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Message-ID: <AANLkTikP-u1adVItGDgUZdee14Z13k8MLRC5V7UN-gpV@mail.gmail.com>
Date: Thu, 8 Jul 2010 21:44:09 +0200
From: Michael Kerrisk <mtk.manpages@...il.com>
To: David Howells <dhowells@...hat.com>
Cc: linux-fsdevel@...r.kernel.org, linux-cifs@...r.kernel.org,
linux-kernel@...r.kernel.org, samba-technical@...ts.samba.org,
linux-ext4@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH] Add a pair of system calls to make extended file stats
available [ver #3]
Hi David,
A couple of comments below.
On Thu, Jul 1, 2010 at 1:36 AM, David Howells <dhowells@...hat.com> wrote:
> Add a pair of system calls to make extended file stats available, including
> file creation time, inode version and data version where available through the
> underlying filesystem.
>
> [This depends on the previously posted pair of patches to (a) constify a number
> of syscall string and buffer arguments and (b) rearrange AFS's use of
> i_version and i_generation].
>
> The following structures are defined for their use:
>
> struct xstat_parameters {
> unsigned long long request_mask;
Poor name, since it's a value-result arg? Better maybe something like
"field_mask"?
> };
>
> struct xstat_dev {
> unsigned int major, minor;
> };
>
> struct xstat_time {
> unsigned long long tv_sec, tv_nsec;
> };
>
> struct xstat {
> unsigned int st_mode;
> unsigned int st_nlink;
> unsigned int st_uid;
> unsigned int st_gid;
> struct xstat_dev st_rdev;
> struct xstat_dev st_dev;
> struct xstat_time st_atime;
> struct xstat_time st_mtime;
> struct xstat_time st_ctime;
> struct xstat_time st_btime;
> unsigned long long st_ino;
> unsigned long long st_size;
> unsigned long long st_blksize;
> unsigned long long st_blocks;
> unsigned long long st_gen;
> unsigned long long st_data_version;
> unsigned long long st_result_mask;
> unsigned long long st_extra_results[0];
> };
>
> where st_btime is the file creation time, st_gen is the inode generation
> (i_generation), st_data_version is the data version number (i_version),
> request_mask and st_result_mask are bitmasks of data desired/provided and
> st_extra_results[] is where as-yet undefined fields are appended.
>
> The defined bits in request_mask and st_result_mask are:
>
> XSTAT_REQUEST_MODE Want/got st_mode
> XSTAT_REQUEST_NLINK Want/got st_nlink
> XSTAT_REQUEST_UID Want/got st_uid
> XSTAT_REQUEST_GID Want/got st_gid
> XSTAT_REQUEST_RDEV Want/got st_rdev
> XSTAT_REQUEST_ATIME Want/got st_atime
> XSTAT_REQUEST_MTIME Want/got st_mtime
> XSTAT_REQUEST_CTIME Want/got st_ctime
> XSTAT_REQUEST_INO Want/got st_ino
> XSTAT_REQUEST_SIZE Want/got st_size
> XSTAT_REQUEST_BLOCKS Want/got st_blocks
> XSTAT_REQUEST__BASIC_STATS The stuff in the normal stat struct
> XSTAT_REQUEST_BTIME Want/got st_btime
> XSTAT_REQUEST_GEN Want/got st_gen
> XSTAT_REQUEST_DATA_VERSION Want/got st_data_version
> XSTAT_REQUEST__EXTENDED_STATS The stuff in the xstat struct
> XSTAT_REQUEST__ALL_STATS The defined set of requestables
>
> The system calls are:
>
> ssize_t ret = xstat(int dfd,
> const char *filename,
> unsigned flags,
> const struct xstat_parameters *params,
> struct xstat *buffer,
> size_t buflen);
>
> ssize_t ret = fxstat(unsigned fd,
> unsigned flags,
> const struct xstat_parameters *params,
> struct xstat *buffer,
> size_t buflen);
>
>
> The dfd, filename, flags and fd parameters indicate the file to query. There
> is no equivalent of lstat() as that can be emulated with xstat() by passing
> AT_SYMLINK_NOFOLLOW in flags.
>
> AT_FORCE_ATTR_SYNC can also be set in flags. This will require a network
> filesystem to synchronise its attributes with the server.
>
> When the system call is executed, the request_mask bitmask is read from the
> parameter block to work out what the user is requesting. If params is NULL,
> then request_mask will be assumed to be XSTAT_REQUEST__GET_ANYWAY.
There is no XSTAT_REQUEST__GET_ANYWAY, AFAICS. I guess here you meant
XSTAT_REQUEST__EXTENDED_STATS? Or?
> The request_mask should be set by the caller to specify extra results that the
> caller may desire. These come in a number of classes:
>
> (0) dev, blksize.
>
> These are local data and are always available.
>
> (1) mode, nlinks, uid, gid, [amc]time, ino, size, blocks.
>
> These will be returned whether the caller asks for them or not. The
> corresponding bits in result_mask will be set to indicate their presence.
>
> If the caller didn't ask for them, then they may be approximated. For
> example, NFS won't waste any time updating them from the server, unless as
> a byproduct of updating something requested.
>
> (2) rdev.
>
> As for class (1), but this won't be returned if the file is not a blockdev
> or chardev. The bit will be cleared if the value is not returned.
>
> (3) File creation time, inode generation and data version.
>
> These will be returned if available whether the caller asked for them or
> not. The corresponding bits in result_mask will be set or cleared as
> appropriate to indicate their presence.
>
> If the caller didn't ask for them, then they may be approximated. For
> example, NFS won't waste any time updating them from the server, unless
> as a byproduct of updating something requested.
>
> (4) Extra results.
>
> These will only be returned if the caller asked for them by setting their
> bits in request_mask. They will be placed in the buffer after the xstat
> struct in ascending result_mask bit order. Any bit set in request_mask
> mask will be left set in result_mask if the result is available and
> cleared otherwise.
>
> The pointer into the results list will be rounded up to the nearest 8-byte
> boundary after each result is written in. The size of each extra result
> is specific to the definition for that result.
>
> No extra results are currently defined.
>
> If the buffer is insufficiently big, the syscall returns the amount of space it
> will need to write the complete result set and returns a partial result in the
> buffer.
This case is almost certainly a user error, so why not simply return
an error (-1 and ERANGE or E2BIG)? The above approach invites
userspace errors of the form:
if (xtat(...) < 0) { /* How users often check for error */
/* I'll handle the error */
} else {
/* The call succeeded; I'm fine */
}
Instead, more complex error-handling is required for *every* call:
ret = xstat(..., buflen);
if (ret < 0 || ret > buflen)
/* I'll handle the error */
} else {
/* The call succeeded; I'm fine */
}
If you are looking for a way to inform the user about the required
buffer size, I think it would be better to take a leaf from the
getxattr(2) book: if 'buflen' is zero, then do nothing with the output
arg, but return the size that would be required.
Cheers,
Michael
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