Re: [PATCH 07/10] dax: Disable huge page handling

From: Ross Zwisler <ross.zwisler@linux.intel.com>
To: Jan Kara <jack@suse.cz>
Cc: linux-nvdimm@lists.01.org, NeilBrown <neilb@suse.com>, Wilcox,
Subject: Re: [PATCH 07/10] dax: Disable huge page handling
Date: Wed, 23 Mar 2016 14:50:00 -0600	[thread overview]
Message-ID: <20160323205000.GE5544@linux.intel.com> (raw)
In-Reply-To: <1458566575-28063-8-git-send-email-jack@suse.cz>

On Mon, Mar 21, 2016 at 02:22:52PM +0100, Jan Kara 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.

Yep, I agree that we should disable PMD faults until we get the multi-order
radix tree work finished and integrated with this locking.

I do agree with Dan though that it would be preferable to disable PMD faults
by having CONFIG_FS_DAX_PMD depend on BROKEN.  That seems smaller and easier
to switch PMD faults back on for testing.

> 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 0329ec0bee2e..444e9dd079ca 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -719,7 +719,7 @@ 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
> 
_______________________________________________
Linux-nvdimm mailing list
Linux-nvdimm@lists.01.org
https://lists.01.org/mailman/listinfo/linux-nvdimm