Messages in this thread | | | Date | Tue, 1 Sep 2015 09:06:08 +0200 | From | Christoph Hellwig <> | Subject | Re: [PATCH] dax, pmem: add support for msync |
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On Tue, Sep 01, 2015 at 09:38:03AM +1000, Dave Chinner wrote: > On Mon, Aug 31, 2015 at 12:59:44PM -0600, Ross Zwisler wrote: > > For DAX msync we just need to flush the given range using > > wb_cache_pmem(), which is now a public part of the PMEM API. > > This is wrong, because it still leaves fsync() broken on dax. > > Flushing dirty data to stable storage is the responsibility of the > writeback infrastructure, not the VMA/mm infrasrtucture. For non-dax > configurations, msync defers all that to vfs_fsync_range(), because > it has to be implemented there for fsync() to work. > > Even for DAX, msync has to call vfs_fsync_range() for the filesystem to commit > the backing store allocations to stable storage, so there's not > getting around the fact msync is the wrong place to be flushing > DAX mappings to persistent storage.
DAX does call ->fsync before and after this patch. And with all the recent fixes we take care to ensure data is written though the cache for everything but mmap-access. With this patch from Ross we ensure msync writes back the cache before calling ->fsync so that the filesystem can then do it's work like converting unwritten extents.
The only downside is that previously on Linux you could always use fsync as a replaement for msymc, which isn't true anymore for DAX.
But given that we need the virtual address to write back the cache I can't see how to do this differently given that clwb() needs the user virtual address to flush the cache.
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