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    SubjectRe: O_DIRECT wierd behavior..

    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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