Re: [PATCH v3 18/21] vmcore: check if vmcore objects satify mmap()'s page-size boundary requirement
From: Eric W. Biederman
Date: Tue Mar 19 2013 - 17:22:35 EST
Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> writes:
> 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?
Yes. If it simplifies things I don't see a need to support mmapping
everything. But even there I don't see much of an issue.
Today we allocate a buffer to hold the ELF header program headers and
the note segment, and we could easily allocate that buffer in such a way
to make it mmapable.
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