On Wed, Apr 28, 2021 at 05:02:25PM -0700, Michel Lespinasse wrote: > On Wed, Apr 28, 2021 at 09:11:08AM -0700, Paul E. McKenney wrote: > > On Wed, Apr 28, 2021 at 08:13:53AM -0700, Andy Lutomirski wrote: > > > On Wed, Apr 28, 2021 at 8:05 AM Michel Lespinasse <michel@xxxxxxxxxxxxxx> wrote: > > > > > > > > On Wed, Apr 07, 2021 at 08:36:01AM -0700, Andy Lutomirski wrote: > > > > > On 4/6/21 6:44 PM, Michel Lespinasse wrote: > > > > > > The page table tree is walked with local irqs disabled, which prevents > > > > > > page table reclamation (similarly to what fast GUP does). The logic is > > > > > > otherwise similar to the non-speculative path, but with additional > > > > > > restrictions: in the speculative path, we do not handle huge pages or > > > > > > wiring new pages tables. > > > > > > > > > > Not on most architectures. Quoting the actual comment in mm/gup.c: > > > > > > > > > > > * Before activating this code, please be aware that the following assumptions > > > > > > * are currently made: > > > > > > * > > > > > > * *) Either MMU_GATHER_RCU_TABLE_FREE is enabled, and tlb_remove_table() is used to > > > > > > * free pages containing page tables or TLB flushing requires IPI broadcast. > > > > > > > > > > On MMU_GATHER_RCU_TABLE_FREE architectures, you cannot make the > > > > > assumption that it is safe to dereference a pointer in a page table just > > > > > because irqs are off. You need RCU protection, too. > > > > > > > > > > You have the same error in the cover letter. > > > > > > > > Hi Andy, > > > > > > > > Thanks for your comment. At first I thought did not matter, because we > > > > only enable ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT on selected > > > > architectures, and I thought MMU_GATHER_RCU_TABLE_FREE is not set on > > > > these. But I was wrong - MMU_GATHER_RCU_TABLE_FREE is enabled on X86 > > > > with paravirt. So I took another look at fast GUP to make sure I > > > > actually understand it. > > > > > > > > This brings a question about lockless_pages_from_mm() - I see it > > > > disabling interrupts, which it explains is necessary for disabling THP > > > > splitting IPIs, but I do not see it taking an RCU read lock as would > > > > be necessary for preventing paga table freeing on > > > > MMU_GATHER_RCU_TABLE_FREE configs. I figure local_irq_save() > > > > indirectly takes an rcu read lock somehow ? I think this is something > > > > I should also mention in my explanation, and I have not seen a good > > > > description of this on the fast GUP side... > > > > > > Sounds like a bug! That being said, based on my extremely limited > > > understanding of how the common RCU modes work, local_irq_save() > > > probably implies an RCU lock in at least some cases. Hi Paul! > > > > In modern kernels, local_irq_save() does have RCU reader semantics, > > meaning that synchronize_rcu() will wait for pre-exiting irq-disabled > > regions. It will also wait for pre-existing bh-disable, preempt-disable, > > and of course rcu_read_lock() sections of code. > > Thanks Paul for confirming / clarifying this. BTW, it would be good to > add this to the rcu header files, just so people have something to > reference to when they depend on such behavior (like fast GUP > currently does). There is this in the synchronize_rcu() header block comment: * synchronize_rcu() was waiting. RCU read-side critical sections are * delimited by rcu_read_lock() and rcu_read_unlock(), and may be nested. * In addition, regions of code across which interrupts, preemption, or * softirqs have been disabled also serve as RCU read-side critical * sections. This includes hardware interrupt handlers, softirq handlers, * and NMI handlers. I have pulled this into a separate paragraph to increase its visibility, and will check out other locations in comments and documentation. Thanx, Paul > Going back to my patch. I don't need to protect against THP splitting > here, as I'm only handling the small page case. So when > MMU_GATHER_RCU_TABLE_FREE is enabled, I *think* I could get away with > using only an rcu read lock, instead of disabling interrupts which > implicitly creates the rcu read lock. I'm not sure which way to go - > fast GUP always disables interrupts regardless of the > MMU_GATHER_RCU_TABLE_FREE setting, and I think there is a case to be > made for following the fast GUP stes rather than trying to be smarter. > > Andy, do you have any opinion on this ? Or anyone else really ? > > Thanks, > > -- > Michel "walken" Lespinasse
