On Thu, May 09, 2019 at 05:36:29PM +0000, Nadav Amit wrote:
On May 9, 2019, at 3:38 AM, Peter Zijlstra peterz@infradead.org wrote:
diff --git a/mm/mmu_gather.c b/mm/mmu_gather.c index 99740e1dd273..fe768f8d612e 100644 --- a/mm/mmu_gather.c +++ b/mm/mmu_gather.c @@ -244,15 +244,20 @@ void tlb_finish_mmu(struct mmu_gather *tlb, unsigned long start, unsigned long end) { /*
* If there are parallel threads are doing PTE changes on same range* under non-exclusive lock(e.g., mmap_sem read-side) but defer TLB* flush by batching, a thread has stable TLB entry can fail to flush* the TLB by observing pte_none|!pte_dirty, for example so flush TLB* forcefully if we detect parallel PTE batching threads.
* Sensible comment goes here..
- if (mm_tlb_flush_nested(tlb->mm)) {
__tlb_reset_range(tlb);__tlb_adjust_range(tlb, start, end - start);
if (mm_tlb_flush_nested(tlb->mm) && !tlb->full_mm) {
/** Since we're can't tell what we actually should have* flushed flush everything in the given range.*/tlb->start = start;tlb->end = end;tlb->freed_tables = 1;tlb->cleared_ptes = 1;tlb->cleared_pmds = 1;tlb->cleared_puds = 1;tlb->cleared_p4ds = 1;}
tlb_flush_mmu(tlb);
As a simple optimization, I think it is possible to hold multiple nesting counters in the mm, similar to tlb_flush_pending, for freed_tables, cleared_ptes, etc.
The first time you set tlb->freed_tables, you also atomically increase mm->tlb_flush_freed_tables. Then, in tlb_flush_mmu(), you just use mm->tlb_flush_freed_tables instead of tlb->freed_tables.
That sounds fraught with races and expensive; I would much prefer to not go there for this arguably rare case.
Consider such fun cases as where CPU-0 sees and clears a PTE, CPU-1 races and doesn't see that PTE. Therefore CPU-0 sets and counts cleared_ptes. Then if CPU-1 flushes while CPU-0 is still in mmu_gather, it will see cleared_ptes count increased and flush that granularity, OTOH if CPU-1 flushes after CPU-0 completes, it will not and potentiall miss an invalidate it should have had.
This whole concurrent mmu_gather stuff is horrible.
/me ponders more....
So I think the fundamental race here is this:
CPU-0 CPU-1
tlb_gather_mmu(.start=1, tlb_gather_mmu(.start=2, .end=3); .end=4);
ptep_get_and_clear_full(2) tlb_remove_tlb_entry(2); __tlb_remove_page(); if (pte_present(2)) // nope
tlb_finish_mmu();
// continue without TLBI(2) // whoopsie
tlb_finish_mmu(); tlb_flush() -> TLBI(2)
And we can fix that by having tlb_finish_mmu() sync up. Never let a concurrent tlb_finish_mmu() complete until all concurrenct mmu_gathers have completed.
This should not be too hard to make happen.