[PATCH v3 07/25] fsdax: Hold dax lock over mapping insertion

[Dan Williams <dan.j.williams@intel.com>]

In preparation for dax_insert_entry() to start taking page and pgmap
references ensure that page->pgmap is valid by holding the
dax_read_lock() over both dax_direct_access() and dax_insert_entry().

I.e. the code that wants to elevate the reference count of a pgmap page
from 0 -> 1 must ensure that the pgmap is not exiting and will not start
exiting until the proper references have been taken.

Cc: Matthew Wilcox <willy@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Cc: "Darrick J. Wong" <djwong@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
---
 fs/dax.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 1d4f0072e58d..6990a6e7df9f 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -1107,10 +1107,9 @@ static int dax_iomap_direct_access(const struct iomap *iomap, loff_t pos,
 		size_t size, void **kaddr, pfn_t *pfnp)
 {
 	pgoff_t pgoff = dax_iomap_pgoff(iomap, pos);
-	int id, rc = 0;
 	long length;
+	int rc = 0;
 
-	id = dax_read_lock();
 	length = dax_direct_access(iomap->dax_dev, pgoff, PHYS_PFN(size),
 				   DAX_ACCESS, kaddr, pfnp);
 	if (length < 0) {
@@ -1135,7 +1134,6 @@ static int dax_iomap_direct_access(const struct iomap *iomap, loff_t pos,
 	if (!*kaddr)
 		rc = -EFAULT;
 out:
-	dax_read_unlock(id);
 	return rc;
 }
 
@@ -1588,7 +1586,7 @@ static vm_fault_t dax_fault_iter(struct vm_fault *vmf,
 	loff_t pos = (loff_t)xas->xa_index << PAGE_SHIFT;
 	bool write = iter->flags & IOMAP_WRITE;
 	unsigned long entry_flags = pmd ? DAX_PMD : 0;
-	int err = 0;
+	int err = 0, id;
 	pfn_t pfn;
 	void *kaddr;
 
@@ -1608,11 +1606,15 @@ static vm_fault_t dax_fault_iter(struct vm_fault *vmf,
 		return pmd ? VM_FAULT_FALLBACK : VM_FAULT_SIGBUS;
 	}
 
+	id = dax_read_lock();
 	err = dax_iomap_direct_access(iomap, pos, size, &kaddr, &pfn);
-	if (err)
+	if (err) {
+		dax_read_unlock(id);
 		return pmd ? VM_FAULT_FALLBACK : dax_fault_return(err);
+	}
 
 	*entry = dax_insert_entry(xas, vmf, iter, *entry, pfn, entry_flags);
+	dax_read_unlock(id);
 
 	if (write &&
 	    srcmap->type != IOMAP_HOLE && srcmap->addr != iomap->addr) {