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SubjectRe: Zram writeback feature unstable with heavy swap utilization - BUG: Bad page state in process...
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.

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