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From: Andreas Dilger <adilger@dilger.ca>
To: Jan Ziak <0xe2.0x9a.0x9b@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>,
	gregkh@linuxfoundation.org, linux-api@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-kselftest@vger.kernel.org, linux-man@vger.kernel.org,
	mtk.manpages@gmail.com, shuah@kernel.org,
	viro@zeniv.linux.org.uk
Subject: Re: [PATCH 0/3] readfile(2): a new syscall to make open/read/close faster
Date: Sun, 5 Jul 2020 00:32:38 -0600	[thread overview]
Message-ID: <37170CC1-C132-40BE-8ABA-B14E3419975C@dilger.ca> (raw)
In-Reply-To: <CAODFU0qwtPTaBRbA3_ufA6N7fajhi61Sp5iE75Shdk25NSOTLA@mail.gmail.com>

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On Jul 4, 2020, at 8:46 PM, Jan Ziak <0xe2.0x9a.0x9b@gmail.com> wrote:
> 
> On Sun, Jul 5, 2020 at 4:16 AM Matthew Wilcox <willy@infradead.org> wrote:
>> 
>> On Sun, Jul 05, 2020 at 04:06:22AM +0200, Jan Ziak wrote:
>>> Hello
>>> 
>>> At first, I thought that the proposed system call is capable of
>>> reading *multiple* small files using a single system call - which
>>> would help increase HDD/SSD queue utilization and increase IOPS (I/O
>>> operations per second) - but that isn't the case and the proposed
>>> system call can read just a single file.
>>> 
>>> Without the ability to read multiple small files using a single system
>>> call, it is impossible to increase IOPS (unless an application is
>>> using multiple reader threads or somehow instructs the kernel to
>>> prefetch multiple files into memory).
>> 
>> What API would you use for this?
>> 
>> ssize_t readfiles(int dfd, char **files, void **bufs, size_t *lens);
>> 
>> I pretty much hate this interface, so I hope you have something better
>> in mind.
> 
> I am proposing the following:
> 
> struct readfile_t {
>  int dirfd;
>  const char *pathname;
>  void *buf;
>  size_t count;
>  int flags;
>  ssize_t retval; // set by kernel
>  int reserved; // not used by kernel
> };

If you are going to pass a struct from userspace to the kernel, it
should not mix int and pointer types (which may be 64-bit values,
so that there are not structure packing issues, like:

struct readfile {
	int	dirfd;
	int	flags;
	const char *pathname;
	void	*buf;
	size_t	count;
	ssize_t retval;
};

It would be better if "retval" was returned in "count", so that
the structure fits nicely into 32 bytes on a 64-bit system, instead
of being 40 bytes per entry, which adds up over many entries, like.

struct readfile {
	int	dirfd;
	int	flags;
	const char *pathname;
	void	*buf;
	ssize_t count;	/* input: bytes requested, output: bytes read or -errno */
};


However, there is still an issue with passing pointers from userspace,
since they may be 32-bit userspace pointers on a 64-bit kernel.

> int readfiles(struct readfile_t *requests, size_t count);

It's not clear why count is a "size_t" since it is not a size.
An unsigned int is fine here, since it should never be negative.

> Returns zero if all requests succeeded, otherwise the returned value
> is non-zero (glibc wrapper: -1) and user-space is expected to check
> which requests have succeeded and which have failed. retval in
> readfile_t is set to what the single-file readfile syscall would
> return if it was called with the contents of the corresponding
> readfile_t struct.
> 
> The glibc library wrapper of this system call is expected to store the
> errno in the "reserved" field. Thus, a programmer using glibc sees:
> 
> struct readfile_t {
>  int dirfd;
>  const char *pathname;
>  void *buf;
>  size_t count;
>  int flags;
>  ssize_t retval; // set by glibc (-1 on error)
>  int errno; // set by glibc if retval is -1
> };

Why not just return the errno directly in "retval", or in "count" as
proposed?  That avoids further bloating the structure by another field.

> retval and errno in glibc's readfile_t are set to what the single-file
> glibc readfile would return (retval) and set (errno) if it was called
> with the contents of the corresponding readfile_t struct. In case of
> an error, glibc will pick one readfile_t which failed (such as: the
> 1st failed one) and use it to set glibc's errno.


Cheers, Andreas






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  parent reply	other threads:[~2020-07-05  6:32 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-05  2:06 Jan Ziak
2020-07-05  2:16 ` Matthew Wilcox
2020-07-05  2:46   ` Jan Ziak
2020-07-05  3:12     ` Matthew Wilcox
2020-07-05  3:18       ` Jan Ziak
2020-07-05  3:27         ` Matthew Wilcox
2020-07-05  4:09           ` Jan Ziak
2020-07-05 11:58             ` Greg KH
2020-07-06  6:07               ` Jan Ziak
2020-07-06 11:11                 ` Matthew Wilcox
2020-07-06 11:18                 ` Greg KH
2020-07-05  8:07           ` Vito Caputo
2020-07-05 11:44             ` Greg KH
2020-07-05 20:34               ` Vito Caputo
2020-07-05  6:32     ` Andreas Dilger [this message]
2020-07-05  7:25       ` Jan Ziak
2020-07-05 12:00         ` Greg KH
2020-07-05 11:50 ` Greg KH
2020-07-14  6:51   ` Pavel Machek
2020-07-14  8:07     ` Miklos Szeredi
2020-07-14 11:34       ` Pavel Begunkov
2020-07-14 11:55         ` Miklos Szeredi
2020-07-15  8:31           ` Pavel Begunkov
2020-07-15  8:41             ` Miklos Szeredi
2020-07-15  8:49               ` Pavel Begunkov
2020-07-15  9:00                 ` Pavel Begunkov
2020-07-15 11:17                   ` Miklos Szeredi
  -- strict thread matches above, loose matches on Subject: below --
2020-07-04 14:02 Greg Kroah-Hartman
2020-07-04 19:30 ` Al Viro
2020-07-05 11:47   ` Greg Kroah-Hartman
2020-07-06 17:25 ` Dave Martin

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