Re: [PATCH] mm: Make truncate_inode_pages_range() killable

[Bart Van Assche <Bart.VanAssche@sandisk.com>]

On Tue, 2017-04-18 at 10:15 +0200, Michal Hocko wrote:
> On Sat 15-04-17 00:59:46, Bart Van Assche wrote:
> > On Fri, 2017-04-14 at 17:40 -0700, Hugh Dickins wrote:
> > > Changing a fundamental function, silently not to do its essential job,
> > > when something in the kernel has forgotten (or is slow to) unlock_page():
> > > that seems very wrong to me in many ways.  But linux-fsdevel, Cc'ed, will
> > > be a better forum to advise on how to solve the problem you're seeing.
> > 
> > It seems like you have misunderstood the purpose of the patch I posted. It's
> > neither a missing unlock_page() nor slow I/O that I want to address but a
> > genuine deadlock. In case you would not be familiar with the queue_if_no_path
> > multipath configuration option, the multipath.conf man page is available at
> > e.g. https://linux.die.net/man/5/multipath.conf.
> 
> So, who is holding the page lock and why it cannot make forward
> progress? Is the storage gone so that the ongoing IO will never
> terminate? Btw. we have many other places which wait for the page lock
> !killable way. Why they are any different from this case?

Hello Michal,

queue_if_no_path means that if no paths are available that the dm-mpath driver
does not complete an I/O request until a path becomes available. A standard
test for multipathed storage is to alternatingly remove and restore all paths.

If the reported lockup happens at the end of a test I can break the cycle by
running "dmsetup message ${mpath} 0 fail_if_no_path". That command causes
pending I/O requests to fail if no paths are available.

I think it is rather unintuitive that kill -9 does not work for a process that
uses a dm-mpath device for I/O as long as no paths are available.

The call stack I reported in the first e-mail in this thread is what I ran
into while running multipath tests. I'm not sure why I have not yet hit any
other code paths that perform an unkillable wait on a page lock.

Bart.
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