Re: Non-linear mappings and truncate/madvise(MADV_DONTNEED)

[Hugh Dickins <hugh@veritas.com>]

On Mon, 19 Apr 2004, Jamie Lokier wrote:
> A couple of thoughts on non-linear mappings.  Vanilla 2.6.5.
> 
> I'm reading madvise_dontneed() and thinking about that zap_page_range()
> call.  It'll wipe non-linear file offset ptes, won't it?

Yes, at present.

> MADV_DONTNEED is actually a reasonable thing to do with a non-linear
> mapping, when you no longer need some of the pages.  You could argue
> that losing the offsets is acceptable in this case, but I think it's a
> poor argument.  The offsets should be preserved while zapping the ptes.

Yes.  And I think it also implies that the ->populate functions
are wrong to fail beyond EOF, should just set up file ptes there.

> Then there's vmtruncate() and invalidate_mmap_range() which calls
> zap_page_range().  When you call truncate(), the non-linear offsets
> appear to be lost (I'm reading the code, not testing it) for the part
> of each VMA corresponding to where the linear mapping would have been.
> 
> That means (a) a peculiar part of the mapping is lost, and (b) some of
> the truncated pages will stay mapped, if they're in a part of a VMA
> which didn't get wiped by the linear calculation.
> 
> Do any of the latest objrmap patches fix these problems?  Have I
> misdiagnosed these problems?

rmap 6 nonlinear truncation (which never appeared on LKML, though
sent twice) fixed most of this, and went into 2.6.6-rc1-bk4 last
night: please check it out.

But I just converted madvise_dontneed by rote, adding a NULL arg to
zap_page_range, missing your point that it should respect nonlinearity.

And I made the zap_details structure private to mm/memory.c since I
hadn't noticed anything outside needing it: I'll fix that up later
and post a patch.

I'm haven't and don't intend to change the behaviour of ->populate,
without agreement from others - Ingo? Jamie?

Hugh

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