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    SubjectRe: [RFC PATCH 0/8] remove vm_struct list management
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    Hello, Vivek.

    2012/12/7 Vivek Goyal <vgoyal@redhat.com>:
    > On Fri, Dec 07, 2012 at 10:16:55PM +0900, JoonSoo Kim wrote:
    >> 2012/12/7 Andrew Morton <akpm@linux-foundation.org>:
    >> > On Fri, 7 Dec 2012 01:09:27 +0900
    >> > Joonsoo Kim <js1304@gmail.com> wrote:
    >> >
    >> >> I'm not sure that "7/8: makes vmlist only for kexec" is fine.
    >> >> Because it is related to userspace program.
    >> >> As far as I know, makedumpfile use kexec's output information and it only
    >> >> need first address of vmalloc layer. So my implementation reflect this
    >> >> fact, but I'm not sure. And now, I don't fully test this patchset.
    >> >> Basic operation work well, but I don't test kexec. So I send this
    >> >> patchset with 'RFC'.
    >> >
    >> > Yes, this is irritating. Perhaps Vivek or one of the other kexec
    >> > people could take a look at this please - if would obviously be much
    >> > better if we can avoid merging [patch 7/8] at all.
    >>
    >> I'm not sure, but I almost sure that [patch 7/8] have no problem.
    >> In kexec.c, they write an address of vmlist and offset of vm_struct's
    >> address field.
    >> It imply that user for this information doesn't have any other
    >> information about vm_struct,
    >> and they can't use other field of vm_struct. They can use *only* address field.
    >> So, remaining just one vm_struct for vmlist which represent first area
    >> of vmalloc layer
    >> may be safe.
    >
    > I browsed through makedumpfile source quickly. So yes it does look like
    > that we look at first vmlist element ->addr field to figure out where
    > vmalloc area is starting.
    >
    > Can we get the same information from this rb-tree of vmap_area? Is
    > ->va_start field communication same information as vmlist was
    > communicating? What's the difference between vmap_area_root and vmlist.

    Thanks for comment.

    Yes. vmap_area's va_start field represent same information as vm_struct's addr.
    vmap_area_root is data structure for fast searching an area.
    vmap_area_list is address sorted list, so we can use it like as vmlist.

    There is a little difference vmap_area_list and vmlist.
    vmlist is lack of information about some areas in vmalloc address space.
    For example, vm_map_ram() allocate area in vmalloc address space,
    but it doesn't make a link with vmlist. To provide full information
    about vmalloc address space,
    using vmap_area_list is more adequate.

    > So without knowing details of both the data structures, I think if vmlist
    > is going away, then user space tools should be able to traverse vmap_area_root
    > rb tree. I am assuming it is sorted using ->addr field and we should be
    > able to get vmalloc area start from there. It will just be a matter of
    > exporting right fields to user space (instead of vmlist).

    There is address sorted list of vmap_area, vmap_area_list.
    So we can use it for traversing vmalloc areas if it is necessary.
    But, as I mentioned before, kexec write *just* address of vmlist and
    offset of vm_struct's address field.
    It imply that they don't traverse vmlist,
    because they didn't write vm_struct's next field which is needed for traversing.
    Without vm_struct's next field, they have no method for traversing.
    So, IMHO, assigning dummy vm_struct to vmlist which is implemented by [7/8] is
    a safe way to maintain a compatibility of userspace tool. :)

    Thanks.


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