets some
> nontrivial
> exercise, I had to sudo it to avoid:
>
> $ ./memeater 1m
> memeater: mlock: Cannot allocate memory
>
> With the 10k I didn't get that objection, because:
>
> $ ulimit -l
> 64
>
> Should I make the mlock() not be the default, presumably switching to -m to
> enable rather than disable it? (Dunno what your android environment looks
> like
> here...)
>

"works for me" on _my_ hosts, but i haven't actually tested on Android yet,
and that's very likely to be next week at this point...


> Commit 1e04fb285c3c.
>

...especially because this isn't showing in github yet :-)


> Rob
>
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