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. -- 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
