Re: [PATCH 11/12] dax: Disable huge page handling

[Jan Kara <jack@suse.cz>]

On Thu 10-03-16 11:34:39, Dan Williams wrote:
> On Thu, Mar 10, 2016 at 11:18 AM, Jan Kara <jack@suse.cz> wrote:
> > Currently the handling of huge pages for DAX is racy. For example the
> > following can happen:
> >
> > CPU0 (THP write fault)                  CPU1 (normal read fault)
> >
> > __dax_pmd_fault()                       __dax_fault()
> >   get_block(inode, block, &bh, 0) -> not mapped
> >                                         get_block(inode, block, &bh, 0)
> >                                           -> not mapped
> >   if (!buffer_mapped(&bh) && write)
> >     get_block(inode, block, &bh, 1) -> allocates blocks
> >   truncate_pagecache_range(inode, lstart, lend);
> >                                         dax_load_hole();
> >
> > This results in data corruption since process on CPU1 won't see changes
> > into the file done by CPU0.
> >
> > The race can happen even if two normal faults race however with THP the
> > situation is even worse because the two faults don't operate on the same
> > entries in the radix tree and we want to use these entries for
> > serialization. So disable THP support in DAX code for now.
> >
> > Signed-off-by: Jan Kara <jack@suse.cz>
> > ---
> >  fs/dax.c            | 2 +-
> >  include/linux/dax.h | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/dax.c b/fs/dax.c
> > index 3951237ff248..7148fcdb2c92 100644
> > --- a/fs/dax.c
> > +++ b/fs/dax.c
> > @@ -715,7 +715,7 @@ int dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
> >  }
> >  EXPORT_SYMBOL_GPL(dax_fault);
> >
> > -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +#if 0
> >  /*
> >   * The 'colour' (ie low bits) within a PMD of a page offset.  This comes up
> >   * more often than one might expect in the below function.
> > diff --git a/include/linux/dax.h b/include/linux/dax.h
> > index 4b63923e1f8d..fd28d824254b 100644
> > --- a/include/linux/dax.h
> > +++ b/include/linux/dax.h
> > @@ -29,7 +29,7 @@ static inline struct page *read_dax_sector(struct block_device *bdev,
> >  }
> >  #endif
> >
> > -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > +#if 0
> >  int dax_pmd_fault(struct vm_area_struct *, unsigned long addr, pmd_t *,
> >                                 unsigned int flags, get_block_t);
> >  int __dax_pmd_fault(struct vm_area_struct *, unsigned long addr, pmd_t *,
> > --
> > 2.6.2
> >
> 
> Maybe switch to marking FS_DAX_PMD as "depends on BROKEN" again?  That
> way we re-use the same mechanism as the check for the presence of
> ZONE_DEVICE / struct page for the given pfn.

Yeah, maybe I could do that. At this point PMD fault handler would not even
compile but I could possibly massage it so that it will work with the new
locking unless you try mixing PMD and PTE faults...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR
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