On Tue, Feb 2, 2016 at 9:34 AM, Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> wrote: > On Tue, Feb 02, 2016 at 09:10:24AM -0800, Dan Williams wrote: >> On Tue, Feb 2, 2016 at 8:46 AM, Jan Kara <jack@xxxxxxx> wrote: >> > On Tue 02-02-16 08:33:56, Dan Williams wrote: >> >> On Tue, Feb 2, 2016 at 3:17 AM, Jan Kara <jack@xxxxxxx> wrote: >> >> [..] >> >> > I see, thanks for explanation. So I'm OK with changing what is stored in >> >> > the radix tree to accommodate this use case but my reservation that we IHMO >> >> > have other more pressing things to fix remains... >> >> >> >> We don't need pfns in the radix to support XFS RT configurations. >> >> Just call get_blocks() again and use the sector, or am I missing >> >> something? >> > >> > You are correct. But if you decide to pay the cost of additional >> > get_block() call, you only need the dirty tag in the radix tree and nothing >> > else. So my understanding was that the whole point of games with radix tree >> > is avoiding this extra get_block() calls for fsync(). >> > >> >> DAX-fsync() is already a potentially expensive operation to cover data >> durability guarantees for DAX-unaware applications. A DAX-aware >> application is going to skip fsync, and the get_blocks() cost, to do >> cache management itself. >> >> Willy pointed out some other potential benefits, assuming a suitable >> replacement for the protections afforded by the block-device >> percpu_ref counter can be found. However, optimizing for the >> DAX-unaware-application case seems the wrong motivation to me. > > Oh, no, the primary issue with calling get_block() in the fsync path isn't > performance. It's that we don't have any idea what get_block() function to > call. > > The fault handler calls all come from the filesystem directly, so they can > easily give us an appropriate get_block() function pointer. But the > dax_writeback_mapping_range() calls come from the generic code in > mm/filemap.c, and don't know what get_block() to pass in. > > During one iteration I had the calls to dax_writeback_mapping_range() > happening in the filesystem fsync code so that it could pass in get_block(), > but Dave Chinner pointed out that this misses other paths in the filesystem > that need to have things flushed via a call to filemap_write_and_wait_range(). > > In yet another iteration of this series I tried adding get_block() to struct > inode_operations so that I could access it from what is now > dax_writeback_mapping_range(), but this was shot down as well. Ugh, and we can't trigger it from where a filesystem normally syncs a block device, becauDid you tryse we lose track of the inode radix information at that level. What a about a super_operation? That seems the right level, given we're currently doing: inode->i_sb->s_bdev ...it does not seem terrible to instead do: inode->i_sb->s_op->get_block() -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
