On Thu, Sep 17, 2015 at 8:41 AM, Ross Zwisler
[off-list ref] wrote:

On Wed, Sep 16, 2015 at 02:12:18PM +0300, Kirill A. Shutemov wrote:

quoted

On Tue, Sep 15, 2015 at 04:52:42PM -0700, Dan Williams wrote:

quoted

Hi Kirill,

On Fri, Aug 7, 2015 at 4:53 AM, Kirill A. Shutemov
[off-list ref] wrote:

quoted

DAX is not so special: we need i_mmap_lock to protect mapping->i_mmap.

__dax_pmd_fault() uses unmap_mapping_range() shoot out zero page from
all mappings. We need to drop i_mmap_lock there to avoid lock deadlock.

Re-aquiring the lock should be fine since we check i_size after the
point.

Not-yet-signed-off-by: Matthew Wilcox [off-list ref]
Signed-off-by: Kirill A. Shutemov <redacted>
---
 fs/dax.c    | 35 +++++++++++++++++++----------------
 mm/memory.c | 11 ++---------
 2 files changed, 21 insertions(+), 25 deletions(-)

diff --git a/fs/dax.c b/fs/dax.c
index 9ef9b80cc132..ed54efedade6 100644
--- a/fs/dax.c
+++ b/fs/dax.c

@@ -554,6 +554,25 @@ int __dax_pmd_fault(struct vm_area_struct *vma, unsigned long address,
        if (!buffer_size_valid(&bh) || bh.b_size < PMD_SIZE)
                goto fallback;

+       if (buffer_unwritten(&bh) || buffer_new(&bh)) {
+               int i;
+               for (i = 0; i < PTRS_PER_PMD; i++)
+                       clear_page(kaddr + i * PAGE_SIZE);

This patch, now upstream as commit 46c043ede471, moves the call to
clear_page() earlier in __dax_pmd_fault().  However, 'kaddr' is not
set at this point, so I'm not sure this path was ever tested.

Ughh. It's obviously broken.

I took fs/dax.c part of the patch from Matthew. And I'm not sure now we
would need to move this "if (buffer_unwritten(&bh) || buffer_new(&bh)) {"
block around. It should work fine where it was before. Right?
Matthew?

Moving the "if (buffer_unwritten(&bh) || buffer_new(&bh)) {" block back seems
correct to me.  Matthew is out for a while, so we should probably take care of
this without him.

I'd say leave it at its current location and add a local call to
bdev_direct_access() as I'm not sure you'd want to trigger one of the
failure conditions without having zeroed the page.  I.e. right before
vmf_insert_pfn_pmd() is probably too late.

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