Date:   Thu, 29 Jun 2017 17:25:47 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     Tahsin Erdogan <tahsin@...gle.com>
Cc:     Theodore Ts'o <tytso@....edu>, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ext4: have ext4_xattr_set_handle() allocate journal
 credits

On Jun 28, 2017, at 9:06 PM, Tahsin Erdogan <tahsin@...gle.com> wrote:
> 
> Currently, callers of ext4_xattr_set_handle() are expected to allocate
> journal credits for setting extended attributes. ext4_xattr_set_credits()
> helps them figure out necessary credits. ext4_xattr_set_handle()
> performs a sufficient credits check before starting data updates.
> 
> This model works fine for callers that start the transaction handle
> themselves, such as ext4_xattr_set(). Other callers like ext4_set_context()
> and __ext4_set_acl() use transaction handles that were started by outer
> scopes so for this to work properly we would have to involve outer scope
> callers to incorporate xattr credits.
> 
> Also, existing sufficient credits check in ext4_xattr_set_handle()
> ignores the fact that allocated credits may have been intended for
> operations other than setting extended attributes. So in theory
> ext4_xattr_set_handle() may finish successfully but have also depleted
> the journal credits that were supposed to be used by other operations
> afterwards.
> 
> In this patch, responsibility of allocating journal credits is moved to
> ext4_xattr_set_handle(). It tries to extend the journal credits as
> needed or will restart the transaction handle if extend doesn't work.

One problem with this approach is that restarting the transaction handle will
make the xattr update non-atomic, which could be a real problem for some
workloads.  For example, ACLs or SELinux or fscrypt xattrs being added in
a separate transaction from file creation, or being modified (delete in a
separate transaction from add) and then lost completely if the system crashes
before the second transaction is committed.

It isn't clear to me why using the current helper function to precompute the
required transaction credits doesn't get this right in the first place?  It
would just need to add the xattr credits to the original journal handle?



This brings up a potential area for improvement in ext4, namely that instead
of having all of the callers precompute the number of transaction credits, it
would be possible to declare operations (e.g. ext4_declare_create(),
ext4_declare_setxattr(), ext4_declare_add_entry(), ext4_declare_write())
and then have helpers that will compute these values (possibly dependent on
the specific inode being modified) and the caller just adds up the various
credits returned for each operation.  The ext4_declare_create() function would
gain subsidiary calls to ext4_declare_setxattr() that depend on on filesystem
features enabled and/or the parent directory, so that the caller doesn't need
to know all of the details.

That is probably larger than what is needed for this particular patch, but
something to consider as this code gets increasingly complex.

