Re: Hang with v4.15-rc trying to swap back in

[Minchan Kim <minchan@kernel.org>]

On Fri, Dec 29, 2017 at 09:00:16AM +0900, Minchan Kim wrote:
> On Thu, Dec 28, 2017 at 11:00:40AM -0800, James Bottomley wrote:
> > On Thu, 2017-12-28 at 09:41 -0800, James Bottomley wrote:
> > > I'd guess that since they're both in io_schedule, the problem is that
> > > the io_scheduler is taking far too long servicing the requests due to
> > > some priority issue you've introduced.
> > 
> > OK, so after some analysis, that turned out to be incorrect.  The
> > problem seems to be that we're exiting do_swap_page() with locked pages
> > that have been read in from swap.
> > 
> > Your changelogs are entirely unclear on why you changed the swapcache
> > setting logic in this patch:
> > 
> > commit 0bcac06f27d7528591c27ac2b093ccd71c5d0168
> > Author: Minchan Kim <minchan@kernel.org>
> > Date:   Wed Nov 15 17:33:07 2017 -0800
> > 
> >     mm, swap: skip swapcache for swapin of synchronous device
> > 
> > But I think you're using swapcache == NULL as a signal the page came
> > from a synchronous device.  In which case the bug is that you've
> 
> Exactly. Because the patchset aims for skipping swap cache for synchronous
> device and some logics of do_swap_page has has assumed the page is on
> swap cache.
> 
> > forgotten we may already have picked up a page in
> > swap_readahead_detect() which you're wrongly keeping swapcache == NULL
> > for and the fix is this (it works on my system, although I'm still
> > getting an unaccountable shutdown delay).
> 
> SIGH. I missed that.
> 
> > 
> > I still think we should revert this series, because this may not be the
> > only bug lurking in the code, so it should go through a lot more
> > rigorous testing than it has.
> 
> I have no problem. It's not urgent.
> 
> Andrew, this is reverting patch based on 4.15-rc5. And I need to send
> another revert patch against on mmotm because it would have conflict due to
> vma-based readahead restructuring patch. I will send soon.
> 

We should revert (23c47d2ada9f, bdi: introduce BDI_CAP_SYNCHRONOUS_IO) as well.
I resend reverting patch with including 23c47d2ada9f.

Andrew, Pleas take this patch for linus-tree. Sorry for the confusion.