Messages in this thread | | | Date | Fri, 08 Jun 2007 09:09:18 +0200 | From | Eric Dumazet <> | Subject | Re: JIT emulator needs |
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Albert Cahalan a écrit : > Right now, Linux isn't all that friendly to JIT emulators. > Here are the problems and suggestions to improve the situation. > > There is an SE Linux execmem restriction that enforces W^X. > Assuming you don't wish to just disable SE Linux, there are > two ugly ways around the problem. You can mmap a file twice, > or you can abuse SysV shared memory. The mmap method requires > that you know of a filesystem mounted rw,exec where you can > write a very large temporary file. This arbitrary filesystem, > rather than swap space, will be the backing store. The SysV > shared memory method requires an undocumented flag and is > subject to some annoying size limits. Both methods create > objects that will fail to be deleted if the program dies > before marking the objects for deletion. > > Processors often have annoying limits on the immediate values > in instructions. An x86 or x86_64 JIT can go a bit faster if > all allocations are kept to the low 2 GB of address space. > There are also reasons for a 32bit-to-x86_64 JIT to chose > a nearly arbitrary 2 GB region that lies above 4 GB. > Other archs have other limits, such as 32 MB or 256 MB. > > Sometimes it is very helpful to have the read/write mapping > be a fixed offset from the read/exec mapping. A power of 2 > can be especially desirable. > > Emulators often need a cheap way to change page permissions. > One VMA per page is no good. Besides taking up space and making > many things generally slower, having one VMA per page causes > a huge performance loss for snapshot roll-back operations. > Just tearing down all those VMAs takes a good while. > > Additions to better support JIT emulators: > > a. sysctl to set IPC_RMID by default
Not very good, this will break some apps.
> b. shmget() flag to set IPC_RMID by default
This is better :)
> c. open() flag to unlink a file before returning the fd
Well, I assume you would like fd = open("/path/somefile", O_RDWR | O_CREAT | O_UNLINK, 0644)
(ie allocate a file handle but no name ?)
Quite difficult to implement this atomically with current vfs, maybe a new syscall would be better. (Linus will kill me for that :) )
(We dont need to insert "somefile" in one directory, then unlink it, we only need to allocate an unnamed inode to get some backing store)
This is a generalization of anonymous inodes ( fs/anon_inodes.c )
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