On Tue, Jun 10, 2014 at 03:25:42PM -0500, Christoph Lameter wrote: > On Mon, 9 Jun 2014, Kirill A. Shutemov wrote: > > > To be able to split huge page at any point we have to track which tail > > page was pinned. It leads to tricky and expensive get_page() on tail pages > > and also occupy tail_page->_mapcount. > > Maybe we should give up the requirement to be able to split a huge page at > any point?
Yes, that's what the patchset does: we don't allow to split the page if any sub-page is pinned. > This got us into the mess AFAICT. Instead we could use the locking > mechanisms that we have to stop all access to the page and then do the > conversion? I end up with compound_lock to freeze page count. Not sure if it's the best option we have > Page migration can do that so it should be fine with refcounting for > huge pages exclusively in the head page exactly like a regular page. We've discussed "split via migration" with Dave. I need to look more on how migration works. > The problem is then dealing with the locations where we now do rely on > the ability to split at "any point" (notion is weird in itself and > suggests issues with synchronization). As I said, we have only 4 places where we need to split the page (not only PMD): swap out, memory failure, KSM, migration. All of them can tolerate split failure. > Use the standard locking schemes for pages instead? Could you elaborate here? > I thought the idea was that we would modify the relevant code and > that at some point this requirement could go away? > > Huge pages (and other larger order pages) will become increasingly > difficult to handle if relevant page state has to be maintained in tail > pages and if it differs significantly from regular pages. Agreed. The patchset drops tail page refcounting. -- Kirill A. Shutemov -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
