Re: objrmap and vmtruncate

[Andrew Morton <akpm@digeo.com>]

Andrea Arcangeli <andrea@suse.de> wrote:
>
> the worst part IMHO is that it screwup the vma making the vma->vm_file
> totally wrong for the pages in the vma.

Not sure what you mean here.  All pages in the vma are backed by the file at
vm_file.  It is vm_pgoff which is meaningless.

As for your other concerns: yes, I hear you.  I suspect something will have
to give.  Ingo has a better feel for the problems which this code is solving
and hopefully he can comment.

Perhaps it is useful to itemise the prblems which we're trying to solve here:

- ZONE_NORMAL consumption by pte_chains

  Solved by objrmap and presumably page clustering.

- ZONE_NORMAL consumption by VMAs

  Solved by remap_file_pages.  Neither objrmap nor page clustering will
  help here.

- pte_chain setup and teardown CPU cost.

  objrmap does not seem to help.  Page clustering might, but is unlikely to
  be enabled on the machines which actually care about the overhead.

- get_unmapped_area() search complexity.

  Solved by remap_file_pages and by as-yet unimplemented algorithmic rework.

- pagefault frequency and TLB invalidation cost.

  Solved by MAP_POPULATE, could also be solved by MAP_PREFAULT (but it's
  not really a demonstrated problem).

Anything else?


So looking at the above, remap_file_pages() actually has pretty good
coverage.

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