Re: get_zone_device_page() in get_page() and page_cache_get_speculative()

[Kirill A. Shutemov]

On Mon, Apr 24, 2017 at 10:23:59AM -0700, Dan Williams wrote:
> On Sun, Apr 23, 2017 at 4:31 PM, Kirill A. Shutemov
> <kirill@xxxxxxxxxxxxx> wrote:
> > On Thu, Apr 20, 2017 at 02:46:51PM -0700, Dan Williams wrote:
> >> On Sat, Mar 18, 2017 at 2:52 AM, tip-bot for Kirill A. Shutemov
> >> <tipbot@xxxxxxxxx> wrote:
> >> > Commit-ID:  2947ba054a4dabbd82848728d765346886050029
> >> > Gitweb:     http://git.kernel.org/tip/2947ba054a4dabbd82848728d765346886050029
> >> > Author:     Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> >> > AuthorDate: Fri, 17 Mar 2017 00:39:06 +0300
> >> > Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
> >> > CommitDate: Sat, 18 Mar 2017 09:48:03 +0100
> >> >
> >> > x86/mm/gup: Switch GUP to the generic get_user_page_fast() implementation
> >> >
> >> > This patch provides all required callbacks required by the generic
> >> > get_user_pages_fast() code and switches x86 over - and removes
> >> > the platform specific implementation.
> >> >
> >> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> >> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> >> > Cc: Aneesh Kumar K . V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
> >> > Cc: Borislav Petkov <bp@xxxxxxxxx>
> >> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> >> > Cc: Dann Frazier <dann.frazier@xxxxxxxxxxxxx>
> >> > Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
> >> > Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> >> > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> >> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> >> > Cc: Rik van Riel <riel@xxxxxxxxxx>
> >> > Cc: Steve Capper <steve.capper@xxxxxxxxxx>
> >> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >> > Cc: linux-arch@xxxxxxxxxxxxxxx
> >> > Cc: linux-mm@xxxxxxxxx
> >> > Link: http://lkml.kernel.org/r/20170316213906.89528-1-kirill.shutemov@xxxxxxxxxxxxxxx
> >> > [ Minor readability edits. ]
> >> > Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> >>
> >> I'm still trying to spot the bug, but bisect points to this patch as
> >> the point at which my unit tests start failing with the following
> >> signature:
> >>
> >> [   35.423841] WARNING: CPU: 8 PID: 245 at lib/percpu-refcount.c:155
> >> percpu_ref_switch_to_atomic_rcu+0x1f5/0x200
> >
> > Okay, I've tracked it down. The issue is triggered by replacment
> > get_page() with page_cache_get_speculative().
> >
> > page_cache_get_speculative() doesn't have get_zone_device_page(). :-|
> >
> > And I think it's your bug, Dan: it's wrong to have
> > get_/put_zone_device_page() in get_/put_page(). I must be handled by
> > page_ref_* machinery to catch all cases where we manipulate with page
> > refcount.
> 
> The page_ref conversion landed in 4.6 *after* the ZONE_DEVICE
> implementation that landed in 4.5, so there was a missed conversion of
> the zone-device reference counting to page_ref.

Fair enough.

But get_page_unless_zero() definitely predates ZONE_DEVICE. :)

-- 
 Kirill A. Shutemov