On Thu, Oct 13, 2016 at 05:42:24PM +0200, Jan Kara wrote: > On Wed 12-10-16 16:50:20, Ross Zwisler wrote: > > DAX PMDs have been disabled since Jan Kara introduced DAX radix tree based > > locking. This patch allows DAX PMDs to participate in the DAX radix tree > > based locking scheme so that they can be re-enabled using the new struct > > iomap based fault handlers. > > > > There are currently three types of DAX 4k entries: 4k zero pages, 4k DAX > > mappings that have an associated block allocation, and 4k DAX empty > > entries. The empty entries exist to provide locking for the duration of a > > given page fault. > > > > This patch adds three equivalent 2MiB DAX entries: Huge Zero Page (HZP) > > entries, PMD DAX entries that have associated block allocations, and 2 MiB > > DAX empty entries. > > > > Unlike the 4k case where we insert a struct page* into the radix tree for > > 4k zero pages, for HZP we insert a DAX exceptional entry with the new > > RADIX_DAX_HZP flag set. This is because we use a single 2 MiB zero page in > > every 2MiB hole mapping, and it doesn't make sense to have that same struct > > page* with multiple entries in multiple trees. This would cause contention > > on the single page lock for the one Huge Zero Page, and it would break the > > page->index and page->mapping associations that are assumed to be valid in > > many other places in the kernel. > > > > One difficult use case is when one thread is trying to use 4k entries in > > radix tree for a given offset, and another thread is using 2 MiB entries > > for that same offset. The current code handles this by making the 2 MiB > > user fall back to 4k entries for most cases. This was done because it is > > the simplest solution, and because the use of 2MiB pages is already > > opportunistic. > > > > If we were to try to upgrade from 4k pages to 2MiB pages for a given range, > > we run into the problem of how we lock out 4k page faults for the entire > > 2MiB range while we clean out the radix tree so we can insert the 2MiB > > entry. We can solve this problem if we need to, but I think that the cases > > where both 2MiB entries and 4K entries are being used for the same range > > will be rare enough and the gain small enough that it probably won't be > > worth the complexity. > > > > Signed-off-by: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> > > Just one small bug below. Feel free to add: > > Reviewed-by: Jan Kara <jack@xxxxxxx> > > after fixing that. Fixed, thank you for the catch and the review! -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
