On May 13, 2019, at 2:12 AM, Peter Zijlstra peterz@infradead.org wrote:
On Mon, May 13, 2019 at 10:36:06AM +0200, Peter Zijlstra wrote:
On Thu, May 09, 2019 at 09:21:35PM +0000, Nadav Amit wrote:
It may be possible to avoid false-positive nesting indications (when the flushes do not overlap) by creating a new struct mmu_gather_pending, with something like:
struct mmu_gather_pending { u64 start; u64 end; struct mmu_gather_pending *next; }
tlb_finish_mmu() would then iterate over the mm->mmu_gather_pending (pointing to the linked list) and find whether there is any overlap. This would still require synchronization (acquiring a lock when allocating and deallocating or something fancier).
We have an interval_tree for this, and yes, that's how far I got :/
The other thing I was thinking of is trying to detect overlap through the page-tables themselves, but we have a distinct lack of storage there.
We might just use some state in the pmd, there's still 2 _pt_pad_[12] in struct page to 'use'. So we could come up with some tlb generation scheme that would detect conflict.
It is rather easy to come up with a scheme (and I did similar things) if you flush the table while you hold the page-tables lock. But if you batch across page-tables it becomes harder.
Thinking about it while typing, perhaps it is simpler than I think - if you need to flush range that runs across more than a single table, you are very likely to flush a range of more than 33 entries, so anyhow you are likely to do a full TLB flush.
So perhaps just avoiding the batching if only entries from a single table are flushed would be enough.