On Thu, Dec 04, 2014 at 02:10:53PM +0800, Wang, Yalin wrote: > > -----Original Message----- > > From: Kirill A. Shutemov [mailto:kirill@xxxxxxxxxxxxx] > > Sent: Tuesday, December 02, 2014 7:30 PM > > To: Wang, Yalin > > Cc: 'linux-kernel@xxxxxxxxxxxxxxx'; 'linux-mm@xxxxxxxxx'; 'linux-arm- > > kernel@xxxxxxxxxxxxxxxxxxx' > > Subject: Re: [RFC V2] mm:add zero_page _mapcount when mapped into user > > space > > > > On Tue, Dec 02, 2014 at 05:27:36PM +0800, Wang, Yalin wrote: > > > This patch add/dec zero_page's _mapcount to make sure the mapcount is > > > correct for zero_page, so that when read from /proc/kpagecount, > > > zero_page's mapcount is also correct, userspace process like procrank > > > can calculate PSS correctly. > > > > I don't have specific code path to point to, but I would expect zero page > > with non-zero mapcount would cause a problem with rmap. > > > > How do you test the change? > > > I just test it to see the mapcount from /proc/pid/pagemap and /proc/kpagecount , > It works well, I took a closer look and your patch is broken in multiple places: - on zap_pte_range() you don't decrement mapcount; - you don't update rss counters for mm; - copy_one_pte() doesn't increase mapcount; - ... Basically, each and every vm_normal_page() call must be audited. As first step. And you totally skip huge zero page. Proper mapcount handling for zero page would require a lot more work and I don't think it worth it. Gain is too small. NAK. > The problem is that when I see /proc/pid/smaps , > The Rss / Pss don't calculate zero_page map, > Because smaps_pte_entry() --> vm_normal_page( ), > Will return NULL for zero_page, > > But when userspace process cat /proc/pid/pagemap , > It will see zero_page mapped, > And will treat as Rss , > This is weird, should we also omit zero_page in /proc/pid/pagemap ? > Or add zero_page as Rss in /proc/pid/smaps ? > > I think we should add zero_page into Rss , > Because it is really mapped into userspace address space. > And will let userspace memory analysis more accurate . It would be easier for userspace to find out pfn of zero page and take it into account. Note: some architectures have multiple zero page due to coloring. -- Kirill A. Shutemov -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>