Date: Fri, 05 Oct 2012 15:24:12 +0300 From: Andriy Gapon <avg@FreeBSD.org> To: Nikolay Denev <ndenev@gmail.com>, Pawel Jakub Dawidek <pjd@FreeBSD.org> Cc: freebsd-fs <freebsd-fs@FreeBSD.org> Subject: Re: nfs + zfs hangs on RELENG_9 Message-ID: <506ED16C.7000207@FreeBSD.org> In-Reply-To: <506DB5DB.7080302@FreeBSD.org> References: <906543F2-96BD-4519-B693-FD5AFB646F87@gmail.com> <506BF372.1090208@FreeBSD.org> <CF9C7048-15C1-4C7A-8395-2BAB3AE31322@gmail.com> <506C4049.4040100@FreeBSD.org> <D50A4777-BF2E-438A-B15B-661D4CB3C3B6@gmail.com> <506D81A7.8030506@FreeBSD.org> <506DB5DB.7080302@FreeBSD.org>
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on 04/10/2012 19:14 Andriy Gapon said the following:
> BTW, one thing to note here is that the lowmem hook was invoked because of KVA
> space shortage, not because of page shortage.
>
> From practical point of view this may mean that having sufficient KVA size may
> help to not run into this deadlock.
>
> From programming point of view I am tempted to let arc_lowmem block only if
> curproc == pageproc. That should both handle the case where blocking is most
> needed and should prevent the deadlock described above.
A possible patch:
--- a/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c
+++ b/sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c
@@ -3720,8 +3720,16 @@ arc_lowmem(void *arg __unused, int howto __unused)
mutex_enter(&arc_reclaim_thr_lock);
needfree = 1;
cv_signal(&arc_reclaim_thr_cv);
- while (needfree)
- msleep(&needfree, &arc_reclaim_thr_lock, 0, "zfs:lowmem", 0);
+
+ /*
+ * It is unsafe to block here in arbitrary threads, because we can come
+ * here from ARC itself and may hold ARC locks and thus risk a deadlock
+ * with ARC reclaim thread.
+ */
+ if (curproc == pageproc) {
+ while (needfree)
+ msleep(&needfree, &arc_reclaim_thr_lock, 0, "zfs:lowmem", 0);
+ }
mutex_exit(&arc_reclaim_thr_lock);
mutex_exit(&arc_lowmem_lock);
}
--
Andriy Gapon
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