Re: [patch 4/6] mm: merge populate and nopage into fault (fixes nonlinear)

On Wed, Mar 07, 2007 at 02:19:22PM +0100, Peter Zijlstra wrote:
On Wed, 2007-03-07 at 14:08 +0100, Nick Piggin wrote:

The thing is, I don't think anybody who uses these things cares
about any of the 'problems' you want to fix, do they? We are
interested in dirty pages only for the correctness issue, rather
than performance. Same as reclaim.

If so, we can just stick to the dead slow but correct 'scan the full
vma' page_mkclean() and nobody would ever trigger it.

Not if we restricted it to root and mlocked tmpfs. But then why
wouldn't you just do it with the much more efficient msync walk,
so that if root does want to do writeout via these things, it does
not blow up?

This is all used on ram based filesystems right, they all have
BDI_CAP_NO_WRITEBACK afaik, so page_mkclean will never get called
anyway. Mlock doesn't avoid getting page_mkclean called.

Those who use this on a 'real' filesystem will get hit in the face by a
linear scanning page_mkclean(), but AFAIK nobody does this anyway.

But somebody might do it. I just don't know why you'd want to make
this _worse_ when the msync option would work?

Restricting it to root for such filesystems is unwanted, that'd severely
handicap both UML and Oracle as I understand it (are there other users
of this feature around?)

Why? I think they all use tmpfs backings, don't they?

msync() might never get called and then we're back with the old
behaviour where we can surprise the VM with a ton of dirty pages.

But we're root. With your patch, root *can't* do nonlinear writeback
well. Ever. With msync, at least you give them enough rope.

Restricting to root doesn't buy you much, nobody wants to be root.
Restricting to mlock is similarly pointless. UML _will_ want to get
swapped out if there's no activity.

Restricting to tmpfs makes sense, but it's probably not what UML
wants.

Conclusion: there's no good solution for UML in kernel-space.

Miklos
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