On Fri, Aug 11, 2023 at 05:32:37PM +0200, David Hildenbrand wrote: > On 11.08.23 17:18, Peter Xu wrote: > > On Fri, Aug 11, 2023 at 12:27:13AM +0200, David Hildenbrand wrote: > > > On 10.08.23 23:48, Matthew Wilcox wrote: > > > > On Thu, Aug 10, 2023 at 04:57:11PM -0400, Peter Xu wrote: > > > > > AFAICS if that patch was all correct (while I'm not yet sure..), you can > > > > > actually fit your new total mapcount field into page 1 so even avoid the > > > > > extra cacheline access. You can have a look: the trick is refcount for > > > > > tail page 1 is still seems to be free on 32 bits (if that was your worry > > > > > before). Then it'll be very nice if to keep Hugh's counter all in tail 1. > > > > > > > > No, refcount must be 0 on all tail pages. We rely on this in many places > > > > in the MM. > > > > > > Very right. > > > > Obviously I could have missed this in the past.. can I ask for an example > > explaining why refcount will be referenced before knowing it's a head? > > I think the issue is, when coming from a PFN walker (or GUP-fast), you might > see "oh, this is a folio, let's lookup the head page". And you do that. > > Then, you try taking a reference on that head page. (see try_get_folio()). > > But as you didn't hold a reference on the folio yet, it can happily get > freed + repurposed in the meantime, so maybe it's not a head page anymore. > > So if the field would get reused for something else, grabbing a reference > would corrupt whatever is now stored in there. Not an issue before large folios, am I right? Because having a head page reused as tail cannot happen iiuc with current thps if only pmd-sized, because the head page is guaranteed to be pmd aligned physically. I don't really know, where a hugetlb 2M head can be reused by a 1G huge later right during the window of fast-gup walking. But obviously that's not common either if that could ever happen. Maybe Matthew was referring to something else (per "in many places")? Thanks, -- Peter Xu
