linux-kernel - RE: [f2fs-dev] [PATCH 6/6] f2fs: avoid double lock for cp

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Date:	Tue, 23 Dec 2014 15:01:36 +0800
From:	Chao Yu <chao2.yu@...sung.com>
To:	'Jaegeuk Kim' <jaegeuk@...nel.org>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	linux-f2fs-devel@...ts.sourceforge.net
Subject: RE: [f2fs-dev] [PATCH 6/6] f2fs: avoid double lock for cp_rwsem

Hi Jaegeuk,

> -----Original Message-----
> From: Jaegeuk Kim [mailto:jaegeuk@...nel.org]
> Sent: Monday, December 22, 2014 12:35 AM
> To: linux-kernel@...r.kernel.org; linux-fsdevel@...r.kernel.org;
> linux-f2fs-devel@...ts.sourceforge.net
> Cc: Jaegeuk Kim
> Subject: [f2fs-dev] [PATCH 6/6] f2fs: avoid double lock for cp_rwsem
> 
> The __f2fs_add_link is covered by cp_rwsem all the time.
> This calls init_inode_metadata, which conducts some acl operations including
> memory allocation with GFP_KERNEL previously.
> But, under memory pressure, f2fs_write_data_page can be called, which also
> grabs cp_mutex too.

grabs cp_rwsem.

> Basically, it's safe since down_read was used in both of cases, but it'd
> better avoid this situation in advance.

If checkpoint intend to hold down_write in the middle of the two down_read
caller, it will cause a deadlock, so it's not safe.

Could you update the comments?

> 
> Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>

Anyway, Nice catch and please add:

Reviewed-by: Chao Yu <chao2.yu@...sung.com>

> ---
>  fs/f2fs/acl.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/f2fs/acl.c b/fs/f2fs/acl.c
> index 1ccb26b..7f12d28 100644
> --- a/fs/f2fs/acl.c
> +++ b/fs/f2fs/acl.c
> @@ -62,7 +62,7 @@ static struct posix_acl *f2fs_acl_from_disk(const char *value, size_t size)
>  	if (count == 0)
>  		return NULL;
> 
> -	acl = posix_acl_alloc(count, GFP_KERNEL);
> +	acl = posix_acl_alloc(count, GFP_NOFS);
>  	if (!acl)
>  		return ERR_PTR(-ENOMEM);
> 
> @@ -116,7 +116,7 @@ static void *f2fs_acl_to_disk(const struct posix_acl *acl, size_t *size)
>  	int i;
> 
>  	f2fs_acl = kmalloc(sizeof(struct f2fs_acl_header) + acl->a_count *
> -			sizeof(struct f2fs_acl_entry), GFP_KERNEL);
> +			sizeof(struct f2fs_acl_entry), GFP_NOFS);
>  	if (!f2fs_acl)
>  		return ERR_PTR(-ENOMEM);
> 
> --
> 2.1.1
> 
> 
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