On Mon 17-10-16 13:29:49, Ross Zwisler wrote:
> On Tue, Sep 27, 2016 at 06:08:14PM +0200, Jan Kara wrote:
> > Move final handling of COW faults from generic code into DAX fault
> > handler. That way generic code doesn't have to be aware of peculiarities
> > of DAX locking so remove that knowledge.
> >
> > Signed-off-by: Jan Kara <jack@xxxxxxx>
> > ---
> > fs/dax.c | 22 ++++++++++++++++------
> > include/linux/dax.h | 7 -------
> > include/linux/mm.h | 9 +--------
> > mm/memory.c | 14 ++++----------
> > 4 files changed, 21 insertions(+), 31 deletions(-)
> >
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 0dc251ca77b8..b1c503930d1d 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -876,10 +876,15 @@ int dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
> > goto unlock_entry;
> > if (!radix_tree_exceptional_entry(entry)) {
> > vmf->page = entry;
> > - return VM_FAULT_LOCKED;
> > + if (unlikely(PageHWPoison(entry))) {
> > + put_locked_mapping_entry(mapping, vmf->pgoff,
> > + entry);
> > + return VM_FAULT_HWPOISON;
> > + }
> > }
> > - vmf->entry = entry;
> > - return VM_FAULT_DAX_LOCKED;
> > + error = finish_fault(vmf);
> > + put_locked_mapping_entry(mapping, vmf->pgoff, entry);
> > + return error ? error : VM_FAULT_DONE_COW;
> > }
> >
> > if (!buffer_mapped(&bh)) {
> > @@ -1430,10 +1435,15 @@ int iomap_dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
> > goto unlock_entry;
> > if (!radix_tree_exceptional_entry(entry)) {
> > vmf->page = entry;
>
> In __do_fault() we explicitly clear vmf->page in the case where PageHWPoison()
> is set. I think we can get the same behavior here by moving the call that
> sets vmf->page after the PageHWPoison() check.
Actually, the whole HWPoison checking was non-sensical for DAX. We want to
check for HWPoison to avoid reading from poisoned pages. However for DAX we
either use copy_user_dax() which takes care of IO errors / poisoning itself
or we use clear_user_highpage() which doesn't touch the source page. So we
don't have to check for HWPoison at all. Fixed.
> > - return VM_FAULT_LOCKED;
> > + if (unlikely(PageHWPoison(entry))) {
> > + put_locked_mapping_entry(mapping, vmf->pgoff,
> > + entry);
> > + return VM_FAULT_HWPOISON;
> > + }
> > }
> > - vmf->entry = entry;
> > - return VM_FAULT_DAX_LOCKED;
>
> I think we're missing a call to
>
> __SetPageUptodate(new_page);
> before finish_fault()? This call currently lives in do_cow_fault(), and
> is part of the path that we don't skip as part of the VM_FAULT_DAX_LOCKED
> logic.
Ah, great catch. I wonder how the DAX COW test could have passed with this?
Maybe PageUptodate is not used much for anon pages... Anyway thanks for
spotting this.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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