On Sun, Jun 30, 2019 at 01:01:04AM -0700, Dan Williams wrote:
On Sun, Jun 30, 2019 at 12:27 AM Dan Williams dan.j.williams@intel.com wrote:
On Sat, Jun 29, 2019 at 9:03 AM Matthew Wilcox willy@infradead.org wrote:
On Thu, Jun 27, 2019 at 07:39:37PM -0700, Dan Williams wrote:
On Thu, Jun 27, 2019 at 12:59 PM Matthew Wilcox willy@infradead.org wrote:
On Thu, Jun 27, 2019 at 12:09:29PM -0700, Dan Williams wrote:
> This bug feels like we failed to unlock, or unlocked the wrong entry > and this hunk in the bisected commit looks suspect to me. Why do we > still need to drop the lock now that the radix_tree_preload() calls > are gone?
Nevermind, unmapp_mapping_pages() takes a sleeping lock, but then I wonder why we don't restart the lookup like the old implementation.
We have the entry locked:
/* * Make sure 'entry' remains valid while we drop * the i_pages lock. */ dax_lock_entry(xas, entry); /* * Besides huge zero pages the only other thing that gets * downgraded are empty entries which don't need to be * unmapped. */ if (dax_is_zero_entry(entry)) { xas_unlock_irq(xas); unmap_mapping_pages(mapping, xas->xa_index & ~PG_PMD_COLOUR, PG_PMD_NR, false); xas_reset(xas); xas_lock_irq(xas); }If something can remove a locked entry, then that would seem like the real bug. Might be worth inserting a lookup there to make sure that it hasn't happened, I suppose?
Nope, added a check, we do in fact get the same locked entry back after dropping the lock.
The deadlock revolves around the mmap_sem. One thread holds it for read and then gets stuck indefinitely in get_unlocked_entry(). Once that happens another rocksdb thread tries to mmap and gets stuck trying to take the mmap_sem for write. Then all new readers, including ps and top that try to access a remote vma, then get queued behind that write.
It could also be the case that we're missing a wake up.
OK, I have a Theory.
get_unlocked_entry() doesn't check the size of the entry being waited for. So dax_iomap_pmd_fault() can end up sleeping waiting for a PTE entry, which is (a) foolish, because we know it's going to fall back, and (b) can lead to a missed wakeup because it's going to sleep waiting for the PMD entry to come unlocked. Which it won't, unless there's a happy accident that happens to map to the same hash bucket.
Let's see if I can steal some time this weekend to whip up a patch.
Theory seems to have some evidence... I instrumented fs/dax.c to track outstanding 'lock' entries and 'wait' events. At the time of the hang we see no locks held and the waiter is waiting on a pmd entry:
[ 4001.354334] fs/dax locked entries: 0 [ 4001.358425] fs/dax wait entries: 1 [ 4001.362227] db_bench/2445 index: 0x0 shift: 6 [ 4001.367099] grab_mapping_entry+0x17a/0x260 [ 4001.371773] dax_iomap_pmd_fault.isra.43+0x168/0x7a0 [ 4001.377316] ext4_dax_huge_fault+0x16f/0x1f0 [ 4001.382086] __handle_mm_fault+0x411/0x1390 [ 4001.386756] handle_mm_fault+0x172/0x360
In fact, this naive fix is holding up so far:
@@ -215,7 +216,7 @@ static wait_queue_head_t *dax_entry_waitqueue(struct xa_state *xas, * queue to the start of that PMD. This ensures that all offsets in * the range covered by the PMD map to the same bit lock. */
if (dax_is_pmd_entry(entry))
//if (dax_is_pmd_entry(entry)) index &= ~PG_PMD_COLOUR; key->xa = xas->xa; key->entry_start = index;
Hah, that's a great naive fix! Thanks for trying that out.
I think my theory was slightly mistaken, but your fix has the effect of fixing the actual problem too.
The xas->xa_index for a PMD is going to be PMD-aligned (ie a multiple of 512), but xas_find_conflict() does _not_ adjust xa_index (... which I really should have mentioned in the documentation). So we go to sleep on the PMD-aligned index instead of the index of the PTE. Your patch fixes this by using the PMD-aligned index for PTEs too.
I'm trying to come up with a clean fix for this. Clearly we shouldn't wait for a PTE entry if we're looking for a PMD entry. But what should get_unlocked_entry() return if it detects that case? We could have it return an error code encoded as an internal entry, like grab_mapping_entry() does. Or we could have it return the _locked_ PTE entry, and have callers interpret that.
At least get_unlocked_entry() is static, but it's got quite a few callers. Trying to discern which ones might ask for a PMD entry is a bit tricky. So this seems like a large patch which might have bugs.
Thoughts?