On Fri, Oct 30, 2015 at 11:34:07AM -0700, Dan Williams wrote: > On Thu, Oct 29, 2015 at 1:12 PM, Ross Zwisler > <ross.zwisler@xxxxxxxxxxxxxxx> wrote: > > This patch series adds support for fsync/msync to DAX. > > > > Patches 1 through 8 add various utilities that the DAX code will eventually > > need, and the DAX code itself is added by patch 9. Patches 10 and 11 are > > filesystem changes that are needed after the DAX code is added, but these > > patches may change slightly as the filesystem fault handling for DAX is > > being modified ([1] and [2]). > > > > I've marked this series as RFC because I'm still testing, but I wanted to > > get this out there so people would see the direction I was going and > > hopefully comment on any big red flags sooner rather than later. > > > > I realize that we are getting pretty dang close to the v4.4 merge window, > > but I think that if we can get this reviewed and working it's a much better > > solution than the "big hammer" approach that blindly flushes entire PMEM > > namespaces [3]. > > > > [1] http://oss.sgi.com/archives/xfs/2015-10/msg00523.html > > [2] http://marc.info/?l=linux-ext4&m=144550211312472&w=2 > > [3] https://lists.01.org/pipermail/linux-nvdimm/2015-October/002614.html > > > > Ross Zwisler (11): > > pmem: add wb_cache_pmem() to the PMEM API > > mm: add pmd_mkclean() > > pmem: enable REQ_FLUSH handling > > dax: support dirty DAX entries in radix tree > > mm: add follow_pte_pmd() > > mm: add pgoff_mkclean() > > mm: add find_get_entries_tag() > > fs: add get_block() to struct inode_operations > > dax: add support for fsync/sync > > xfs, ext2: call dax_pfn_mkwrite() on write fault > > ext4: add ext4_dax_pfn_mkwrite() > > This is great to have when the flush-the-world solution ends up > killing performance. However, there are a couple mitigating options > for workloads that dirty small amounts and flush often that we need to > collect data on: > > 1/ Using cache management and pcommit from userspace to skip calls to > msync / fsync. Although, this does not eliminate all calls to > blkdev_issue_flush as the fs may invoke it for other reasons. I > suspect turning on REQ_FUA support eliminates a number of those > invocations, and pmem already satisfies REQ_FUA semantics by default. Sure, I'll turn on REQ_FUA in addition to REQ_FLUSH - I agree that PMEM already handles the requirements of REQ_FUA, but I didn't realize that it might reduce the number of REQ_FLUSH bios we receive. > 2/ Turn off DAX and use the page cache. As Dave mentions [1] we > should enable this control on a per-inode basis. I'm folding in this > capability as a blkdev_ioctl for the next version of the raw block DAX > support patch. Umm...I think you just said "the way to avoid this delay is to just not use DAX". :) I don't think this is where we want to go - we are trying to make DAX better, not abandon it. > It's entirely possible these mitigations won't eliminate the need for > a mechanism like this, but I think we have a bit more work to do to > find out how bad this is in practice as well as the crossover point > where walking the radix becomes prohibitive. I'm guessing a single run through xfstests will be enough to convince you that the "big hammer" approach is untenable. Tests that used to take a second now take several minutes, at least in my VM testing environment... And that's only using a tiny 4GiB namespace. Yes, we can distribute the cost over multiple CPUs, but that just distributes the problem and doesn't reduce the overall work that needs to be done. Ultimately I think that looping through multiple GiB or even TiB of cache lines and blindly writing them back individually on every REQ_FLUSH is going to be a deal breaker. -- 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
