kernel: system_info.used_pages don't include mapped files

[X512]

This is hrev54741.

As discussed ​here, mapping files to read-only areas almost not affect system_info.used_pages that is used by most memory inspecting utilities, but it actually consumes memory and make allocations not possible if too many files are mapped. system_info.free_memory provides correct information about free memory but is also seems to include disk cache that can be freed to release memory for allocations.

If memory mapped files are consuming memory, they should be included in system_info.used_pages.

Actually it is not required to reserve memory for read-only mapped files because it can be handled in similar way as disk cache: load file contents on demand and unload when memory is needed by someone else. This approach do not violate no overcommit policy. If program change area protection and enable copy-on-write, attempt may failed with out of memory if memory can't be reserved.

system_info.used_pages is calculated ​here.

[X512]

Test program.

[X512]

I attached test program that reproduce problem.

Steps to reproduce.

1. Run dd if=/dev/random of=TestFile bs=1M count=32
2. Run gcc MMap.cpp -o MMapTest
3. Run MMapTest
4. MMapTest will get out of memory and freeze. system_info.free_memory will be almost zero, system_info.needed_memory will be 32 MB, system_info.used_pages will be almost unaffected. System will be in OOM state, running applications and doing other tasks that need to allocate memory will fail.
5. Press Ctrl+C to terminate.

[X512]

In test program it is also possible to use shared memory for multiple memory mappings of same file.

[waddlesplash]

This is probably an accounting/flags mix-up somewhere. Regular disk cache memory (not mapped) is part of the "cache" memory, which the VM will readily steal (non-dirty) pages from when it needs to allocate actual memory. mmap'ed files actually utilize the disk cache subsystem, so somehow I guess when files are mmap'ed, their pages are actually reserved and can no longer be stolen. Seems like some kind of a bug indeed.