On Thu, May 12, 2016 at 06:29:18PM +0200, Jan Kara wrote:
> Currently DAX page fault locking is racy.
>
> CPU0 (write fault) CPU1 (read fault)
>
> __dax_fault() __dax_fault()
> get_block(inode, block, &bh, 0) -> not mapped
> get_block(inode, block, &bh, 0)
> -> not mapped
> if (!buffer_mapped(&bh))
> if (vmf->flags & FAULT_FLAG_WRITE)
> get_block(inode, block, &bh, 1) -> allocates blocks
> if (page) -> no
> if (!buffer_mapped(&bh))
> if (vmf->flags & FAULT_FLAG_WRITE) {
> } else {
> dax_load_hole();
> }
> dax_insert_mapping()
>
> And we are in a situation where we fail in dax_radix_entry() with -EIO.
>
> Another problem with the current DAX page fault locking is that there is
> no race-free way to clear dirty tag in the radix tree. We can always
> end up with clean radix tree and dirty data in CPU cache.
>
> We fix the first problem by introducing locking of exceptional radix
> tree entries in DAX mappings acting very similarly to page lock and thus
> synchronizing properly faults against the same mapping index. The same
> lock can later be used to avoid races when clearing radix tree dirty
> tag.
>
> Reviewed-by: NeilBrown <neilb@xxxxxxxx>
> Reviewed-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
> Signed-off-by: Jan Kara <jack@xxxxxxx>
> ---
<>
> @@ -897,13 +1166,10 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
> * the write to insert a dirty entry.
> */
> if (write) {
> - error = dax_radix_entry(mapping, pgoff, dax.sector,
> - true, true);
> - if (error) {
> - dax_pmd_dbg(&bh, address,
> - "PMD radix insertion failed");
> - goto fallback;
> - }
> + /*
> + * We should insert radix-tree entry and dirty it here.
> + * For now this is broken...
> + */
With this change the 'error' variable in __dax_pmd_fault() is now unused,
resulting in a compiler warning.
fs/dax.c: In function ‘__dax_pmd_fault’:
fs/dax.c:1019:6: warning: unused variable ‘error’ [-Wunused-variable]
int error, result = 0;
^
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
