Why would some process swap in place of reclaiming free(cached) memory

[Soham Chakraborty]

Hi all,

I posted this question in linux-mm list as well but it didn't spawn much
interest. So, I am putting the same question here also. Would love if
someone can put some traction.

The question is salivating and simple. When we have free and lotsa cached
memory in a system (irrespective of distro and kernel), why would some
process end up in swap space, I understand the overcommit mode of vm, the
default value of 0 and what it does. I also know that the cache might be
dirty and we don't might not want to allocate free pages thinking that we
might go under the watermark of low. But, I mainly end up getting this
question from end users - why would things swap if I have free memory. I
guess, technically we can't stop swapping even if we set vm.swappiness to 0
but well, I think I have said what I need to said. If you need data, I can
provide those.

Any reference to source where it gives the condition to transfer some pages
to swap, despite of having free memory, would be highly appreciated. I
tried to find in mm/vmscan.c but didn't get much success.

Soham
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