[LTP] [PATCH v3] lib: memutils: respect minimum memory watermark when polluting memory

[Li Wang]

> >> Therically this is correct, and I believe it will work on your tested
> >> machine.
> >>
> >> But my concern is ioctl_sg01 still fails on the special system which
> >> MemAvai < MemFree.
> >>
> >> Just like the one Xinpeng mentioned before:
> >> https://lists.linux.it/pipermail/ltp/2021-January/020817.html
> >> <https://lists.linux.it/pipermail/ltp/2021-January/020817.html>
> >>
> >> [root@test-env-nm05-compute-14e5e72e38
> >> <mailto:root@test-env-nm05-compute-14e5e72e38>~]# cat /proc/meminfo
> >>
> >> MemTotal:       526997420 kB
> >> MemFree:        520224908 kB
> >> MemAvailable:   519936744 kB
> >> ...
> >>
> >> [root@test-env-nm05-compute-14e5e72e38 <mailto:
> root@test-env-nm05-compute-14e5e72e38> ~]# cat
> /proc/sys/vm/min_free_kbytes
> >> 90112
> >>
> >>
> >> There even reserve the safety to the 128MB, still less than the gap
> >> between MemFree and MemAvailable.
> >>
> >> MemFree (520224908 kB) - MemAvailable (520224908 kB) = 288164 kB  >
> safety
> >
> > I don't have such case and I am not sure it is reasonable.
> >
> > As mentioned in the thread there it looks unusual to have less available
> > memory than free. Maybe the system has some weird memory accounting
> > because MemAvailable is counted from MemFree by adding memory which can
> > be reclaimed. When adding a non-negative number, you should not end up
> > with lower MemAvailable than MemFree. :)
> >
> > Maybe that's the reason why that patch was not accepted - the system is
> > not vanilla, not common?
>
> OK, I found a possible explanation (on vanilla kernel) - the
> totalreserve_pages. This is the only subtraction from free memory when
> counting available. This could happen if someone was setting sysctl
> lowmem_reserve_ratio or min_free_kbytes.
>

That's exactly, beside the two controllers, you could also get such
a system with enabling smaller swap space on aarch64/x86_64.

(I did that and found that 'MemFree > MemAvail' is common to see)


> When setting min_free_kbytes, this will be reflected in
> /proc/sys/vm/min_free_kbytes, so we are good.
>
> When setting vm.lowmem_reserve_ratio, this will be missed by my patch
> and MemAvailable could be lower than MemFree.
>

Yes, we have to face different kinds of system configuration in testing.

Maybe we'd better keep the (info.freeram/64) in MAX() as well,
Or, do a comparison between MemFree and MemAvail to choose
the smaller one as baseline.

-- 
Regards,
Li Wang
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