Re: [PATCH] mm: take i_mmap_lock in unmap_mapping_range() for DAX

[Kirill A. Shutemov]

Ross Zwisler wrote:
> On Wed, Sep 16, 2015 at 02:12:18PM +0300, Kirill A. Shutemov wrote:
> > On Tue, Sep 15, 2015 at 04:52:42PM -0700, Dan Williams wrote:
> > > Hi Kirill,
> > > 
> > > On Fri, Aug 7, 2015 at 4:53 AM, Kirill A. Shutemov
> > >  wrote:
> > > > 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 
> > > > Signed-off-by: Kirill A. Shutemov 
> > > > ---
> > > >  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.b_size < PMD_SIZE)
> > > > goto fallback;
> > > >
> > > > +   if (buffer_unwritten() || buffer_new()) {
> > > > +   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() || buffer_new()) {"
> > block around. It should work fine where it was before. Right?
> > Matthew?
> 
> Moving the "if (buffer_unwritten() || buffer_new()) {" block back seems
> correct to me.  Matthew is out for a while, so we should probably take care of
> this without him.
> 
> Kirill, do you want to whip up a quick patch?  I'm happy to do it if you're
> busy.

I would be better if you'll prepare the patch. Thanks.

-- 
 Kirill
--
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Re: [PATCH] mm: take i_mmap_lock in unmap_mapping_range() for DAX

[Dan Williams]

On Thu, Sep 17, 2015 at 8:41 AM, Ross Zwisler
 wrote:
> On Wed, Sep 16, 2015 at 02:12:18PM +0300, Kirill A. Shutemov wrote:
>> On Tue, Sep 15, 2015 at 04:52:42PM -0700, Dan Williams wrote:
>> > Hi Kirill,
>> >
>> > On Fri, Aug 7, 2015 at 4:53 AM, Kirill A. Shutemov
>> >  wrote:
>> > > 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 
>> > > Signed-off-by: Kirill A. Shutemov 
>> > > ---
>> > >  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.b_size < PMD_SIZE)
>> > > goto fallback;
>> > >
>> > > +   if (buffer_unwritten() || buffer_new()) {
>> > > +   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() || buffer_new()) {"
>> block around. It should work fine where it was before. Right?
>> Matthew?
>
> Moving the "if (buffer_unwritten() || buffer_new()) {" 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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Re: [PATCH] mm: take i_mmap_lock in unmap_mapping_range() for DAX

[Ross Zwisler]

On Wed, Sep 16, 2015 at 02:12:18PM +0300, Kirill A. Shutemov wrote:
> On Tue, Sep 15, 2015 at 04:52:42PM -0700, Dan Williams wrote:
> > Hi Kirill,
> > 
> > On Fri, Aug 7, 2015 at 4:53 AM, Kirill A. Shutemov
> >  wrote:
> > > 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 
> > > Signed-off-by: Kirill A. Shutemov 
> > > ---
> > >  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.b_size < PMD_SIZE)
> > > goto fallback;
> > >
> > > +   if (buffer_unwritten() || buffer_new()) {
> > > +   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() || buffer_new()) {"
> block around. It should work fine where it was before. Right?
> Matthew?

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

Kirill, do you want to whip up a quick patch?  I'm happy to do it if you're
busy.
--
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Re: [PATCH] mm: take i_mmap_lock in unmap_mapping_range() for DAX

[Kirill A. Shutemov]

Ross Zwisler wrote:
> On Wed, Sep 16, 2015 at 02:12:18PM +0300, Kirill A. Shutemov wrote:
> > On Tue, Sep 15, 2015 at 04:52:42PM -0700, Dan Williams wrote:
> > > Hi Kirill,
> > > 
> > > On Fri, Aug 7, 2015 at 4:53 AM, Kirill A. Shutemov
> > >  wrote:
> > > > 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 
> > > > Signed-off-by: Kirill A. Shutemov 
> > > > ---
> > > >  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.b_size < PMD_SIZE)
> > > > goto fallback;
> > > >
> > > > +   if (buffer_unwritten() || buffer_new()) {
> > > > +   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() || buffer_new()) {"
> > block around. It should work fine where it was before. Right?
> > Matthew?
> 
> Moving the "if (buffer_unwritten() || buffer_new()) {" block back seems
> correct to me.  Matthew is out for a while, so we should probably take care of
> this without him.
> 
> Kirill, do you want to whip up a quick patch?  I'm happy to do it if you're
> busy.

I would be better if you'll prepare the patch. Thanks.

