Re: [PATCH 00/07][RFC] Remove mapcount from struct page

[Hugh Dickins]

On Thu, 8 Dec 2005, Magnus Damm wrote:
> This patchset tries to remove page->_mapcount.

Interesting.  I share your feeling that it ought to be possible to
get along without page->_mapcount, but I've not succeeded yet.  And
perhaps the system without page->_mapcount would perform worse.

Unfortunately, I don't have time to study your patches at the moment,
nor get into a discussion on them.  Sorry if that sounds dismissive:
not my intention, I hope others will take up the discussion instead.

But it looked to me as if you've done the easy part without doing the
hard part yet: vmscanning can get along very well with an approximate
idea of page_mapped, but can_share_swap_page really needs to know.

At present you're just saying "no" there, which appears safe but
slow; but there's a get_user_pages fork case where it's very bad
for it to say "no" when it should say "yes".  See try_to_unmap_one
comment on get_user_pages in 2.6.12 mm/rmap.c.

It looked as if you were doing a separate scan to update PG_mapped,
which would better be incorporated in the page_referenced scan.
I found locking to be a problem.  lock_page is held at many of
the right points, but not all, and may be bad to extend its use.

Your patches looked over-split to me (a rare criticism!): you don't
need a separate patch to delete each little thing that's no longer
used, nor a separate patch to introduce each new definition before
it's used.

Hugh
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