linux-ext4 - Re: [PATCH 2/2] ext4: mark fc as ineligible using an handle in ext4_xattr

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Message-ID: <20240919214730.gza4j3gkrn34tcyn@quack3>
Date: Thu, 19 Sep 2024 23:47:30 +0200
From: Jan Kara <jack@...e.cz>
To: "Luis Henriques (SUSE)" <luis.henriques@...ux.dev>
Cc: Theodore Ts'o <tytso@....edu>, Andreas Dilger <adilger@...ger.ca>,
	Jan Kara <jack@...e.cz>,
	Harshad Shirwadkar <harshadshirwadkar@...il.com>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] ext4: mark fc as ineligible using an handle in
 ext4_xattr_set()

On Thu 19-09-24 10:38:48, Luis Henriques (SUSE) wrote:
> Calling ext4_fc_mark_ineligible() with a NULL handle is racy and may result
> in a fast-commit being done before the filesystem is effectively marked as
> ineligible.  This patch reduces the risk of this happening in function
> ext4_xattr_set() by using an handle if one is available.
> 
> Suggested-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Luis Henriques (SUSE) <luis.henriques@...ux.dev>

One comment below:

> diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
> index 46ce2f21fef9..dbe4d11cd332 100644
> --- a/fs/ext4/xattr.c
> +++ b/fs/ext4/xattr.c
> @@ -2554,11 +2554,15 @@ ext4_xattr_set(struct inode *inode, int name_index, const char *name,
>  	handle = ext4_journal_start(inode, EXT4_HT_XATTR, credits);
>  	if (IS_ERR(handle)) {
>  		error = PTR_ERR(handle);
> +		ext4_fc_mark_ineligible(inode->i_sb, EXT4_FC_REASON_XATTR,
> +					NULL);

So when starting a transaction fails:

a) We have a big problem, the journal is aborted so marking fs ineligible
is moot.

b) We don't set anything and bail with error to userspace so again marking
fs as ineligible is pointless.

So there's no need to do anything in this case.

								Honza

>  	} else {
>  		int error2;
>  
>  		error = ext4_xattr_set_handle(handle, inode, name_index, name,
>  					      value, value_len, flags);
> +		ext4_fc_mark_ineligible(inode->i_sb, EXT4_FC_REASON_XATTR,
> +					handle);
>  		error2 = ext4_journal_stop(handle);
>  		if (error == -ENOSPC &&
>  		    ext4_should_retry_alloc(sb, &retries))
> @@ -2566,7 +2570,6 @@ ext4_xattr_set(struct inode *inode, int name_index, const char *name,
>  		if (error == 0)
>  			error = error2;
>  	}
> -	ext4_fc_mark_ineligible(inode->i_sb, EXT4_FC_REASON_XATTR, NULL);
>  
>  	return error;
>  }
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR