Re: Can get_user_pages( ,write=1, force=1, ) result in a read-only pte and _count=2?

[Hugh Dickins <hugh@veritas.com>]

On Thu, 19 Jun 2008, Nick Piggin wrote:
> On Thursday 19 June 2008 21:39, Hugh Dickins wrote:
> 
> > I've had a quick look at my collection of uncompleted/unpublished
> > swap patches, and here's a hunk from one of them which is trying
> > to address that point.  But I'll have to look back and see what
> > else this depends upon.
> >
> > -		if (!TestSetPageLocked(old_page)) {
> > -			reuse = can_share_swap_page(old_page);
> > -			unlock_page(old_page);
> > +		if (page_mapcount(old_page) == 1) {
> > +			extern int page_swapcount(struct page *);
> > +			if (!PageSwapCache(old_page))
> > +				reuse = 1;
> > +			else if (!TestSetPageLocked(old_page)) {
> > +				reuse = !page_is_shared(old_page);
> > +				unlock_page(old_page);
> > +			} else if (!page_swapcount(old_page))
> > +				reuse = 1;
> >
> > I probably won't get back to this today.  And there are also good
> > reasons in -mm for me to check back on all these swapcount issues.
> 
> I don't see how you can get an accurate page_swapcount without
> the page lock.

I doubt it's an accurate swapcount, just a case where one can be
sure of !page_swapcount.  It's certainly not something to take on
trust, patches I need to be sceptical about and refresh my mind on.

> Anyway, if you volunteer to take a look at the problem, great.

I do.

> I expect Robin could just as well fix it for
> their code in the meantime by using force=0...

Sorry, please explain, I don't see that: though their driver happens
to say force=1, I don't think it's needed and I don't think it's
making any difference in this case.

Hugh

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