linux-ext4 - Re: [PATCH 13/18] dax: Make huge page handling depend of CONFIG

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Date:	Fri, 29 Apr 2016 13:53:40 -0600
From:	Ross Zwisler <ross.zwisler@...ux.intel.com>
To:	Jan Kara <jack@...e.cz>
Cc:	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-mm@...ck.org, Ross Zwisler <ross.zwisler@...ux.intel.com>,
	Dan Williams <dan.j.williams@...el.com>,
	linux-nvdimm@...ts.01.org, Matthew Wilcox <willy@...ux.intel.com>
Subject: Re: [PATCH 13/18] dax: Make huge page handling depend of
 CONFIG_BROKEN

On Mon, Apr 18, 2016 at 11:35:36PM +0200, 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 make THP support in DAX code depend on CONFIG_BROKEN
> for now.
> 
> Signed-off-by: Jan Kara <jack@...e.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 d7addfab2094..388327f56fa8 100644
> --- a/fs/dax.c
> +++ b/fs/dax.c
> @@ -707,7 +707,7 @@ int dax_fault(struct vm_area_struct *vma, struct vm_fault *vmf,
>  }
>  EXPORT_SYMBOL_GPL(dax_fault);
>  
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && defined(CONFIG_BROKEN)
>  /*
>   * 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 7c45ac7ea1d1..0591f4853228 100644
> --- a/include/linux/dax.h
> +++ b/include/linux/dax.h
> @@ -23,7 +23,7 @@ static inline struct page *read_dax_sector(struct block_device *bdev,
>  }
>  #endif
>  
> -#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +#if defined(CONFIG_TRANSPARENT_HUGEPAGE) && defined(CONFIG_BROKEN)
>  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 *,
> -- 
> 2.6.6

Ah, sorry, I think by "make PMD depend on CONFIG_BROKEN" Dan & I meant this:

diff --git a/fs/Kconfig b/fs/Kconfig
index 6725f59..b8fcb41 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -52,6 +52,7 @@ config FS_DAX_PMD
        depends on FS_DAX
        depends on ZONE_DEVICE
        depends on TRANSPARENT_HUGEPAGE
+       depends on BROKEN
 
 endif # BLOCK

This has the benefit that you have only one place to quickly reenable PMD code
if/when we want to work on it.
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