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Date: Sun, 16 Dec 2001 14:57:53 +0100 (CET)
From: Terje Eggestad <terje.eggestad@scali.no>
To: Suresh Gopalakrishnan <gsuresh@cs.rutgers.edu>
cc: Andrew Morton <akpm@zip.com.au>, GOTO Masanori <gotom@debian.org>,
        <linux-kernel@vger.kernel.org>, Andrea Arcangeli <andrea@suse.de>
Subject: Re: O_DIRECT wierd behavior..
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The problem is that the kernel that don't support O_DIRECT has erronous
handling of the O_DIRECT flag. Meaning they happily accept it.

In order to figure out ifthe running kernel support O_DIRECT you MUST
attempt an unaligned read/write, if it succed the kernel DON'T
support O_DIRECT.

TJ


On Sun, 16 Dec 2001, Suresh Gopalakrishnan wrote:

>
> On Sun, 16 Dec 2001, Andrew Morton wrote:
> > GOTO Masanori wrote:
> > > At Sat, 15 Dec 2001 21:59:06 -0800,
> > > Andrew Morton wrote:
> > > ...
> > > > --- linux-2.4.17-rc1/mm/filemap.c     Thu Dec 13 14:07:55 2001
> > > > +++ linux-akpm/mm/filemap.c   Sat Dec 15 21:52:06 2001
> > > > @@ -3038,8 +3038,11 @@ unlock:
> > > >       /* For now, when the user asks for O_SYNC, we'll actually
> > > >        * provide O_DSYNC. */
> > > >       if (status >= 0) {
> > > > -             if ((file->f_flags & O_SYNC) || IS_SYNC(inode))
> > > > +             if ((file->f_flags & O_SYNC) || IS_SYNC(inode)) {
> > > >                       status = generic_osync_inode(inode, OSYNC_METADATA|OSYNC_DATA);
> > > > +                     if (status < 0)
> > > > +                             written = 0;    /* Return the right thing */
> > > > +             }
> > > >       }
> > >
> > > Right. If generic_osync_inode returns error, it must be needed.
> > > This patch seems ok than my patch...
> >
> > Actually, I preferred your approach :)
> >
> > Also, note how if ->commit_write() or ->prepare_write() return an
> > error, and we have already written some data, the function returns the
> > number of bytes written and no indication that there was an error.
> > According to the write(2) manpage, that's wrong.
> >
> > Probably it is sufficient to make `written' a signed quantity and to
> > do:
> >
> > out_status:
> > -       err = written ? written : status;
> > +       err = status ? status : written;
> >
> > I think that fixes the five or six bugs we've found so far in this
> > function.  err..  make that six or seven.  What is it trying do if
> > ->prepare_write() returns a non-zero, positive value?
> >
> > It needs a big spring-clean.  I'm afraid I don't have time to do that
> > for several days.
>
> Thanks for the patches. There seems to be one more fix required: the test
> program below works in 2.4.16 only if the write size is a multiple of 4K.
> (Why) are all writes expected to be page size, in addition to being page
> aligned? (It works fine on 2.4.2 for all sizes). Any quick fixes? :)
>
> --suresh
>
> #include <stdio.h>
> #include <stdlib.h>
> #include <fcntl.h>
> #include <sys/stat.h>
> #include <sys/types.h>
> #include <unistd.h>
>
> #define O_DIRECT         040000 /* direct disk access hint */
>
> int main()
> {
>         char *buf;
>         int fd;
>
>         buf = valloc(16384);
>         fd = open("/tmp/blah", O_CREAT | O_RDWR | O_DIRECT, 0755 );
>
>         printf("write returns %i\n", write(fd, buf, 4096));
>
>         return 0;
> }
>
>
> -
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-- 
_________________________________________________________________________

Terje Eggestad                  terje.eggestad@scali.no
Scali Scalable Linux Systems    http://www.scali.com

Olaf Helsets Vei 6              tel:    +47 22 62 89 61 (OFFICE)
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