Re: [PATCH] Memory management livelock

[Nick Piggin]

On Friday 03 October 2008 21:43, Mikulas Patocka wrote:
> On Thu, 2 Oct 2008, Andrew Morton wrote:
> > On Fri, 3 Oct 2008 13:47:21 +1000 Nick Piggin <nickpiggin@xxxxxxxxxxxx> 
wrote:
> > > > I expect there's no solution which avoids blocking the writers at
> > > > some stage.
> > >
> > > See my other email. Something roughly like this would do the trick
> > > (hey, it actually boots and runs and does fix the problem too).
> >
> > It needs exclusion to protect all those temp tags.  Is do_fsync()'s
> > i_mutex sufficient?  It's qute unobvious (and unmaintainable?) that all
> > the callers of this stuff are running under that lock.
>
> That filemap_fdatawrite and filemap_fdatawait in fsync() aren't really
> called under i_mutex (see do_fsync).
>
> So the possible solutions are:
>
> 1. Add jiffies when the page was diried and wroteback to struct page
> + no impact on locking and concurrency
> - increases the structure by 8 bytes

This one is not practical.


> 2. Stop the writers when the starvation happens (what I did)
> + doesn't do any locking if the livelock doesn't happen
> - locks writers when the livelock happens (I think it's not really serious
> --- because very few people complained about the livelock, very few people
> will see performance degradation from blocking the writers).

Maybe it is because not much actually does sequential writes to a massive
file or block device while trying to fsync it as well? I don't know. You
could still have cases where fsync takes much longer than expected but it
is still not long enough for a user to report it as a "livelock" bug.


> 3. Add another bit to radix tree (what Nick did)
> + doesn't ever block writers
> - unconditionally takes the lock on fsync path and serializates concurrent
> syncs/fsyncs. Probably low overhead too ... or I don't know, is there any
> possible situation when more processes execute sync() in parallel and user
> would see degradations if those syncs were serialized?

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