Re: [PATCH RFC v4 40/42] kmsan: ext4: skip block merging logic in ext4_mpage_readpages for KMSAN

[Eric Biggers <ebiggers@google.com>]

On Fri, Dec 20, 2019 at 07:49:53PM +0100, glider@google.com wrote:
> KMSAN doesn't allow treating adjacent memory pages as such, if they were
> allocated by different alloc_pages() calls.
> ext4_mpage_readpages() however does so: adjacent pages end up being passed
> together to dma_direct_map_sg().
> To prevent this, jump directly to the buffer_head-based read function in
> KMSAN builds.
> 
> Signed-off-by: Alexander Potapenko <glider@google.com>
> Cc: "Theodore Ts'o" <tytso@mit.edu>
> Cc: Andreas Dilger <adilger.kernel@dilger.ca>
> Cc: Vegard Nossum <vegard.nossum@oracle.com>
> Cc: Dmitry Vyukov <dvyukov@google.com>
> Cc: Marco Elver <elver@google.com>
> Cc: Andrey Konovalov <andreyknvl@google.com>
> Cc: linux-mm@kvack.org
> ---
> 
> v4:
>  - use IS_ENABLED, as requested by Marco Elver
> 
> Change-Id: I54ae8af536626a988e6398ff18a06c179b0ce034
> ---
>  fs/ext4/readpage.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/fs/ext4/readpage.c b/fs/ext4/readpage.c
> index fef7755300c3..1e46ddf7a854 100644
> --- a/fs/ext4/readpage.c
> +++ b/fs/ext4/readpage.c
> @@ -252,6 +252,16 @@ int ext4_mpage_readpages(struct address_space *mapping,
>  		if (page_has_buffers(page))
>  			goto confused;
>  
> +		/*
> +		 * The following code may treat adjacent pages allocated
> +		 * separately as a bigger contiguous allocation.
> +		 * KMSAN doesn't allow this, as the corresponding metadata
> +		 * pages may be separated.
> +		 * Skip this complex logic for KMSAN builds.
> +		 */
> +		if (IS_ENABLED(CONFIG_KMSAN))
> +			goto confused;
> +
>  		block_in_file = (sector_t)page->index << (PAGE_SHIFT - blkbits);
>  		last_block = block_in_file + nr_pages * blocks_per_page;
>  		last_block_in_file = (ext4_readpage_limit(inode) +
> -- 

Why is ext4 special here?  Wouldn't other filesystems need this too?

Is it possible the problem is the page merging logic in bio_add_page()?
Maybe we need something like:

diff --git a/block/bio.c b/block/bio.c
index a5d75f6bf4c7e..9449a1e571ee7 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -646,6 +646,8 @@ static inline bool page_is_mergeable(const struct bio_vec *bv,
 	*same_page = ((vec_end_addr & PAGE_MASK) == page_addr);
 	if (!*same_page && pfn_to_page(PFN_DOWN(vec_end_addr)) + 1 != page)
 		return false;
+	if (!*same_page && IS_ENABLED(CONFIG_KMSAN))
+		return false;
 	return true;
 }