From: Eric Dumazet <dada1@cosmosbay.com>
To: Albert Cahalan <acahalan@gmail.com>
Cc: linux-kernel <linux-kernel@vger.kernel.org>,
Davide Libenzi <davidel@xmailserver.org>
Subject: Re: JIT emulator needs
Date: Fri, 08 Jun 2007 09:09:18 +0200 [thread overview]
Message-ID: <4669009E.3000702@cosmosbay.com> (raw)
In-Reply-To: <787b0d920706072335v10d6025cwe1437194b6c60d84@mail.gmail.com>
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 )
next prev parent reply other threads:[~2007-06-08 7:09 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-06-08 6:35 JIT emulator needs Albert Cahalan
2007-06-08 7:09 ` Eric Dumazet [this message]
2007-06-09 4:12 ` Albert Cahalan
2007-06-08 11:10 ` Alan Cox
2007-06-08 16:35 ` Nicholas Miell
2007-06-09 5:17 ` Albert Cahalan
2007-06-09 20:00 ` H. Peter Anvin
2007-06-19 15:08 ` William Lee Irwin III
2007-06-20 3:16 ` Albert Cahalan
2007-06-20 16:01 ` William Lee Irwin III
2007-06-20 16:37 ` H. Peter Anvin
2007-06-20 17:54 ` William Lee Irwin III
2007-06-20 18:23 ` H. Peter Anvin
2007-06-20 18:25 ` Albert Cahalan
2007-06-20 18:51 ` H. Peter Anvin
2007-06-21 3:21 ` Albert Cahalan
2007-06-21 3:32 ` H. Peter Anvin
2007-06-21 7:38 ` Albert Cahalan
2007-06-20 18:43 ` Albert Cahalan
2007-06-23 3:52 ` Kyle Moffett
2007-06-24 4:14 ` William Lee Irwin III
2007-06-21 17:44 ` Arjan van de Ven
2007-06-22 5:56 ` Albert Cahalan
2007-06-22 13:43 ` Arjan van de Ven
2007-06-22 14:32 ` Albert Cahalan
2007-06-22 14:42 ` Arjan van de Ven
2007-06-23 2:30 ` Albert Cahalan
[not found] <8tGiE-2Hv-1@gated-at.bofh.it>
[not found] ` <8xNvm-2Tw-29@gated-at.bofh.it>
[not found] ` <8xYTM-3So-13@gated-at.bofh.it>
2007-06-21 11:08 ` Bodo Eggert
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