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    SubjectRe: vm_ops.page_mkwrite() fails with vmalloc on 2.6.23
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    On Mon, 2007-10-29 at 19:17 +0100, Peter Zijlstra wrote:
    > On Mon, 2007-10-29 at 13:51 -0400, Jaya Kumar wrote:
    > > On 10/29/07, Peter Zijlstra <peterz@infradead.org> wrote:
    > > > On Mon, 2007-10-29 at 01:17 -0700, Jaya Kumar wrote:
    > > > > An aside, I just tested that deferred IO works fine on 2.6.22.10/pxa255.
    > > > >
    > > > > I understood from the thread that PeterZ is looking into page_mkclean
    > > > > changes which I guess went into 2.6.23. I'm also happy to help in any
    > > > > way if the way we're doing fb_defio needs to change.
    > > >
    > > > OK, seems I can't read. Or at least, I missed a large part of the
    > > > problem.
    > > >
    > > > page_mkclean() hasn't changed, it was ->page_mkwrite() that changed. And
    > > > looking at the fb_defio code, I'm not sure I understand how its
    > > > page_mkclean() use could ever have worked.
    > > >
    > > > The proposed patch [1] only fixes the issue of ->page_mkwrite() on
    > > > vmalloc()'ed memory. Not page_mkclean(), and that has never worked from
    > > > what I can make of it.
    > > >
    > > > Jaya, could you shed some light on this? I presume you had your display
    > > > working.
    > > >
    > >
    > > I thought I had it working. I saw the display update after each
    > > mmap/write sequence to the framebuffer. I need to check if there's an
    > > munmap or anything else going on in between write sequences that would
    > > cause it to behave like page_mkclean was working.
    > >
    > > Is it correct to assume that page_mkclean should mark the pages
    > > read-only so that the next write would again trigger mkwrite?
    >
    > Well, yes, that is the intended behaviour.
    >
    > > Even if the page was from a vmalloc_to_page()?
    >
    > That is the crux, I only ever implemented it for file pages.

    Hmm, so these vmalloc pages are mapped into user-space with
    remap_pfn_range(), which doesn't have any form of rmap. That is, given a
    pfn there is no way to obtain all ptes for it. So the interface to
    page_mkclean() could never work for these (as it only provides a struct
    page *).

    [ also, remap_vmalloc_range() suffers similar issues, only file and anon
    have proper rmap ]

    I'm not sure we want full rmap for remap_pfn/vmalloc_range, but perhaps
    we could assist drivers in maintaining and using vma lists.

    I think page_mkclean_one() would work if you'd manually set page->index
    and iterate the vmas yourself. Although atm I'm not sure of anything so
    don't pin me on it.

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