In https://github.com/mongodb/mongo/blob/master/src/mongo/db/db.cpp#L614
the function checkReadAhead() introduced by Core/SERVER-7335
https://jira.mongodb.org/browse/SERVER-7335
uses boost::filesystem::exists() without checks for exceptions.

An exception will be thrown if the path can't be accessed (due to insufficient access rights in my case). This will crash the startup.

I gather that checking the readahead is informational only. It would be an improvement to just log the error and carry on.

The attached patch will catch the filesystem exception and log a line of
Wed Oct 2 13:52:40.434 [initandlisten] boost::filesystem::status: Permission denied: "/sys/dev/block/9:0/queue/read_ahead_kb"

See here for the changes:
https://github.com/zuckschwerdt/mongo/commit/f0b286380b4647ebd7a1e084b84309f12bc8e087

And here for the complete patch with indentation
https://github.com/zuckschwerdt/mongo/commit/581f3abe6b1faa2a65168c3044eb1e8f737c07b8