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    SubjectRe: No more bits in vm_area_struct's vm_flags.
    Hi.

    KAMEZAWA Hiroyuki wrote:
    > On Fri, 25 Sep 2009 18:34:56 +1000
    > Nigel Cunningham <ncunningham@crca.org.au> wrote:
    >> Hi.
    >>
    >> KAMEZAWA Hiroyuki wrote:
    >>>> I have some code in TuxOnIce that needs a bit too (explicitly mark the
    >>>> VMA as needing to be atomically copied, for GEM objects), and am not
    >>>> sure what the canonical way to proceed is. Should a new unsigned long be
    >>>> added? The difficulty I see with that is that my flag was used in
    >>>> shmem_file_setup's flags parameter (drm_gem_object_alloc), so that
    >>>> function would need an extra parameter too..
    >>> Hmm, how about adding vma->vm_flags2 ?
    >> The difficulty there is that some functions pass these flags as arguments.
    >>
    > Ah yes. But I wonder some special flags, which is rarey used, can be moved
    > to vm_flags2...
    >
    > For example,
    >
    > #define VM_SEQ_READ 0x00008000 /* App will access data sequentially */
    > #define VM_RAND_READ 0x00010000 /* App will not benefit from clustered reads */
    > are all capsuled under
    > mm.h
    > 117 #define VM_READHINTMASK (VM_SEQ_READ | VM_RAND_READ)
    > 118 #define VM_ClearReadHint(v) (v)->vm_flags &= ~VM_READHINTMASK
    > 119 #define VM_NormalReadHint(v) (!((v)->vm_flags & VM_READHINTMASK))
    > 120 #define VM_SequentialReadHint(v) ((v)->vm_flags & VM_SEQ_READ)
    > 121 #define VM_RandomReadHint(v) ((v)->vm_flags & VM_RAND_READ)
    >
    > Or
    >
    > 105 #define VM_PFN_AT_MMAP 0x40000000 /* PFNMAP vma that is fully mapped at mmap time */
    > is only used under special situation.
    >
    > etc..
    >
    > They'll be able to be moved to other(new) flag field, IIUC.

    I'm working on a patch to do this, and am looking at is_mergeable_vma,
    which is invoked via can_vma_merge_after from vma_merge from
    madvise_behaviour (which potentially modifies these hint flags). Should
    those hints be considered in that function? (Do I need to pass the hints
    in as well and check they're equal?)

    By the way, VM_ClearReadHint and VM_NormalReadHint are currently unused.
    madvise_behaviour manipulates the flags directly (in preparing
    potential replacement values). Not sure if something should be done
    about that.

    By the way #2, in response to the later message in this thread, I'm
    calling the new var vma->vm_hints, and have put it at the end of the
    struct at the moment. Is that a good place?

    Regards,

    Nigel


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