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SubjectRe: Zram writeback feature unstable with heavy swap utilization - BUG: Bad page state in process...
On Tue, Jul 24, 2018 at 07:35:32PM -0700, Matthew Wilcox wrote:
> On Wed, Jul 25, 2018 at 11:16:57AM +0900, Minchan Kim wrote:
> > On Tue, Jul 24, 2018 at 06:55:02PM -0700, Matthew Wilcox wrote:
> > > On Wed, Jul 25, 2018 at 10:32:50AM +0900, Minchan Kim wrote:
> > > > Hi Tino,
> > > >
> > > > On Tue, Jul 24, 2018 at 09:30:34AM +0200, Tino Lehnig wrote:
> > > > > Hi,
> > > > >
> > > > > The first build I used was from the master branch of the mainline kernel,
> > > > > somewhere between rc5 and rc6. I have just reproduced the bug with 4.17.9
> > > > > and 4.18-rc6. Kernel messages below.
> > > > >
> > > > > The bug does not appear on 4.14.57. I can test more versions if it helps.
> > > >
> > > > Could you try 4.15?
> > > >
> > > > I think it's a regression of struct page field reordring and it started from
> > > > v4.16.
> > > >
> > > > page->units for zsmalloc is used as offset of first object on the zspage,
> > > > However, below patch unified it with page->_refcount.
> > >
> > > No it didn't. It's in a union with _mapcount, which is where it was before
> > > my patches.
> > >
> > > It's entiely possible that my patches caused this, but the explanation
> > > you're offering is wrong.
> >
> > Before your patch, _mapcount and _refcount is separated space so we can use
> > _mapcount for page->units for non-mapped pages because units is unified with
> > _mapcount. However, with your patch, now units is unified with _refcount.
>
> No. That's completely untrue.
>
> union { /* This union is 4 bytes in size. */
> atomic_t _mapcount;
> unsigned int page_type;
> unsigned int active; /* SLAB */
> int units; /* SLOB */
> };
>
> atomic_t _refcount;
>
> There is NOTHING in a union with _refcount.
>


Confusing. Matthew, what am I missing?

Before:

counters consumes 8 bytes
units and _refcount consumes each 4 bytes so memory space is not overlapped.

union {
unsigned long counters;
struct {

union {
atomic_t _mapcount;
unsigned int active; /* SLAB */
struct { /* SLUB */
unsigned inuse:16;
unsigned objects:15;
unsigned frozen:1;
};
int units; /* SLOB */
};
/*
* Usage count, *USE WRAPPER FUNCTION* when manual
* accounting. See page_ref.h
*/
atomic_t _refcount;
};
};


After:

Now, units is overlapped with _refcount and _mapcount.

union {
unsigned long counters;
unsigned int active; /* SLAB */
struct { /* SLUB */
unsigned inuse:16;
unsigned objects:15;
unsigned frozen:1;
};
int units; /* SLOB */
struct { /* Page cache */
atomic_t _mapcount;
atomic_t _refcount;
};
};


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