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SubjectRe: [PATCH] dax: fix radix tree insertion race
On Thu 06-04-17 15:29:44, Ross Zwisler wrote:
> While running generic/340 in my test setup I hit the following race. It can
> happen with kernels that support FS DAX PMDs, so v4.10 thru v4.11-rc5.
>
> Thread 1 Thread 2
> -------- --------
> dax_iomap_pmd_fault()
> grab_mapping_entry()
> spin_lock_irq()
> get_unlocked_mapping_entry()
> 'entry' is NULL, can't call lock_slot()
> spin_unlock_irq()
> radix_tree_preload()
> dax_iomap_pmd_fault()
> grab_mapping_entry()
> spin_lock_irq()
> get_unlocked_mapping_entry()
> ...
> lock_slot()
> spin_unlock_irq()
> dax_pmd_insert_mapping()
> <inserts a PMD mapping>
> spin_lock_irq()
> __radix_tree_insert() fails with -EEXIST
> <fall back to 4k fault, and die horribly
> when inserting a 4k entry where a PMD exists>
>
> The issue is that we have to drop mapping->tree_lock while calling
> radix_tree_preload(), but since we didn't have a radix tree entry to lock
> (unlike in the pmd_downgrade case) we have no protection against Thread 2
> coming along and inserting a PMD at the same index. For 4k entries we
> handled this with a special-case response to -EEXIST coming from the
> __radix_tree_insert(), but this doesn't save us for PMDs because the
> -EEXIST case can also mean that we collided with a 4k entry in the radix
> tree at a different index, but one that is covered by our PMD range.
>
> So, correctly handle both the 4k and 2M collision cases by explicitly
> re-checking the radix tree for an entry at our index once we reacquire
> mapping->tree_lock.
>
> This patch has made it through a clean xfstests run with the current
> v4.11-rc5 based linux/master, and it also ran generic/340 500 times in a
> loop. It used to fail within the first 10 iterations.
>
> Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
> Cc: <stable@vger.kernel.org> [4.10+]

The patch looks good to me (and I can see Andrew already sent it to Linus),
I'm just worndering where did things actually go wrong? I'd expect we would
return VM_FAULT_FALLBACK from dax_iomap_pmd_fault() and then do PTE fault
for the address which should just work out fine...

Honza

> ---
> fs/dax.c | 35 ++++++++++++++++++++++-------------
> 1 file changed, 22 insertions(+), 13 deletions(-)
>
> diff --git a/fs/dax.c b/fs/dax.c
> index de622d4..85abd74 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -373,6 +373,22 @@ static void *grab_mapping_entry(struct address_space *mapping, pgoff_t index,
> }
> spin_lock_irq(&mapping->tree_lock);
>
> + if (!entry) {
> + /*
> + * We needed to drop the page_tree lock while calling
> + * radix_tree_preload() and we didn't have an entry to
> + * lock. See if another thread inserted an entry at
> + * our index during this time.
> + */
> + entry = __radix_tree_lookup(&mapping->page_tree, index,
> + NULL, &slot);
> + if (entry) {
> + radix_tree_preload_end();
> + spin_unlock_irq(&mapping->tree_lock);
> + goto restart;
> + }
> + }
> +
> if (pmd_downgrade) {
> radix_tree_delete(&mapping->page_tree, index);
> mapping->nrexceptional--;
> @@ -388,19 +404,12 @@ static void *grab_mapping_entry(struct address_space *mapping, pgoff_t index,
> if (err) {
> spin_unlock_irq(&mapping->tree_lock);
> /*
> - * Someone already created the entry? This is a
> - * normal failure when inserting PMDs in a range
> - * that already contains PTEs. In that case we want
> - * to return -EEXIST immediately.
> - */
> - if (err == -EEXIST && !(size_flag & RADIX_DAX_PMD))
> - goto restart;
> - /*
> - * Our insertion of a DAX PMD entry failed, most
> - * likely because it collided with a PTE sized entry
> - * at a different index in the PMD range. We haven't
> - * inserted anything into the radix tree and have no
> - * waiters to wake.
> + * Our insertion of a DAX entry failed, most likely
> + * because we were inserting a PMD entry and it
> + * collided with a PTE sized entry at a different
> + * index in the PMD range. We haven't inserted
> + * anything into the radix tree and have no waiters to
> + * wake.
> */
> return ERR_PTR(err);
> }
> --
> 2.9.3
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR

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