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authorDan Williams <dan.j.williams@intel.com>2018-03-09 17:44:31 -0800
committerDan Williams <dan.j.williams@intel.com>2018-05-22 07:19:08 -0700
commit5fac7408d828719db6d3fdba63e3c3726a6d1ee5 (patch)
tree46d0b7891fa01a63292d9a2dd58012830160694d /include/linux/dax.h
parenta9b6de77b1a3ff729f7bfc54b2e17711776a416c (diff)
mm, fs, dax: handle layout changes to pinned dax mappings
Background: get_user_pages() in the filesystem pins file backed memory pages for access by devices performing dma. However, it only pins the memory pages not the page-to-file offset association. If a file is truncated the pages are mapped out of the file and dma may continue indefinitely into a page that is owned by a device driver. This breaks coherency of the file vs dma, but the assumption is that if userspace wants the file-space truncated it does not matter what data is inbound from the device, it is not relevant anymore. The only expectation is that dma can safely continue while the filesystem reallocates the block(s). Problem: This expectation that dma can safely continue while the filesystem changes the block map is broken by dax. With dax the target dma page *is* the filesystem block. The model of leaving the page pinned for dma, but truncating the file block out of the file, means that the filesytem is free to reallocate a block under active dma to another file and now the expected data-incoherency situation has turned into active data-corruption. Solution: Defer all filesystem operations (fallocate(), truncate()) on a dax mode file while any page/block in the file is under active dma. This solution assumes that dma is transient. Cases where dma operations are known to not be transient, like RDMA, have been explicitly disabled via commits like 5f1d43de5416 "IB/core: disable memory registration of filesystem-dax vmas". The dax_layout_busy_page() routine is called by filesystems with a lock held against mm faults (i_mmap_lock) to find pinned / busy dax pages. The process of looking up a busy page invalidates all mappings to trigger any subsequent get_user_pages() to block on i_mmap_lock. The filesystem continues to call dax_layout_busy_page() until it finally returns no more active pages. This approach assumes that the page pinning is transient, if that assumption is violated the system would have likely hung from the uncompleted I/O. Cc: Jeff Moyer <jmoyer@redhat.com> Cc: Dave Chinner <david@fromorbit.com> Cc: Matthew Wilcox <mawilcox@microsoft.com> Cc: Alexander Viro <viro@zeniv.linux.org.uk> Cc: "Darrick J. Wong" <darrick.wong@oracle.com> Cc: Ross Zwisler <ross.zwisler@linux.intel.com> Cc: Dave Hansen <dave.hansen@linux.intel.com> Cc: Andrew Morton <akpm@linux-foundation.org> Reported-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Christoph Hellwig <hch@lst.de> Reviewed-by: Jan Kara <jack@suse.cz> Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Diffstat (limited to 'include/linux/dax.h')
-rw-r--r--include/linux/dax.h7
1 files changed, 7 insertions, 0 deletions
diff --git a/include/linux/dax.h b/include/linux/dax.h
index f9eb22ad341e..25bab6abb695 100644
--- a/include/linux/dax.h
+++ b/include/linux/dax.h
@@ -83,6 +83,8 @@ static inline void fs_put_dax(struct dax_device *dax_dev)
struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev);
int dax_writeback_mapping_range(struct address_space *mapping,
struct block_device *bdev, struct writeback_control *wbc);
+
+struct page *dax_layout_busy_page(struct address_space *mapping);
#else
static inline int bdev_dax_supported(struct super_block *sb, int blocksize)
{
@@ -103,6 +105,11 @@ static inline struct dax_device *fs_dax_get_by_bdev(struct block_device *bdev)
return NULL;
}
+static inline struct page *dax_layout_busy_page(struct address_space *mapping)
+{
+ return NULL;
+}
+
static inline int dax_writeback_mapping_range(struct address_space *mapping,
struct block_device *bdev, struct writeback_control *wbc)
{