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    SubjectRe: [RFC][PATCH] vmalloc: simplify vread()/vwrite()
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    On Thu, 2010-01-07 at 10:50 +0800, Wu, Fengguang wrote:
    > On Thu, Jan 07, 2010 at 09:38:25AM +0800, KAMEZAWA Hiroyuki wrote:
    > > On Thu, 7 Jan 2010 09:24:59 +0800
    > > Wu Fengguang <fengguang.wu@intel.com> wrote:
    > >
    > > > vread()/vwrite() is only called from kcore/kmem to access one page at
    > > > a time. So the logic can be vastly simplified.
    > > >
    > > I recommend you to rename the function because safety of function is
    > > changed and you can show what callers are influenced.
    >
    > OK.
    >
    > > > The changes are:
    > > > - remove the vmlist walk and rely solely on vmalloc_to_page()
    > > > - replace the VM_IOREMAP check with (page && page_is_ram(pfn))
    > > >
    > > > The VM_IOREMAP check is introduced in commit d0107eb07320b for per-cpu
    > > > alloc. Kame, would you double check if this change is OK for that
    > > > purpose?
    > > >
    > > I think VM_IOREMAP is for avoiding access to device configuration area and
    > > unexpected breakage in device. Then, VM_IOREMAP are should be skipped by
    > > the caller. (My patch _just_ moves the avoidance of callers to vread()/vwrite())
    >
    > "device configuration area" is not RAM, so testing of RAM would be
    > able to skip them?
    >
    > >
    > > > The page_is_ram() check is necessary because kmap_atomic() is not
    > > > designed to work with non-RAM pages.
    > > >
    > > I think page_is_ram() is not a complete method...on x86, it just check
    > > e820's memory range. checking VM_IOREMAP is better, I think.
    >
    > (double check) Not complete or not safe?
    >
    > EFI seems to not update e820 table by default. Ying, do you know why?

    In EFI system, E820 table is constructed from EFI memory map in boot
    loader, so I think you can rely on E820 table.

    Best Regards,
    Huang Ying




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