Cheers, Andreas

> Fixes: 021264a98d700e8 ("ext4: improve journal credit handling in set xattr paths")
> 
> Signed-off-by: Tahsin Erdogan <tahsin@...gle.com>
> ---
> v2: Allocate 1 journal credit in ext4_set_acl() and ext4_set_context() for modifying
>    inode outside ext4_xattr_set_handle() call.
> 
> fs/ext4/acl.c   | 14 +++++++-------
> fs/ext4/super.c | 13 +++++++------
> fs/ext4/xattr.c | 39 +++++++--------------------------------
> fs/ext4/xattr.h |  2 --
> 4 files changed, 21 insertions(+), 47 deletions(-)
> 
> diff --git a/fs/ext4/acl.c b/fs/ext4/acl.c
> index 8db03e5c78bc..6a4c2b831699 100644
> --- a/fs/ext4/acl.c
> +++ b/fs/ext4/acl.c
> @@ -231,18 +231,18 @@ int
> ext4_set_acl(struct inode *inode, struct posix_acl *acl, int type)
> {
> 	handle_t *handle;
> -	int error, credits, retries = 0;
> -	size_t acl_size = acl ? ext4_acl_size(acl->a_count) : 0;
> +	int error, retries = 0;
> 
> 	error = dquot_initialize(inode);
> 	if (error)
> 		return error;
> retry:
> -	error = ext4_xattr_set_credits(inode, acl_size, &credits);
> -	if (error)
> -		return error;
> -
> -	handle = ext4_journal_start(inode, EXT4_HT_XATTR, credits);
> +	/*
> +	 * Allocate 1 journal credit to allow __ext4_set_acl() update inode
> +	 * before calling ext4_xattr_set_handle(). Extended attribute related
> +	 * credits are allocated by ext4_xattr_set_handle() itself.
> +	 */
> +	handle = ext4_journal_start(inode, EXT4_HT_XATTR, 1 /* credits */);
> 	if (IS_ERR(handle))
> 		return PTR_ERR(handle);
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 56c971807df5..74036c382b63 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1150,7 +1150,7 @@ static int ext4_set_context(struct inode *inode, const void *ctx, size_t len,
> 							void *fs_data)
> {
> 	handle_t *handle = fs_data;
> -	int res, res2, credits, retries = 0;
> +	int res, res2, retries = 0;
> 
> 	/*
> 	 * Encrypting the root directory is not allowed because e2fsck expects
> @@ -1194,11 +1194,12 @@ static int ext4_set_context(struct inode *inode, const void *ctx, size_t len,
> 	if (res)
> 		return res;
> retry:
> -	res = ext4_xattr_set_credits(inode, len, &credits);
> -	if (res)
> -		return res;
> -
> -	handle = ext4_journal_start(inode, EXT4_HT_MISC, credits);
> +	/*
> +	 * Allocate 1 journal credit for ext4_mark_inode_dirty() call in this
> +	 * function. Extended attribute related credits are allocated by
> +	 * ext4_xattr_set_handle() itself.
> +	 */
> +	handle = ext4_journal_start(inode, EXT4_HT_MISC, 1 /* credits */);
> 	if (IS_ERR(handle))
> 		return PTR_ERR(handle);
> 
> diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
> index 062756b4e6d8..6b909804aa5e 100644
> --- a/fs/ext4/xattr.c
> +++ b/fs/ext4/xattr.c
> @@ -2253,7 +2253,11 @@ ext4_xattr_set_handle(handle_t *handle, struct inode *inode, int name_index,
> 		credits = __ext4_xattr_set_credits(inode->i_sb, bh, value_len);
> 		brelse(bh);
> 
> -		if (!ext4_handle_has_enough_credits(handle, credits)) {
> +		if (ext4_xattr_ensure_credits(
> +					handle, inode,
> +					handle->h_buffer_credits + credits,
> +					NULL /* bh */, false /* dirty */,
> +					false /* block_csum */)) {
> 			error = -ENOSPC;
> 			goto cleanup;
> 		}
> @@ -2357,31 +2361,6 @@ ext4_xattr_set_handle(handle_t *handle, struct inode *inode, int name_index,
> 	return error;
> }
> 
> -int ext4_xattr_set_credits(struct inode *inode, size_t value_len, int *credits)
> -{
> -	struct buffer_head *bh;
> -	int err;
> -
> -	*credits = 0;
> -
> -	if (!EXT4_SB(inode->i_sb)->s_journal)
> -		return 0;
> -
> -	down_read(&EXT4_I(inode)->xattr_sem);
> -
> -	bh = ext4_xattr_get_block(inode);
> -	if (IS_ERR(bh)) {
> -		err = PTR_ERR(bh);
> -	} else {
> -		*credits = __ext4_xattr_set_credits(inode->i_sb, bh, value_len);
> -		brelse(bh);
> -		err = 0;
> -	}
> -
> -	up_read(&EXT4_I(inode)->xattr_sem);
> -	return err;
> -}
> -
> /*
>  * ext4_xattr_set()
>  *
> @@ -2397,18 +2376,14 @@ ext4_xattr_set(struct inode *inode, int name_index, const char *name,
> 	handle_t *handle;
> 	struct super_block *sb = inode->i_sb;
> 	int error, retries = 0;
> -	int credits;
> 
> 	error = dquot_initialize(inode);
> 	if (error)
> 		return error;
> 
> retry:
> -	error = ext4_xattr_set_credits(inode, value_len, &credits);
> -	if (error)
> -		return error;
> -
> -	handle = ext4_journal_start(inode, EXT4_HT_XATTR, credits);
> +	/* ext4_xattr_set_handle() allocates journal credits if necessary. */
> +	handle = ext4_journal_start(inode, EXT4_HT_XATTR, 0 /* credits */);
> 	if (IS_ERR(handle)) {
> 		error = PTR_ERR(handle);
> 	} else {
> diff --git a/fs/ext4/xattr.h b/fs/ext4/xattr.h
> index 26119a67c8c3..f81cee5c21cc 100644
> --- a/fs/ext4/xattr.h
> +++ b/fs/ext4/xattr.h
> @@ -152,8 +152,6 @@ extern ssize_t ext4_listxattr(struct dentry *, char *, size_t);
> extern int ext4_xattr_get(struct inode *, int, const char *, void *, size_t);
> extern int ext4_xattr_set(struct inode *, int, const char *, const void *, size_t, int);
> extern int ext4_xattr_set_handle(handle_t *, struct inode *, int, const char *, const void *, size_t, int);
> -extern int ext4_xattr_set_credits(struct inode *inode, size_t value_len,
> -				  int *credits);
> 
> extern int ext4_xattr_delete_inode(handle_t *handle, struct inode *inode,
> 				   struct ext4_xattr_inode_array **array,
> --
> 2.13.2.725.g09c95d1e9-goog
> 


Cheers, Andreas






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