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

[Nick Piggin <nickpiggin@yahoo.com.au>]

On Thursday 19 June 2008 22:34, Hugh Dickins wrote:
> 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 I've had a quick look at my collection of uncompleted/unpublished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.

I don't know if you can be sure of that, because after checking
page_mapcount, but before checking page_swapcount, can't another
process have moved their swapcount to mapcount?


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

Thanks


> > 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.

Oh, I missed that. You're now thinking they do have VM_WRITE on
the vma and hence your patch isn't going to work (and neither
force=0). OK, that sounds right to me.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>