On Thu, Feb 11, 2016 at 12:46 PM, Dave Chinner <david@xxxxxxxxxxxxx> wrote: [..] >> It seems to me we need to modify the >> metadata i/o paths to bypass the page cache, > > XFS doesn't use the block device page cache for it's metadata - it > has it's own internal metadata cache structures and uses get_pages > or heap memory to back it's metadata. But that doesn't make mixing > DAX and pages in the block device mapping tree sane. > > What you are missing here is that the underlying architecture of > journalling filesystems mean they can't use DAX for their metadata. > Modifications have to be buffered, because they have to be written > to the journal first before they are written back in place. IOWs, we > need to buffer changes in volatile memory for some time, and that > means we can't use DAX during transactional modifications. > > And to put the final nail in that coffin, metadata in XFS can be > discontiguous multi-block objects - in those situations we vmap the > underlying pages so they appear to the code to be a contiguous > buffer, and that's something we can't do with DAX.... Sorry, I wasn't clear when I said "bypass page cache" I meant a solution similar to commit d1a5f2b4d8a1 "block: use DAX for partition table reads". However, I suspect that is broken if the filesystem is not ready to see a new page allocated for every I/O. I assume one thread will want to insert a page in the radix for another thread to find/manipulate before metadata gets written back to storage. >> or teach the fsync code >> how to flush populated data pages out of the radix. > > That doesn't solve the problem. Filesystems free and reallocate > filesystem blocks without intermediate block device mapping > invalidation calls, so what is one minute a data block accessed by > DAX may become a metadata block that accessed via buffered IO. It > all goes to crap very quickly.... > > However, I'd say fsync is not the place to address this. This block > device cache aliasing issue is supposed to be what > unmap_underlying_metadata() solves, right? I'll take a look at this. Right now I'm trying to implement the "clear block-device-inode S_DAX on fs mount" approach. My concern though is that we need to disable block device mmap while a filesystem is mounted... Maybe I don't need to worry because it's already the case that a mmap of the raw device may not see the most up to date data for a file that has dirty fs-page-cache data. -- 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
