On Wed, Nov 19, 2014 at 2:56 PM, Frederic Weisbecker <fweis...@gmail.com> wrote: > On Wed, Nov 19, 2014 at 10:56:26PM +0100, Thomas Gleixner wrote: >> On Wed, 19 Nov 2014, Frederic Weisbecker wrote: >> > I got a report lately involving context tracking. Not sure if it's >> > the same here but the issue was that context tracking uses per cpu data >> > and per cpu allocation use vmalloc and vmalloc'ed area can fault due to >> > lazy paging. >> >> This is complete nonsense. pcpu allocations are populated right >> away. Otherwise no single line of kernel code which uses dynamically >> allocated per cpu storage would be safe. > > Note this isn't faulting because part of the allocation is swapped. No > it's all reserved in the physical memory, but it's a lazy allocation. > Part of it isn't yet addressed in the P[UGM?]D. That's what vmalloc_fault() > is for. > > So it's a non-blocking/sleeping fault which is why it's probably fine > most of the time except on code that isn't fault-safe. And I suspect that > most people assume that kernel data won't fault so probably some other > places have similar issues. > > That's a long standing issue. We even had to convert the perf callchain > allocation to ad-hoc kmalloc() based per cpu allocation to get over vmalloc > faults. At that time, NMIs couldn't handle faults and many callchains were > populated in NMIs. We had serious crashes because of per cpu memory faults.
Is there seriously more than 512GB of per-cpu virtual space or whatever's needed to exceed a single pgd on x86_64? And there are definitely placed that access per-cpu data in contexts in which a non-IST fault is not allowed. Maybe not dynamic per-cpu data, though. --Andy > > I think that lazy adressing is there for allocation performance reasons. But > still having faultable per cpu memory is insame IMHO. > -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
