linux-kernel - Re: [PATCH v2] vfs: avoid recopying file names in getname

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Date:	Sun, 29 Mar 2015 12:13:29 +0200
From:	Richard Weinberger <richard.weinberger@...il.com>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Paul Moore <pmoore@...hat.com>
Subject: Re: [PATCH v2] vfs: avoid recopying file names in getname_flags

On Wed, Mar 25, 2015 at 7:45 PM, Boqun Feng <boqun.feng@...il.com> wrote:
> In the current implementation of getname_flags, a file name in the
> user-space will be recopied if it takes more space that
> EMBEDDED_NAME_MAX, however, at this moment, EMBEDDED_NAME_MAX bytes of
> the file name are already copied into kernel address space, the only
> reason why the recopy is needed is that "kname", which points to the
> string of the file name, needs to be relocated.
>
> And the recopy can be avoided if we change the memory layout of the
> `names_cachep` allocation. By putting `struct filename` at the tail of
> the allocation instead of the head, relocation of kname is avoided.
> Once putting the struct at the tail, each byte in the user space will be
> copied only one time, so the recopy is avoided.
>
> Of course, other functions aware of the layout of the `names_cachep`
> allocation, i.e., getname_kernel and putname also need to be modified to
> adapt to the new layout.
>
> Since we change the layout of `names_cachep` allocation, in order to
> figure out whether kname and the struct are separate, we now need to
> check whether the file name string starts at the address
> EMBEDDED_NAME_MAX bytes before the address of the struct.  As a result,
> `iname`, which points the end of `struct filename`, is not needed
> anymore.
>
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> ---
> v1 --> v2:
> Rebase the patch onto the for-next branch of Al's vfs repo.
>
>
>  fs/namei.c         | 45 ++++++++++++++++++++++++++++-----------------
>  include/linux/fs.h |  1 -
>  2 files changed, 28 insertions(+), 18 deletions(-)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 7a11ec1..6d04029 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -119,7 +119,7 @@
>   * PATH_MAX includes the nul terminator --RR.
>   */
>
> -#define EMBEDDED_NAME_MAX      (PATH_MAX - offsetof(struct filename, iname))
> +#define EMBEDDED_NAME_MAX      (PATH_MAX - sizeof(struct filename))

*confused*
 EMBEDDED_NAME_MAX is currently PATH_MAX - sizeof(struct filename)

Did you mix original and new source while creating the patch?

-- 
Thanks,
//richard
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