linux-kernel - Re: [PATCH] fs/ext2: Replace kmap_atomic() with kmap_local

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Message-ID: <20230102123411.a7xgfocrbr56qruh@quack3>
Date:   Mon, 2 Jan 2023 13:34:11 +0100
From:   Jan Kara <jack@...e.cz>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Jan Kara <jack@...e.com>, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Ira Weiny <ira.weiny@...el.com>
Subject: Re: [PATCH] fs/ext2: Replace kmap_atomic() with kmap_local_page()

On Sat 31-12-22 18:42:05, 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 mapping and un-mapping in ext2_make_empty()
> 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>
> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>

Thanks, the patch looks good and I'll queue it in my tree. I'm not sure why
it got missed during the initial conversion by Ira :).

								Honza


> ---
> 
> 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/ext2/dir.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext2/dir.c b/fs/ext2/dir.c
> index e5cbc27ba459..0f144c5c7861 100644
> --- a/fs/ext2/dir.c
> +++ b/fs/ext2/dir.c
> @@ -646,7 +646,7 @@ int ext2_make_empty(struct inode *inode, struct inode *parent)
>  		unlock_page(page);
>  		goto fail;
>  	}
> -	kaddr = kmap_atomic(page);
> +	kaddr = kmap_local_page(page);
>  	memset(kaddr, 0, chunk_size);
>  	de = (struct ext2_dir_entry_2 *)kaddr;
>  	de->name_len = 1;
> @@ -661,7 +661,7 @@ int ext2_make_empty(struct inode *inode, struct inode *parent)
>  	de->inode = cpu_to_le32(parent->i_ino);
>  	memcpy (de->name, "..\0", 4);
>  	ext2_set_de_type (de, inode);
> -	kunmap_atomic(kaddr);
> +	kunmap_local(kaddr);
>  	ext2_commit_chunk(page, 0, chunk_size);
>  	err = ext2_handle_dirsync(inode);
>  fail:
> -- 
> 2.39.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR