> From: Akshay Karle [mailto:akshay.a.karle@xxxxxxxxx] > Subject: RE: [RFC 1/2] kvm: host-side changes for tmem on KVM > > > > From: Akshay Karle [mailto:akshay.a.karle@xxxxxxxxx] > > > Subject: Re: [RFC 1/2] kvm: host-side changes for tmem on KVM > > > > > > >> @@ -669,7 +670,6 @@ static struct zv_hdr *zv_create(struct x > > > >> int chunks = (alloc_size + (CHUNK_SIZE - 1)) >> CHUNK_SHIFT; > > > >> int ret; > > > >> > > > >> - BUG_ON(!irqs_disabled()); > > > > > > > > Can you explain why? > > > > > > Zcache is by default used in the non-virtualized environment for page compression. Whenever > > > a page is to be evicted from the page cache the spin_lock_irq is held on the page mapping. > > > To ensure that this is done, the BUG_ON(!irqs_disabled()) was used. > > > But now the situation is different, we are using zcache functions for kvm VM's. > > > So if any page of the guest is to be evicted the irqs should be disabled in just that > > > guest and not the host, so we removed the BUG_ON(!irqs_disabled()); line. > > > > I think irqs may still need to be disabled (in your code by the caller) > > since the tmem code (in tmem.c) takes spinlocks with this assumption. > > I'm not sure since I don't know what can occur with scheduling a > > kvm guest during an interrupt... can a different vcpu of the same guest > > be scheduled on this same host pcpu? > > The irqs are disabled but only in the guest kernel not in the host. We > tried adding the spin_lock_irq code into the host but that was resulting > in host panic as the lock is being taken on the entire mapping. If the > irqs are disabled in the guest, is there a need to disable them on the > host as well? Because the mappings maybe different in the host and the > guest. The issue is that interrupts MUST be disabled in code this is called by zcache_put_page() and by zv_create() because the called code (tmem_put and xv_malloc) takes locks. This may be difficult to reproduce, but if an interrupt occurs during a critical region, a deadlock is possible. You don't need to do a spin_lock_irq. You just need to do a local_irq_save and restore in zcache_put_page if kvm_tmem_enabled. Look at zcache_get_page as an example... the code in zcache_put_page would be something like: { if (kvm_tmem_enabled) local_irq_save(flags); : : out: if (kvm_tmem_enabled) local_irq_restore(flags); return ret; } -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
