On Fri, Oct 30, 2015 at 12:43 PM, Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> wrote: > 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. I'll let Dave chime in, but a lot of the flush requirements come from guaranteeing the state of the metadata, if metadata updates can be done with REQ_FUA then there is no subsequent need to flush. >> 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. That's a bit of an exaggeration. Avoiding DAX where it is not necessary is not "abandoning DAX", it's using the right tool for the job. Page cache is fine for many cases. >> 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. Right, part of the problem is that the driver doesn't know which blocks are actively DAX mapped. I think we can incrementally fix that without requiring DAX specific fsync/msync handling code for each fs that supports DAX. -- 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
