linux-kernel - Re: [PATCH 1/3] readfile: implement readfile syscall

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Date:   Sat, 4 Jul 2020 21:16:36 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Michael Kerrisk <mtk.manpages@...il.com>, shuah@...nel.org,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-man <linux-man@...r.kernel.org>,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 1/3] readfile: implement readfile syscall

On Sat, Jul 04, 2020 at 09:12:06PM +0100, Al Viro wrote:
> On Sat, Jul 04, 2020 at 09:41:09PM +0200, Miklos Szeredi wrote:
> > And "If the size of file is smaller than the value provided in count
> > then the whole file will be copied into buf", which is simply a lie;
> > for example seq_file will happily return a smaller-than-PAGE_SIZE
> > chunk if at least one record fits in there.  You'll have a very hard
> > time explaining that in the man page.  So I think there are two
> > possible ways forward:
> > 
> >  1) just leave the first explanation (it's an open + read + close
> > equivalent) and leave out the rest
> > 
> >  2) add a loop around the vfs_read() in the code.
> 
> 3) don't bother with the entire thing, until somebody manages to demonstrate
> a setup where it does make a real difference (compared to than the obvious
> sequence of syscalls, that is).  At which point we'll need to figure out
> what's going on and deal with the underlying problem of that setup.

Incidentally, if that's intended for use on _sysfs_, I would like to see the
effects of that sucker being called by many processes in parallel, seeing that
sysfs has, er, certain scalability problems in its lookups.  And I would be
very surprised if they were not heavier than said overhead of two extra syscalls.