From: "Aneesh Kumar K.V" Subject: Re: [PATCH v7 15/17] dax: add struct iomap based DAX PMD support Date: Mon, 17 Oct 2016 11:36:55 +0530 Message-ID: <87a8e3tsow.fsf@linux.vnet.ibm.com> References: <20161013154224.GB30680@quack2.suse.cz> <1476386619-2727-1-git-send-email-ross.zwisler@linux.intel.com> Mime-Version: 1.0 Content-Type: text/plain Cc: Ross Zwisler , "Theodore Ts'o" , Alexander Viro , Andreas Dilger , Andrew Morton , Christoph Hellwig , Dan Williams , Dave Chinner , Jan Kara , Matthew Wilcox , linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-nvdimm@lists.01.org, linux-xfs@vger.kernel.org To: Ross Zwisler , linux-kernel@vger.kernel.org Return-path: In-Reply-To: <1476386619-2727-1-git-send-email-ross.zwisler@linux.intel.com> Sender: owner-linux-mm@kvack.org List-Id: linux-ext4.vger.kernel.org Ross Zwisler writes: > 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 > Reviewed-by: Jan Kara > --- > fs/dax.c | 378 +++++++++++++++++++++++++++++++++++++++++++++++----- > include/linux/dax.h | 55 ++++++-- > mm/filemap.c | 3 +- > 3 files changed, 386 insertions(+), 50 deletions(-) > > diff --git a/fs/dax.c b/fs/dax.c > index 0582c7c..153cfd5 100644 > --- a/fs/dax.c > +++ b/fs/dax.c > @@ -76,6 +76,26 @@ static void dax_unmap_atomic(struct block_device *bdev, > blk_queue_exit(bdev->bd_queue); > } > > +static int dax_is_pmd_entry(void *entry) > +{ > + return (unsigned long)entry & RADIX_DAX_PMD; > +} > + > +static int dax_is_pte_entry(void *entry) > +{ > + return !((unsigned long)entry & RADIX_DAX_PMD); > +} > + > +static int dax_is_zero_entry(void *entry) > +{ > + return (unsigned long)entry & RADIX_DAX_HZP; > +} How about dax_is_pmd_zero_entry() ? > + > +static int dax_is_empty_entry(void *entry) > +{ > + return (unsigned long)entry & RADIX_DAX_EMPTY; > +} > + -aneesh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org