dax: provide diagnostics for pmd mapping failures

There is a wide gamut of conditions that can trigger the dax pmd path to
fallback to pte mappings.  Ideally we'd have a syscall interface to
determine mapping characteristics after the fact.  In the meantime
provide debug messages.

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Suggested-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 57 insertions, 8 deletions

diff --git a/fs/dax.c b/fs/dax.c
index 96ac3072463d..e1f251dc9654 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -558,6 +558,24 @@ EXPORT_SYMBOL_GPL(dax_fault);
  */
 #define PG_PMD_COLOUR   ((PMD_SIZE >> PAGE_SHIFT) - 1)
 
+static void __dax_dbg(struct buffer_head *bh, unsigned long address,
+                const char *reason, const char *fn)
+{
+        if (bh) {
+                char bname[BDEVNAME_SIZE];
+                bdevname(bh->b_bdev, bname);
+                pr_debug("%s: %s addr: %lx dev %s state %lx start %lld "
+                        "length %zd fallback: %s\n", fn, current->comm,
+                        address, bname, bh->b_state, (u64)bh->b_blocknr,
+                        bh->b_size, reason);
+        } else {
+                pr_debug("%s: %s addr: %lx fallback: %s\n", fn,
+                        current->comm, address, reason);
+        }
+}
+
+#define dax_pmd_dbg(bh, address, reason)        __dax_dbg(bh, address, reason, "dax_pmd")
+
 int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
                 pmd_t *pmd, unsigned int flags, get_block_t get_block,
                 dax_iodone_t complete_unwritten)
@@ -581,21 +599,29 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
         /* Fall back to PTEs if we're going to COW */
         if (write && !(vma->vm_flags & VM_SHARED)) {
                 split_huge_pmd(vma, pmd, address);
+                dax_pmd_dbg(NULL, address, "cow write");
                 return VM_FAULT_FALLBACK;
         }
         /* If the PMD would extend outside the VMA */
-        if (pmd_addr < vma->vm_start)
+        if (pmd_addr < vma->vm_start) {
+                dax_pmd_dbg(NULL, address, "vma start unaligned");
                 return VM_FAULT_FALLBACK;
-        if ((pmd_addr + PMD_SIZE) > vma->vm_end)
+        }
+        if ((pmd_addr + PMD_SIZE) > vma->vm_end) {
+                dax_pmd_dbg(NULL, address, "vma end unaligned");
                 return VM_FAULT_FALLBACK;
+        }
 
         pgoff = linear_page_index(vma, pmd_addr);
         size = (i_size_read(inode) + PAGE_SIZE - 1) >> PAGE_SHIFT;
         if (pgoff >= size)
                 return VM_FAULT_SIGBUS;
         /* If the PMD would cover blocks out of the file */
-        if ((pgoff | PG_PMD_COLOUR) >= size)
+        if ((pgoff | PG_PMD_COLOUR) >= size) {
+                dax_pmd_dbg(NULL, address,
+                                "offset + huge page size > file size");
                 return VM_FAULT_FALLBACK;
+        }
 
         memset(&bh, 0, sizeof(bh));
         block = (sector_t)pgoff << (PAGE_SHIFT - blkbits);
@@ -611,8 +637,10 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
          * just fall back to PTEs.  Calling get_block 512 times in a loop
          * would be silly.
          */
-        if (!buffer_size_valid(&bh) || bh.b_size < PMD_SIZE)
+        if (!buffer_size_valid(&bh) || bh.b_size < PMD_SIZE) {
+                dax_pmd_dbg(&bh, address, "allocated block too small");
                 goto fallback;
+        }
 
         /*
          * If we allocated new storage, make sure no process has any
@@ -635,23 +663,33 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
                 result = VM_FAULT_SIGBUS;
                 goto out;
         }
-        if ((pgoff | PG_PMD_COLOUR) >= size)
+        if ((pgoff | PG_PMD_COLOUR) >= size) {
+                dax_pmd_dbg(&bh, address, "pgoff unaligned");
                 goto fallback;
+        }
 
         if (!write && !buffer_mapped(&bh) && buffer_uptodate(&bh)) {
                 spinlock_t *ptl;
                 pmd_t entry;
                 struct page *zero_page = get_huge_zero_page();
 
-                if (unlikely(!zero_page))
+                if (unlikely(!zero_page)) {
+                        dax_pmd_dbg(&bh, address, "no zero page");
                         goto fallback;
+                }
 
                 ptl = pmd_lock(vma->vm_mm, pmd);
                 if (!pmd_none(*pmd)) {
                         spin_unlock(ptl);
+                        dax_pmd_dbg(&bh, address, "pmd already present");
                         goto fallback;
                 }
 
+                dev_dbg(part_to_dev(bdev->bd_part),
+                                "%s: %s addr: %lx pfn: <zero> sect: %llx\n",
+                                __func__, current->comm, address,
+                                (unsigned long long) to_sector(&bh, inode));
+
                 entry = mk_pmd(zero_page, vma->vm_page_prot);
                 entry = pmd_mkhuge(entry);
                 set_pmd_at(vma->vm_mm, pmd_addr, pmd, entry);
@@ -668,8 +706,13 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
                         result = VM_FAULT_SIGBUS;
                         goto out;
                 }
-                if (length < PMD_SIZE
-                                || (pfn_t_to_pfn(dax.pfn) & PG_PMD_COLOUR)) {
+                if (length < PMD_SIZE) {
+                        dax_pmd_dbg(&bh, address, "dax-length too small");
+                        dax_unmap_atomic(bdev, &dax);
+                        goto fallback;
+                }
+                if (pfn_t_to_pfn(dax.pfn) & PG_PMD_COLOUR) {
+                        dax_pmd_dbg(&bh, address, "pfn unaligned");
                         dax_unmap_atomic(bdev, &dax);
                         goto fallback;
                 }
@@ -680,6 +723,7 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
                  */
                 if (pfn_t_has_page(dax.pfn)) {
                         dax_unmap_atomic(bdev, &dax);
+                        dax_pmd_dbg(&bh, address, "pfn not in memmap");
                         goto fallback;
                 }
 
@@ -692,6 +736,11 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
                 }
                 dax_unmap_atomic(bdev, &dax);
 
+                dev_dbg(part_to_dev(bdev->bd_part),
+                                "%s: %s addr: %lx pfn: %lx sect: %llx\n",
+                                __func__, current->comm, address,
+                                pfn_t_to_pfn(dax.pfn),
+                                (unsigned long long) dax.sector);
                 result |= vmf_insert_pfn_pmd(vma, address, pmd,
                                 dax.pfn, write);
         }