[Woart]

Summary

Replace env_malloc() / env_free() with env_vmalloc() / env_vfree() for temporary metadata segment buffers.

The existing kmalloc() allocation can fail with ENOMEM under severe physical memory fragmentation, preventing Open CAS from loading cache metadata despite sufficient available RAM.

Temporary metadata copies do not require physically contiguous memory. The change also avoids unnecessary buffer zeroing.

Validation

Tested on Open CAS 26.09, Ubuntu 24.04, Linux 6.8.0-147-generic:

• WT and WA cache modes
• Successful cache Stop → Load recovery
• 32/32 functional tests passed
• Simulated SSD EIO and 200 ms latency
• Cache eviction, concurrent I/O and SHA256 integrity verified

The fix follows the approach suggested by the maintainer in the related bug report.

Related issue: Open-CAS/open-cas-linux#1837

[Woart]

I've submitted the fix as #993, using env_vmalloc() / env_vfree() as suggested.
The patch has passed our WT/WA functional and fault-injection tests.
Thank you for reviewing it!