[Question] Can we release vma that include code when one process is running?

[Question] Can we release vma that include code when one process is running?

[liyu@WAN]

Hi, All:

    I am writing a patch to swap process memory to swap partition or 
swap file.
We think this make system speed up on PC that more little memory. It have
a bit like with page replacement algorithm depend application hit, but 
ours idea is
more easier.We found, if we try swap out openoffice.org, this way  can 
save more
than 50M memory, and when switch task to run openoffice.org again, it do 
not read
all data that did swapped from swap space at most time.

    According to result of experimmnt, the anonymous memory of one 
process are more than code
and data, even, it look some shared library as private code.

    As general rule, we can only some swap anonymous memory. this's OK! 
I also done that.
However my question is if we release vma that include code when one 
process is running?
Of course, if the refcnt of vma is more than one, it can not be released 
yet. In fact,
I writing one patch to do this, some it do successfully on some simple 
process, but
in more time, I will get a SIGSEGV!

    I am running IA32 architecture, I used one little python script to 
try this. and
get SIGSEGV each time. I dump memory that include that code, they are 
too simple:
   
    ...
    nop
    int $0x80
    ret
    nop
    ...

    It seem that code in other place jump here to enter kernel. this is 
in a anonymous
code area.
    At first time, I think this SIGSEGV will trigger by anonymous code 
that is swapped,
but I wrote one specical condition check to filte out this sort of code, 
IOW, I do
not swap out it. but I still get SIGSEGV.

    May be, we can not be release the vma that include code? or, Is 
there have some errors
 in my words for page fault?

   
   
sailor

Re: [Question] Can we release vma that include code when one process is running?

[Nick Piggin]

liyu@WAN wrote:

>    It seem that code in other place jump here to enter kernel. this is 
> in a anonymous
> code area.
>    At first time, I think this SIGSEGV will trigger by anonymous code 
> that is swapped,
> but I wrote one specical condition check to filte out this sort of code, 
> IOW, I do
> not swap out it. but I still get SIGSEGV.
> 
>    May be, we can not be release the vma that include code? or, Is there 
> have some errors
> in my words for page fault?
> 

That's right, you cannot release the VMA if the application still
expects to use memory in that area. The page fault handler will
see that no VMA exists in that region and raise a SIGSEGV.

See arch/i386/mm/fault.c:do_page_fault


Nick

-- 
SUSE Labs, Novell Inc.

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Re: [Question] Can we release vma that include code when one process is running?

[liyu@WAN]

Sorry, I perhaps didn't said clearly.

As I knwon, if we remove vma from vma tree of task, the SIGSEGV must be got!
but I am not removed them , I just unmapped them. and the SIGSEGV occurs 
some times,
not alway.

I doublt on it.

Any clearly idea?

many thanks.

> liyu@WAN wrote:
>
>>    It seem that code in other place jump here to enter kernel. this 
>> is in a anonymous
>> code area.
>>    At first time, I think this SIGSEGV will trigger by anonymous code 
>> that is swapped,
>> but I wrote one specical condition check to filte out this sort of 
>> code, IOW, I do
>> not swap out it. but I still get SIGSEGV.
>>
>>    May be, we can not be release the vma that include code? or, Is 
>> there have some errors
>> in my words for page fault?
>>
>
> That's right, you cannot release the VMA if the application still
> expects to use memory in that area. The page fault handler will
> see that no VMA exists in that region and raise a SIGSEGV.
>
> See arch/i386/mm/fault.c:do_page_fault
>
>
> Nick
>

Re: [Question] Can we release vma that include code when one process is running?

[Nick Piggin]

liyu@WAN wrote:
> 
> 
> Sorry, I perhaps didn't said clearly.
> 
> As I knwon, if we remove vma from vma tree of task, the SIGSEGV must be 
> got!
> but I am not removed them , I just unmapped them. and the SIGSEGV occurs 
> some times,
> not alway.
> 
> I doublt on it.
> 
> Any clearly idea?
> 

No clear ideas, no. Are you doing the proper synchronization and
flushing required when unmapping those regions?

If you can post your unmapping code to lkml, someone might be able
to spot a bug in it.

Thanks,
Nick

-- 
SUSE Labs, Novell Inc.

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