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SubjectRe: O_DIRECT wierd behavior..
Suresh Gopalakrishnan wrote:
>
> I tried this small piece of code from an old post in the archive:
>
> #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[16384];
> int fd;
> char *p;
>
> p = (char *)((((unsigned long)buf) + 8191) & ~8191L);
> fd = open("/tmp/blah", O_CREAT | O_RDWR | O_DIRECT);
>
> printf("write returns %i\n", write(fd, buf, 8192));
> printf("write returns %i\n", write(fd, p, 1));
>
> return 0;
> }
>

The app has a bug in it (I think); but the kernel has four.
Your first write fails because `buf' is not page-aligned.

Then the kernel screws up the error handling, and ends up
setting the file size to -EINVAL (ie: rather large).



1: We're testing `written >= 0', but it is unsigned (!). In two
places.

This one, IMO is a gcc shortcoming. The compiler is capable of warning
about expressions which always evaluate to true or false in `if' statements,
but turning this on also enables lots of things you don't want it to warn about.
gcc needs to provide finer control of its warning capabilities. I patched
gcc-2.7.2.3 to do this ages back and it was very useful.

2: If generic_osync_inode() returns an error, we fail to report it. In
two places.

Here's a quick fix. It needs a review.



--- 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 */
+ }
}

out_status:
@@ -3054,7 +3057,8 @@ fail_write:

o_direct:
written = generic_file_direct_IO(WRITE, file, (char *) buf, count, pos);
- if (written > 0) {
+ status = written;
+ if (status > 0) {
loff_t end = pos + written;
if (end > inode->i_size && !S_ISBLK(inode->i_mode)) {
inode->i_size = end;
@@ -3067,8 +3071,11 @@ o_direct:
* Sync the fs metadata but not the minor inode changes and
* of course not the data as we did direct DMA for the IO.
*/
- if (written >= 0 && file->f_flags & O_SYNC)
+ if (status >= 0 && file->f_flags & O_SYNC) {
status = generic_osync_inode(inode, OSYNC_METADATA);
+ if (status < 0)
+ written = 0; /* Return the right thing */
+ }
goto out_status;
}
-
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