On 15/02/2016 00:27, Alan Cox wrote: > > On Sun, Feb 14, 2016 at 8:09 AM, Andriy Gapon <a...@freebsd.org > <mailto:a...@freebsd.org>> wrote: > > On 10/02/2016 23:28, Andriy Gapon wrote: > > > > Over a span of approximately 3 weeks I have got two slightly different > panics of > > the same kind. The affected system is a several months old amd64 head. > > I added a small assertion and it got tripped: [snip] > > So, it seems that the code allows modification of read_ahead field of an > entry > without holding a map lock. I'd guess that that should be mostly > harmless as > read_ahead value is not critical, but it is still not nice. > > Not intentionally. The nearby code to the read_ahead assignment expects the > map > to be locked, so I wouldn't categorize this as mostly harmless. > > In general, you shouldn't get to the read_ahead assignment without the map > being > locked, and almost all of the code paths that unlock the map jump to > RetryFault > shortly thereafter, so it's hard to imagine how the map lock wouldn't be > reacquired. I speculate that the root cause of your panic is a case where > vm_pager_get_pages() fails, and in such a case we might loop around, > descending > to the next backing object without reacquiring the map lock. In other words, > your above assertion failure is happening on the next iteration after an > initial > vm_pager_get_pages() failure, and we're about to do another > vm_pager_get_pages() > call on a different object. > > In summary, I have no trouble believing that the code that handles a failure > by > vm_pager_get_pages() in vm_fault() is not quite right, and I think that's > what's > biting you.
Alan, thank you very much for the very insightful analysis. Indeed, I see that in this case the object chain is default -> swap -> swap. I am not sure how such chain was created. It seems that going default -> swap is not a problem as the map lock is not dropped in this case. But going swap -> swap the way you described (pager error, e.g. the page is just not found) has exactly the problem that you suggested. -- Andriy Gapon _______________________________________________ freebsd-current@freebsd.org mailing list https://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"
