Re: mmap implementation advice needed.

[christos%astron.com@localhost (Christos Zoulas)]

In article <1ce8eac5-3639-aec1-0e0c-fe857f49bc2c%ludd.ltu.se@localhost>,
Anders Magnusson  <ragge%ludd.ltu.se@localhost> wrote:
>Hi tech-kern,
>
>I'm trying to solve PR#28379 and ran into a problem and I don't really 
>understand how it is supposed to work:
>If a process tries to mmap for example a file with a length of just over 
>1GB it will always succeed as I understand the code, but that may not be 
>true depending on the underlying hardware, and I cannot find any way to 
>control this from the MD code...?
>
>On vax, for example, large mmap's cannot be done due to hardware 
>constraints.
>In the above example it will cause the mmap() to succeed, but when 
>touching the pages it will hang forever since there will never be 
>available pte's.
>
>So, any advice how a max size of allowed mmap'able memory be controlled?
>
>Notes about vax memory management if someone is wondering:
>- 2 areas (P0 and P1) of size 1G each, P0 grows from bottom, P1 grows 
>from top (intended for stack).
>- The PTEs for KVM must be in contiguous physical memory, hence the 
>allocation for one process with all of P0 and P1 mapped takes 128k.
>- Vax uses VM_MAP_TOPDOWN so that not too much of KVM space is needed 
>for mmap.

Perhaps we should add a resource limit for contiguous memory allocations.
RLIMIT_MEMCONT?  The actual value can be MD.

christos