Re: [PATCH 0/3] readfile(2): a new syscall to make open/read/close faster

[Andreas Dilger]

On Jul 4, 2020, at 8:46 PM, Jan Ziak <0xe2.0x9a.0x9b@xxxxxxxxx> wrote:
> 
> On Sun, Jul 5, 2020 at 4:16 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> 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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