-- 
 Kirill
--
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Re: [PATCH] mm: take i_mmap_lock in unmap_mapping_range() for DAX

[Dan Williams]

On Thu, Sep 17, 2015 at 8:41 AM, Ross Zwisler
 wrote:
> On Wed, Sep 16, 2015 at 02:12:18PM +0300, Kirill A. Shutemov wrote:
>> On Tue, Sep 15, 2015 at 04:52:42PM -0700, Dan Williams wrote:
>> > Hi Kirill,
>> >
>> > On Fri, Aug 7, 2015 at 4:53 AM, Kirill A. Shutemov
>> >  wrote:
>> > > 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 
>> > > Signed-off-by: Kirill A. Shutemov 
>> > > ---
>> > >  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.b_size < PMD_SIZE)
>> > > goto fallback;
>> > >
>> > > +   if (buffer_unwritten() || buffer_new()) {
>> > > +   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() || buffer_new()) {"
>> block around. It should work fine where it was before. Right?
>> Matthew?
>
> Moving the "if (buffer_unwritten() || buffer_new()) {" 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.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH] mm: take i_mmap_lock in unmap_mapping_range() for DAX

[Ross Zwisler]

On Wed, Sep 16, 2015 at 02:12:18PM +0300, Kirill A. Shutemov wrote:
> On Tue, Sep 15, 2015 at 04:52:42PM -0700, Dan Williams wrote:
> > Hi Kirill,
> > 
> > On Fri, Aug 7, 2015 at 4:53 AM, Kirill A. Shutemov
> >  wrote:
> > > 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 
> > > Signed-off-by: Kirill A. Shutemov 
> > > ---
> > >  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.b_size < PMD_SIZE)
> > > goto fallback;
> > >
> > > +   if (buffer_unwritten() || buffer_new()) {
> > > +   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() || buffer_new()) {"
> block around. It should work fine where it was before. Right?
> Matthew?

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

Kirill, do you want to whip up a quick patch?  I'm happy to do it if you're
busy.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH] mm: take i_mmap_lock in unmap_mapping_range() for DAX

[Kirill A. Shutemov]

On Tue, Sep 15, 2015 at 04:52:42PM -0700, Dan Williams wrote:
> Hi Kirill,
> 
> On Fri, Aug 7, 2015 at 4:53 AM, Kirill A. Shutemov
>  wrote:
> > 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 
> > Signed-off-by: Kirill A. Shutemov 
> > ---
> >  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.b_size < PMD_SIZE)
> > goto fallback;
> >
> > +   if (buffer_unwritten() || buffer_new()) {
> > +   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() || buffer_new()) {"
block around. It should work fine where it was before. Right?
Matthew?

> I'm also not sure why the compiler is not complaining about an
> uninitialized variable?

No idea.

-- 
 Kirill A. Shutemov
--
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Re: [PATCH] mm: take i_mmap_lock in unmap_mapping_range() for DAX

[Kirill A. Shutemov]

On Tue, Sep 15, 2015 at 04:52:42PM -0700, Dan Williams wrote:
> Hi Kirill,
> 
> On Fri, Aug 7, 2015 at 4:53 AM, Kirill A. Shutemov
>  wrote:
> > 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 
> > Signed-off-by: Kirill A. Shutemov 
> > ---
> >  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.b_size < PMD_SIZE)
> > goto fallback;
> >
> > +   if (buffer_unwritten() || buffer_new()) {
> > +   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() || buffer_new()) {"
block around. It should work fine where it was before. Right?
Matthew?

> I'm also not sure why the compiler is not complaining about an
> uninitialized variable?

No idea.

-- 
 Kirill A. Shutemov
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH] mm: take i_mmap_lock in unmap_mapping_range() for DAX

[Dan Williams]

Hi Kirill,

On Fri, Aug 7, 2015 at 4:53 AM, Kirill A. Shutemov
 wrote:
> 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 
> Signed-off-by: Kirill A. Shutemov 
> ---
>  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.b_size < PMD_SIZE)
> goto fallback;
>
> +   if (buffer_unwritten() || buffer_new()) {
> +   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.  I'm
also not sure why the compiler is not complaining about an
uninitialized variable?
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Re: [PATCH] mm: take i_mmap_lock in unmap_mapping_range() for DAX

[Dan Williams]

Hi Kirill,

On Fri, Aug 7, 2015 at 4:53 AM, Kirill A. Shutemov
 wrote:
> 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 
> Signed-off-by: Kirill A. Shutemov 
> ---
>  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.b_size < PMD_SIZE)
> goto fallback;
>
> +   if (buffer_unwritten() || buffer_new()) {
> +   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.  I'm
also not sure why the compiler is not complaining about an
uninitialized variable?
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