On Sun, September 27, 2009 1:13 am, Christoph Hellwig wrote: > On Thu, Sep 24, 2009 at 08:52:36AM +1000, Neil Brown wrote: >> On Wednesday September 23, hch@xxxxxx wrote: >> > It's a very awkward way to write out all data and wait for it, so just >> > call filemap_write_and_wait. I still can't figure what the point of >> > all this is, so a comment would surely be helpful. >> >> When md/bitmap accesses a file, it uses bmap to find addresses and then >> submit_bh to do IO, so it completely by-passes the page cache. >> So this code is present to ensure that the page cache has no dirty >> pages for the file before we start using the file. >> >> I don't recall exactly why I used do_sync_mapping_range. I suspect >> that I looked at what "sys_fsync" used (do_fsync?) and found that >> wasn't exported, so I looked at what sys_sync_file_range used, found >> that was exported, and so used that. >> >> Looking at the current state of the VFS, I think I would rather use >> vfs_fsync. > > Using vfs_fsync is better, but using bmap is extremly dangerous, I'm > kinda suprised how this managed to get sneaked in without notice. bmap > really is just an advisory interface as the mappings by change > underneath all the time. Some modern filesystems like btrfs really > can't implement it at all. > I used bmap because I wanted writeout characteristics similar to swapout, and mm/swapfile.c uses bmap. I copied the ideas from that code. If bmap is unsafe for bitmap files, then presumably it is unsafe for swap files too. This is exactly the argument that I used when Andrew Morton queried the code on the way in. The swap-over-NFS patches introduce a new interface to the filesystem that supports swapout (i.e. writeout that is guaranteed not to block on mem allocation etc). If they ever get upstream, I will change bitmap files to use that interface when it is present. > >> So would you be happy with something like the following? > > It's at least a small improvement.. Thanks. NeilBrown -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
