Messages in this thread | | | Date | Thu, 4 Dec 2014 14:28:13 +0200 | From | "Kirill A. Shutemov" <> | Subject | Re: [RFC V2] mm:add zero_page _mapcount when mapped into user space |
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On Thu, Dec 04, 2014 at 02:10:53PM +0800, Wang, Yalin wrote: > > -----Original Message----- > > From: Kirill A. Shutemov [mailto:kirill@shutemov.name] > > Sent: Tuesday, December 02, 2014 7:30 PM > > To: Wang, Yalin > > Cc: 'linux-kernel@vger.kernel.org'; 'linux-mm@kvack.org'; 'linux-arm- > > kernel@lists.infradead.org' > > 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
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