[Devel] Re: [RFC] checkpoint: un-blacklist VM_NORESERVE

[Oren Laadan]

On 06/17/2010 05:07 PM, Nathan Lynch wrote:
> While a region mapped with MAP_NORESERVE may segv on a write in
> low-memory conditions, reading it (as we do during checkpoint) is
> fine.  Remove the VM_NORESERVE flag from CKPT_VMA_NOT_SUPPORTED.
> 
> ---
> 
> Oren, what do you think about this?  I've tested this lightly with no
> apparent problems; is there some reason I'm missing to not support
> MAP_NORESERVE regions?

Sounds reasonable. I originally left it out because I wasn't sure.
In particular, my concern was whether a user may cause troubles by
specifying this flag. Now I looked deeper, and I think it's safe.

Note, however, it isn't enough to allow the flag, we also need to
modify calc_map_flags_bits() to reintroduce MAP_NORESERVE for the
flags argument to do_mmap_pgoff(). Otherwise it won't be restored
at restart...

No need to repost the patch. I'll do it.

Oren.

> 
>  include/linux/checkpoint.h |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/checkpoint.h b/include/linux/checkpoint.h
> index 84bb7a9..f3e0d11 100644
> --- a/include/linux/checkpoint.h
> +++ b/include/linux/checkpoint.h
> @@ -301,7 +301,7 @@ extern int restore_memory_contents(struct ckpt_ctx *ctx, struct inode *inode);
>  
>  #define CKPT_VMA_NOT_SUPPORTED						\
>  	(VM_IO | VM_HUGETLB | VM_NONLINEAR | VM_PFNMAP |		\
> -	 VM_RESERVED | VM_NORESERVE | VM_HUGETLB | VM_NONLINEAR |	\
> +	 VM_RESERVED | VM_HUGETLB | VM_NONLINEAR |	\
>  	 VM_MAPPED_COPY | VM_INSERTPAGE | VM_MIXEDMAP | VM_SAO)
>  
>  /* signals */
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