Re: [PATCH v3 18/21] vmcore: check if vmcore objects satifymmap()'s page-size boundary requirement

From: Andrew Morton
Date: Tue Mar 19 2013 - 16:02:35 EST

On Sat, 16 Mar 2013 13:02:29 +0900 HATAYAMA Daisuke <d.hatayama@xxxxxxxxxxxxxx> wrote:

> If there's some vmcore object that doesn't satisfy page-size boundary
> requirement, remap_pfn_range() fails to remap it to user-space.
> Objects that posisbly don't satisfy the requirement are ELF note
> segments only. The memory chunks corresponding to PT_LOAD entries are
> guaranteed to satisfy page-size boundary requirement by the copy from
> old memory to buffer in 2nd kernel done in later patch.
> This patch doesn't copy each note segment into the 2nd kernel since
> they amount to so large in total if there are multiple CPUs. For
> example, current maximum number of CPUs in x86_64 is 5120, where note
> segments exceed 1MB with NT_PRSTATUS only.

I don't really understand this. Why does the number of or size of
note segments affect their alignment?

> --- a/fs/proc/vmcore.c
> +++ b/fs/proc/vmcore.c
> @@ -38,6 +38,8 @@ static u64 vmcore_size;
> static struct proc_dir_entry *proc_vmcore = NULL;
> +static bool support_mmap_vmcore;

This is quite regrettable. It means that on some kernels/machines,
mmap(vmcore) simply won't work. This means that people might write
code which works for them, but which will fail for others when deployed
on a small number of machines.

Can we avoid this? Why can't we just copy the notes even if there are
a large number of them?

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