Re: unuse_pte: set pte dirty if the page is dirty

[Hugh Dickins <hugh@veritas.com>]

On Mon, 27 Feb 2006, Christoph Lameter wrote:
> 
> unuse_pte is used:
> 
> 1. To switch off a swap device.
> 
> 2. To reestablish ptes for a migrated anonymous page.
> 
> In both cases we are only dealing with anonymous pages. The only writer 
> can be the swap code and as far as I can tell the only risk is writing a 
> swap page out once again. That is if it would be cleaned by pageout().

I shared Andrew's unease, but couldn't put my finger on any actual
problem.  But in the course of writing a much more hesitant reply,
came to realize the patch is just bogus.  Did you ever measure any
improvement from it, on any architecture?  0% is my estimate.

I was recommending that the VM_WRITE test be replaced by a pte_write
test, when I remembered that vm_page_prot on any vma which contains
anonymous pages (excepting the very rare Linus ptrace case) will not
grant write access (see comment above unuse_pte).  So if this pte is
actually written to afterwards, you'll have to handle a write fault
on it, won't you?  No saving whatever from presetting dirty - or am
I misunderstanding how the architecture closest to your heart works?

I guess you could work around that by checking mapcount+swapcount
and granting write access in the common uniquely-mapped case; but
swapoff has never bothered to do so.  Unless you can come up with
convincing numbers, I'd say let it die - halve the time of a
significant migration testcase?  yes, we should make a patch;
shave 5% off it?  no, for peace of mind let's not worry about it.

Hugh

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