Re: jemalloc SEGV problem

[James Chacon <jmc%netbsd.org@localhost>]

On Nov 17, 2008, at 11:09 PM, Channa wrote:

> Hi,
> mmap does not fail.
> mamap does allocate memory for the variable "chunk"
>
> and then when the program tries to modify "chunk"  SEGV is recieved.
>
> I checked the problem is below:
> The variable
> 'pagesize_2pow' is calculated as
>
>                result = sysconf(_SC_PAGESIZE);
>                assert(result != -1);
>                pagesize = (unsigned) result;
>
>                /*
>                 * We assume that pagesize is a power of 2 when  
> calculating
>                 * pagesize_mask and pagesize_2pow.
>                 */
>                assert(((result - 1) & result) == 0);
>                pagesize_mask = result - 1;
>                pagesize_2pow = ffs((int)result) - 1;
>
> In my case case pagesize_2pow comes as 11.
>
> Since sysconf(_SC_PAGESIZE) returns 4096.
>
> If i retain the macro value CHUNK_2POW_DEFAULT as 20 and initialize
>  pagesize_2pow = 19
> which means my pagesize is 2^20..
>
> jemalloc() works fine , does not give any SEGV.
>
> In anycase mamp () is not failing.
> I feel the problem is with the alignment of the memory which is not  
> happening
> properly..
> Also i checked none of the variables are overflowing.


Does jemalloc assume that mmap'ing a given sized chunk will result in  
getting back a chunk aligned to that size?

i.e. if I mmap 1M is it assuming it gets back something on a 1M  
boundary?

ptmalloc2/3 do this sort of thing for speed for instance so that  
getting handed any given chunk of memory into free() it's able to find  
it's internal structures by storing them at the top of the aligned  
segment.

James

> 2008/11/17 Christos Zoulas <christos%zoulas.com@localhost>:
> > On Nov 17, 12:42pm, channa.kad%gmail.com@localhost (Channa) wrote:
> > -- Subject: Re: jemalloc SEGV problem
> >
> > | Hi,
> > | Ya i understand but when i changed the pagesize to 64K in mips arch
> > |  and retained the macro CHUNK_2POW_DEFAULT as present in jemalloc.c
> > | original file.I dint get any seg fault.
> > |
> > | But when i changed the pagesize again to 4K i got SEGV.
> > |
> > | Regards,
> > | Channa
> >
> > What does the ktrace of your program show? Does the mmap fail?
> >
> > christos