On Wed, 2011-01-05 at 13:30 -0800, Linus Torvalds wrote: > On Wed, Jan 5, 2011 at 1:16 PM, Trond Myklebust > <Trond.Myklebust@xxxxxxxxxx> wrote: > > > > So what should be the preferred way to ensure data gets flushed when > > you've written directly to a page, and then want to read through the > > vm_map_ram() virtual range? Should we be adding new semantics to > > flush_kernel_dcache_page()? > > The "preferred way" is actually simple: "don't do that". IOW, if some > page is accessed through a virtual mapping you've set up, then > _always_ access it through that virtual mapping. > > Now, when that is impossible (and yes, it sometimes is), then you > should flush after doing all writes. And if you do the write through > the regular kernel mapping, you should use flush_dcache_page(). And if > you did it through the virtual mapping, you should use > "flush_kernel_vmap_range()" or whatever. > > NOTE! I really didn't look those up very closely, and if the accesses > can happen concurrently you are basically screwed, so you do need to > do locking or something else to guarantee that there is some nice > sequential order. And maybe I forgot something. Which is why I do > suggest "don't do that" as a primary approach to the problem if at all > possible. > > Oh, and you may need to flush before reading too (and many writes do > end up being "read-modify-write" cycles) in case it's possible that > you have stale data from a previous read that was then invalidated by > a write to the aliasing address. Even if that write was flushed out, > the stale read data may exist at the virtual address. I forget what > all we required - in the end the only sane model is "virtual caches > suck so bad that anybody who does them should be laughed at for being > a retard". Yes. The fix I sent out was a call to invalidate_kernel_vmap_range(), which takes care of invalidating the cache prior to a virtual address read. My question was specifically about the write through the regular kernel mapping: according to Russell and my reading of the cachetlb.txt documentation, flush_dcache_page() is only guaranteed to have an effect on page cache pages. flush_kernel_dcache_page() (not to be confused with flush_dcache_page) would appear to be the closest fit according to my reading of the documentation, however the ARM implementation appears to be a no-op... -- Trond Myklebust Linux NFS client maintainer NetApp Trond.Myklebust@xxxxxxxxxx www.netapp.com -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
