[londons_explore]

Id be interested to know if any program relies on freeing memory in one thread, then (with some lock for synchronization) deliberately accessing it from another thread causing a segfault.

Sure, it's possible, but if no programs do it, it might be worth breaking the behaviour for the performance increase.

[kentonv]

If the program is just using mmap/munmap for memory allocation, then I agree with you.

But some programs do crazier things. Imagine a program that implements mmap of a file entirely in userspace. For example, say I have a file that's stored on a distributed filesystem that dosen't have kernel drivers. So I catch SIGSEGV in userspace and populate pages on-demand by fetching them from the remote server. Later on, the page hasn't been used in a while, so I munmap() it to reclaim memory. I expect that any further uses after this point (especially writes, which maybe I need to sync back?) will raise a new SIGSEGV which I can handle.

I think something like that, unfortunately, could be broken by changing all munmaps to be lazy...

[markjdb]

I was wondering how this scheme might break applications that do clever things with a SIGBUS/SIGSEGV handler. Do you happen to know of any OSS that actually does something like this?

[saagarjha]

FUSE?