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@xxxxxxxxx>
---
fs/dax.c | 47 ++++++++++++++++++++++++++++++++++++++---------
1 file changed, 38 insertions(+), 9 deletions(-)
diff --git a/fs/dax.c b/fs/dax.c
index 9eb46f4b6e38..a429a00628c5 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -567,8 +567,9 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
unsigned blkbits = inode->i_blkbits;
unsigned long pmd_addr = address & PMD_MASK;
bool write = flags & FAULT_FLAG_WRITE;
- struct block_device *bdev;
+ struct block_device *bdev = NULL;
pgoff_t size, pgoff;
+ const char *reason;
sector_t block;
int result = 0;
@@ -579,21 +580,28 @@ 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_page_pmd(vma, address, pmd);
+ reason = "cow write";
return VM_FAULT_FALLBACK;
}
/* If the PMD would extend outside the VMA */
- if (pmd_addr < vma->vm_start)
- return VM_FAULT_FALLBACK;
- if ((pmd_addr + PMD_SIZE) > vma->vm_end)
- return VM_FAULT_FALLBACK;
+ if (pmd_addr < vma->vm_start) {
+ reason = "vma start unaligned";
+ goto fallback;
+ }
+ if ((pmd_addr + PMD_SIZE) > vma->vm_end) {
+ reason = "vma end unaligned";
+ goto 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) {
+ reason = "offset + huge page size > file size";
return VM_FAULT_FALLBACK;
+ }
memset(&bh, 0, sizeof(bh));
block = (sector_t)pgoff << (PAGE_SHIFT - blkbits);
@@ -609,8 +617,10 @@ 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) {
+ reason = "block allocation size invalid";
goto fallback;
+ }
/*
* If we allocated new storage, make sure no process has any
@@ -633,23 +643,33 @@ 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) {
+ reason = "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)) {
+ reason = "no zero page";
goto fallback;
+ }
ptl = pmd_lock(vma->vm_mm, pmd);
if (!pmd_none(*pmd)) {
spin_unlock(ptl);
+ reason = "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);
@@ -678,6 +698,7 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
*/
if (pfn_t_has_page(dax.pfn)) {
dax_unmap_atomic(bdev, &dax);
+ reason = "pfn not in memmap";
goto fallback;
}
@@ -690,6 +711,11 @@ 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);
}
@@ -703,6 +729,9 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
return result;
fallback:
+ pr_debug("%s%s %s: %s addr: %lx fallback: %s\n", bdev
+ ? dev_name(part_to_dev(bdev->bd_part)) : "", bdev
+ ? ": " : "", __func__, current->comm, address, reason);
count_vm_event(THP_FAULT_FALLBACK);
result = VM_FAULT_FALLBACK;
goto out;
--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@xxxxxxxxx. For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>