Re: [PATCH] dax: Release PMD lock even when there is no PMD support in DAX

[Matthew Wilcox <willy@infradead.org>]

On Thu, Jan 18, 2018 at 06:07:39AM -0800, Matthew Wilcox wrote:
> On Thu, Jan 18, 2018 at 02:38:39PM +0100, Jan H. Sch�nherr wrote:
> > The function follow_pte_pmd() can theoretically return after having
> > acquired a PMD lock, even when DAX was not compiled with
> > CONFIG_FS_DAX_PMD.
> 
> I don't think it can.  How would a PMD entry get into a DAX VMA if we
> compiled the kernel without CONFIG_FS_DAX_PMD?

How about this patch instead?  Should shut up sparse nicely.

diff --git a/fs/dax.c b/fs/dax.c
index 78b72c48374e..fea1b64d111b 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -586,6 +586,12 @@ pgoff_address(pgoff_t pgoff, struct vm_area_struct *vma)
 	return address;
 }
 
+#ifdef CONFIG_FS_DAX_PMD
+#define dax_pmd(pmdp)	pmdp
+#else
+#define dax_pmd(pmdp)	0
+#endif
+
 /* Walk all mappings of a given index of a file and writeprotect them */
 static void dax_mapping_entry_mkclean(struct address_space *mapping,
 				      pgoff_t index, unsigned long pfn)
@@ -621,8 +627,7 @@ static void dax_mapping_entry_mkclean(struct address_space *mapping,
 		 *
 		 * See Documentation/vm/mmu_notifier.txt
 		 */
-		if (pmdp) {
-#ifdef CONFIG_FS_DAX_PMD
+		if (dax_pmd(pmdp)) {
 			pmd_t pmd;
 
 			if (pfn != pmd_pfn(*pmdp))
@@ -638,7 +643,6 @@ static void dax_mapping_entry_mkclean(struct address_space *mapping,
 			set_pmd_at(vma->vm_mm, address, pmdp, pmd);
 unlock_pmd:
 			spin_unlock(ptl);
-#endif
 		} else {
 			if (pfn != pte_pfn(*ptep))
 				goto unlock_pte;