On Thu, Dec 4, 2014 at 3:28 PM, Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote: > 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. Instead of tweaking mapcount you could mark zero-pages in /proc/kpageflags and handle them accordingly in userspace. Or mark zero pages with special magic _mapcount and detect it in /proc/kpagecount. >> > >> > 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> -- 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>