linux-kernel - Re: [PATCH] fs/ext4: Replace kmap_atomic() with kmap_local

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Message-ID: <Y7OvUjWtYPTJYjbU@iweiny-mobl>
Date:   Mon, 2 Jan 2023 20:30:10 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
CC:     Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Tom Rix <trix@...hat.com>, <linux-ext4@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <llvm@...ts.linux.dev>,
        <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] fs/ext4: Replace kmap_atomic() with kmap_local_page()

On Sat, Dec 31, 2022 at 06:44:39PM +0100, Fabio M. De Francesco wrote:
> kmap_atomic() is deprecated in favor of kmap_local_page(). Therefore,
> replace kmap_atomic() with kmap_local_page().
> 
> kmap_atomic() is implemented like a kmap_local_page() which also disables
> page-faults and preemption (the latter only for !PREEMPT_RT kernels).
> 
> However, the code within the mappings and un-mappings in ext4/inline.c
> does not depend on the above-mentioned side effects.
> 
> Therefore, a mere replacement of the old API with the new one is all it
> is required (i.e., there is no need to explicitly add any calls to
> pagefault_disable() and/or preempt_disable()).
> 
> Suggested-by: Ira Weiny <ira.weiny@...el.com>

Reviewed-by: Ira Weiny <ira.weiny@...el.com>

> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
> ---
> 
> I tried my best to understand the code within mapping and un-mapping.
> However, I'm not an expert. Therefore, although I'm pretty confident, I
> cannot be 100% sure that the code between the mapping and the un-mapping
> does not depend on pagefault_disable() and/or preempt_disable().
> 
> Unfortunately, I cannot currently test this changes to check the
> above-mentioned assumptions. However, if I'm required to do the tests
> with (x)fstests, I have no problems with doing them in the next days.
> 
> If so, I'll test in a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel with
> HIGHMEM64GB enabled.
> 
> I'd like to hear whether or not the maintainers require these tests
> and/or other tests.
> 
>  fs/ext4/inline.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/ext4/inline.c b/fs/ext4/inline.c
> index 2b42ececa46d..bfb044425d8a 100644
> --- a/fs/ext4/inline.c
> +++ b/fs/ext4/inline.c
> @@ -490,10 +490,10 @@ static int ext4_read_inline_page(struct inode *inode, struct page *page)
>  		goto out;
>  
>  	len = min_t(size_t, ext4_get_inline_size(inode), i_size_read(inode));
> -	kaddr = kmap_atomic(page);
> +	kaddr = kmap_local_page(page);
>  	ret = ext4_read_inline_data(inode, kaddr, len, &iloc);
>  	flush_dcache_page(page);
> -	kunmap_atomic(kaddr);
> +	kunmap_local(kaddr);
>  	zero_user_segment(page, len, PAGE_SIZE);
>  	SetPageUptodate(page);
>  	brelse(iloc.bh);
> @@ -763,9 +763,9 @@ int ext4_write_inline_data_end(struct inode *inode, loff_t pos, unsigned len,
>  		 */
>  		(void) ext4_find_inline_data_nolock(inode);
>  
> -		kaddr = kmap_atomic(page);
> +		kaddr = kmap_local_page(page);
>  		ext4_write_inline_data(inode, &iloc, kaddr, pos, copied);
> -		kunmap_atomic(kaddr);
> +		kunmap_local(kaddr);
>  		SetPageUptodate(page);
>  		/* clear page dirty so that writepages wouldn't work for us. */
>  		ClearPageDirty(page);
> @@ -831,9 +831,9 @@ ext4_journalled_write_inline_data(struct inode *inode,
>  	}
>  
>  	ext4_write_lock_xattr(inode, &no_expand);
> -	kaddr = kmap_atomic(page);
> +	kaddr = kmap_local_page(page);
>  	ext4_write_inline_data(inode, &iloc, kaddr, 0, len);
> -	kunmap_atomic(kaddr);
> +	kunmap_local(kaddr);
>  	ext4_write_unlock_xattr(inode, &no_expand);
>  
>  	return iloc.bh;
> -- 
> 2.39.0
>