On Thu, Oct 06, 2016 at 03:34:24PM -0600, Ross Zwisler wrote: > Interesting - adding iomap_end() calls to the DAX PTE fault handler causes an > AA deadlock because we try and retake ei->dax_sem. We take dax_sem in > ext2_dax_fault() before calling into the DAX code, then if we end up going > through the error path in ext2_iomap_end(), we call > ext2_write_failed() > ext2_truncate_blocks() > dax_sem_down_write() > > Where we try and take dax_sem again. This error path is really only valid for > I/O operations, but we happen to call it for page faults because 'written' in > ext2_iomap_end() is just 0. > > So...how should we handle this? A few ideas: > > 1) Just continue to omit the calls to iomap_end() in the DAX page fault > handlers for now, and add them when there is useful work to be done in one of > the filesystems. > > 2) Add an IOMAP_FAULT flag to the flags passed into iomap_begin() and > iomap_end() so make it explicit that we are calling as part of a fault handler > and not an I/O operation, and use this to adjust the error handling in > ext2_iomap_end(). > > 3) Just work around the existing error handling in ext2_iomap_end() by either > unsetting IOMAP_WRITE or by setting 'written' to the size of the fault. > > For #2 or #3, probably add a comment explaining the deadlock and why we need > to never call ext2_write_failed() while handling a page fault. > > Thoughts? Never mind, #3 it is, I think it was just a plain bug to call iomap_end() with 'length' != 'written'. -- 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
