Re: Hugepages for shm page cache (defrag)

[Hugh Dickins <hughd@google.com>]

On Thu, 14 Jul 2011, mail@rsmogura.net wrote:
> I working to remove errors from patch, and I integrated it with current THP
> infrastructure a little bit,

Great, thank you.

> but I want ask if following I do following -
> it's about get_page, put_page, get_page_unless_zero, put_page_test_zero.
> 
> I want following logic I think it may be better (in x86):
> 1) Each THP page will start with 512 refcount (self + 511 tails)
> 2) Each get/put will increment usage count only on this page, same test
> variants will do (currently those do not make this, so split is broken)
> 3) On compounds put page will call put_page_test_zero, if true, it will do
> compound lock, ask again if it has 0, if yes it will decrease refcount of
> head, if it will fall to zero compound will be freed (double check lock).
> 4) Compound lock is this what caller will need to establish if it needs to
> operate on transparent huge page in whole.
> 
> Motivation:
> I operate on page cache, many assumptions about concurrent call of
> put/get_page are and plain using those causes memory leaks, faults, dangling
> pointers, etc when I'm going to split compound page.
> 
> Is this acceptable?

Sounds plausible, but I really don't know.

I do remember that refcounting compounds by head or by tail always raises
questions (and access via get_user_pages() is an easily-overlooked path
that needs to be kept in mind).  But where THP stands today, and how it
needs to be changed for this, I have no idea - whereas Andrea, perhaps,
will recognize some of your points above and have a more useful response.

It's clear that you have much more of a grip on these details than I
have at present, so just be guided by the principle of not slowing
down the common paths.

Hugh

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