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    SubjectRe: [PATCH v3 18/21] vmcore: check if vmcore objects satify mmap()'s page-size boundary requirement
    Andrew Morton <akpm@linux-foundation.org> writes:

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

    Eric


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