Re: [uml-devel] was is the max. memory for a 32 bit UML decreased ?

[Richard Weinberger <richard@nod.at>]

Am 10.01.2015 um 19:11 schrieb Toralf Förster:
> On 01/10/2015 06:45 PM, Richard Weinberger wrote:
>> Am 10.01.2015 um 18:37 schrieb Toralf Förster:
>>> On 01/10/2015 06:25 PM, Richard Weinberger wrote:
>>>> Am 10.01.2015 um 17:08 schrieb Toralf Förster:
>>>>> yes, it never worked with values above 2026M (tested at an old ThinkPad T420, running a 32 bit Gentoo Linux with host kernel 3.17.4 ) 
>>>>
>>>> The theoretical max. was always 2GiB due to the limited address space on ia32.
>>>> 2GiB - UML binary - ASLR overhead/padding is what you can use.
>>>> But the "write: File too large" issue is something new. I bet it origins from tmpfs or shm on the host side.
>>>>
>>>> Thanks,
>>>> //richard
>>>>
>>> Well, I already had explicitly to define the mount, b/c Gentoo's default is "noexec":
>>>
>>> tfoerste@n22 ~ $ mount | grep shm
>>> shm on /dev/shm type tmpfs (rw,nosuid,nodev,relatime)
>>
>> Can it carry a 2GiB file?
>>
> 
> Ick, good hint, I set now the size to 3 GB :
> tfoerste@t44 ~ $ ssh tfoerste@n22 grep shm /etc/fstab 
> shm                     /dev/shm        tmpfs           nodev,nosuid,size=3G 
> 
> and tried after re-mounting with 2026M, but got :
> 
> $ start_uml.sh -r trinity                
> + /home/tfoerste/devel/linux/linux earlyprintk ubda=/home/tfoerste/virtual/uml/trinity ubdb=/mnt/ramdisk/trinity_swap eth0=tuntap,tap0,72:ef:3d:9f:c3:5a mem=2026M con0=fd:0,fd:1 con=pts umid=uml_trinity rootfstype=ext4 softlockup_all_cpu_backtrace=1
> Locating the bottom of the address space ... 0x1000
> Locating the top of the address space ... 0xc0000000
> Core dump limits :
>         soft - 0
>         hard - NONE
> Checking that ptrace can change system call numbers...OK
> Checking syscall emulation patch for ptrace...OK
> Checking advanced syscall emulation patch for ptrace...OK
> Checking environment variables for a tempdir...none found
> Checking if /dev/shm is on tmpfs...OK
> Checking PROT_EXEC mmap in /dev/shm...OK
> Checking for the skas3 patch in the host:
>   - /proc/mm...not found: No such file or directory
>   - PTRACE_FAULTINFO...not found
>   - PTRACE_LDT...not found
> UML running in SKAS0 mode
> Adding 13238272 bytes to physical memory to account for exec-shield gap
> Kernel virtual memory size shrunk to 944766976 bytes

UML seems only be to able to use 901MiB.

> /home/tfoerste/workspace/bin/start_uml.sh: line 115:  4195 Segmentation fault      $LINUX earlyprintk ubda=$ROOTFS ubdb=$SWAP eth0=$NET mem=$MEM $TTY umid=uml_$NAME rootfstype=ext4 "$ARGS"
> + RC=139
> + set +x
> 
> 
> Nevertheless I tried again, it worked now. The limit is now 2031M as far as I can tell.

Above you wrote 2026MiB?!
Does it work or not? /me us confused.

BTW: Due to ASLR the limit will be still fuzzy.
We definitely need to add more tests into UML to detect too small tmpfs mounts.

Thanks,
//richard

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