[RFC PATCH 05/20] dev_dax_iomap: Add dax_operations for use by fs-dax on devdax

John Groves
2024-02-23T17:42:32Z

Notes about this commit:

* These methods are based somewhat loosely on pmem_dax_ops from
drivers/nvdimm/pmem.c

* dev_dax_direct_access() is returns the hpa, pfn and kva. The kva was
newly stored as dev_dax->virt_addr by dev_dax_probe().

* The hpa/pfn are used for mmap (dax_iomap_fault()), and the kva is used
for read/write (dax_iomap_rw())

* dev_dax_recovery_write() and dev_dax_zero_page_range() have not been
tested yet. I'm looking for suggestions as to how to test those.

Signed-off-by: John Groves <john@groves.net>
---
drivers/dax/bus.c | 107 ++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 107 insertions(+)

diff --git a/drivers/dax/bus.c b/drivers/dax/bus.c
index 664e8c1b9930..06fcda810674 100644
--- a/drivers/dax/bus.c
+++ b/drivers/dax/bus.c
@@ -10,6 +10,12 @@
#include "dax-private.h"
#include "bus.h"

+#if IS_ENABLED(CONFIG_DEV_DAX_IOMAP)
+#include <linux/backing-dev.h>
+#include <linux/pfn_t.h>
+#include <linux/range.h>
+#endif
+
static DEFINE_MUTEX(dax_bus_lock);

#define DAX_NAME_LEN 30
@@ -1349,6 +1355,101 @@ __weak phys_addr_t dax_pgoff_to_phys(struct dev_dax *dev_dax, pgoff_t pgoff,
}
EXPORT_SYMBOL_GPL(dax_pgoff_to_phys);

+#if IS_ENABLED(CONFIG_DEV_DAX_IOMAP)
+
+static void write_dax(void *pmem_addr, struct page *page,
+ unsigned int off, unsigned int len)
+{
+ unsigned int chunk;
+ void *mem;
+
+ while (len) {
+ mem = kmap_local_page(page);
+ chunk = min_t(unsigned int, len, PAGE_SIZE - off);
+ memcpy_flushcache(pmem_addr, mem + off, chunk);
+ kunmap_local(mem);
+ len -= chunk;
+ off = 0;
+ page++;
+ pmem_addr += chunk;
+ }
+}
+
+static long __dev_dax_direct_access(struct dax_device *dax_dev, pgoff_t pgoff,
+ long nr_pages, enum dax_access_mode mode, void **kaddr,
+ pfn_t *pfn)
+{
+ struct dev_dax *dev_dax = dax_get_private(dax_dev);
+ size_t dax_size = dev_dax_size(dev_dax);
+ size_t size = nr_pages << PAGE_SHIFT;
+ size_t offset = pgoff << PAGE_SHIFT;
+ phys_addr_t phys;
+ u64 virt_addr = dev_dax->virt_addr + offset;
+ pfn_t local_pfn;
+ u64 flags = PFN_DEV|PFN_MAP;
+
+ WARN_ON(!dev_dax->virt_addr); /* virt_addr must be saved for direct_access */
+
+ phys = dax_pgoff_to_phys(dev_dax, pgoff, nr_pages << PAGE_SHIFT);
+
+ if (kaddr)
+ *kaddr = (void *)virt_addr;
+
+ local_pfn = phys_to_pfn_t(phys, flags); /* are flags correct? */
+ if (pfn)
+ *pfn = local_pfn;
+
+ /* This the valid size at the specified address */
+ return PHYS_PFN(min_t(size_t, size, dax_size - offset));
+}
+
+static int dev_dax_zero_page_range(struct dax_device *dax_dev, pgoff_t pgoff,
+ size_t nr_pages)
+{
+ long resid = nr_pages << PAGE_SHIFT;
+ long offset = pgoff << PAGE_SHIFT;
+
+ /* Break into one write per dax region */
+ while (resid > 0) {
+ void *kaddr;
+ pgoff_t poff = offset >> PAGE_SHIFT;
+ long len = __dev_dax_direct_access(dax_dev, poff,
+ nr_pages, DAX_ACCESS, &kaddr, NULL);
+ len = min_t(long, len, PAGE_SIZE);
+ write_dax(kaddr, ZERO_PAGE(0), offset, len);
+
+ offset += len;
+ resid -= len;
+ }
+ return 0;
+}
+
+static long dev_dax_direct_access(struct dax_device *dax_dev,
+ pgoff_t pgoff, long nr_pages, enum dax_access_mode mode,
+ void **kaddr, pfn_t *pfn)
+{
+ return __dev_dax_direct_access(dax_dev, pgoff, nr_pages, mode, kaddr, pfn);
+}
+
+static size_t dev_dax_recovery_write(struct dax_device *dax_dev, pgoff_t pgoff,
+ void *addr, size_t bytes, struct iov_iter *i)
+{
+ size_t len, off;
+
+ off = offset_in_page(addr);
+ len = PFN_PHYS(PFN_UP(off + bytes));
+
+ return _copy_from_iter_flushcache(addr, bytes, i);
+}
+
+static const struct dax_operations dev_dax_ops = {
+ .direct_access = dev_dax_direct_access,
+ .zero_page_range = dev_dax_zero_page_range,
+ .recovery_write = dev_dax_recovery_write,
+};
+
+#endif /* IS_ENABLED(CONFIG_DEV_DAX_IOMAP) */
+
struct dev_dax *devm_create_dev_dax(struct dev_dax_data *data)
{
struct dax_region *dax_region = data->dax_region;
@@ -1404,11 +1505,17 @@ struct dev_dax *devm_create_dev_dax(struct dev_dax_data *data)
}
}

+#if IS_ENABLED(CONFIG_DEV_DAX_IOMAP)
+ /* holder_ops currently populated separately in a slightly hacky way */
+ dax_dev = alloc_dax(dev_dax, &dev_dax_ops);
+#else
/*
* No dax_operations since there is no access to this device outside of
* mmap of the resulting character device.
*/
dax_dev = alloc_dax(dev_dax, NULL);
+#endif
+
if (IS_ERR(dax_dev)) {
rc = PTR_ERR(dax_dev);
goto err_alloc_dax;
--
2.43.0