Process Virtual Memory

Limits

Max Number of VMAs

By default, the maximum number of VMAs is: 65530. It is defined by the following variable:

#define MAPCOUNT_ELF_CORE_MARGIN        (5)
#define DEFAULT_MAX_MAP_COUNT   (USHRT_MAX - MAPCOUNT_ELF_CORE_MARGIN)

int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT;

Facts

munmap can split vma

munmap can create a hole with an existing vma, thus divide one existing vma to two new vmas. Do note that, munmap can create hole for both anonymous vma and file-backed vma.

msync() is not atomic

During msync(), pages are being written back to disk one by one (or batched). Consider the case where few pages have been flushed back, while some other few pages are still in the memory. This premature writeback is not atomic and will be affected by failure.

msync() need concurrency control

With a multi-threaded application, does msync() provide the synchronization semantic? The answer is NO. Other threads within the same process are able to write to pages currently under msync(). This implies that application need to handle concurrency by themselves, e.g., rwlocks.

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Yizhou Shan
Created: Feb 19, 2018
Last Updated: Feb 19, 2018