Fixing OOM created by the admission hook

[Patrick-Pataky]

In the context of CS477: Advanced Operating Systems at EPFL, we (@AgeX21, @TizianKurz, and me) reproduced and extended linux-cache-ext as part of a class project. As part of this work, we implemented a custom admission policy based on TinyLFU.

We discovered a critical OOM bug. Specifically, when a page is not admitted into the cache, a temporary page is allocated but never freed correctly. As a result, the memory is filled in a matter of seconds in a memory intensive application.

This pull request fixes the issue by:

• Correcting the reference count handling for temporary pages.
• Cleaning the page allocation and freeing paths, for an easier maintenance.

In our tests, these changes eliminate the memory leak. We are happy to provide additional details if needed.

[tzussman]

Thanks! We were aware of this issue, but hadn't prioritized fixing it, so this is great. Will review and merge in the next few days.

Glad to hear EPFL is testing this out :) Would love to hear some more about how TinyLFU performed