[PATCH] D30010: Improve the robustness of mmap

[Zachary Turner via Phabricator via llvm-commits]

zturner added a comment.

In https://reviews.llvm.org/D30010#679198, @ruiu wrote:

> I wonder what this patch is trying to fix on Unix machines. mmap()'ing a file on NFS is not a special thing, and if something goes wrong with NFS, a process will hang whether a file is being used through read(2)/write(2) or mmap. What am I missing?


If you just `read()` the entire contents, in order for the call to fail the NFS would have to go down during the very short period of time where you are reading the entire file contents into memory.  If you `mmap` the contents and then keep the mapping open for, say several hours, then if the NFS has a hiccup at any point during those several hours, a call to read could crash.  Any time you are dealing with a network file system you of course might encounter problems, I think this just minimizes the likelihood of those problems occuring in practice.

The motivation for this change came when I tried to replace LLDB's mmap code with LLVM's.  But I was told that checking if the file is local is a requirement, because they've had bugs in the past where they crash all of Xcode because they try to mmap some debug info off of a network share and then some time later during debugging, the share goes down and `read` crashes all of Xcode.


https://reviews.llvm.org/D30010