Re: [PATCH v12 00/10] enable bs > ps in XFS

[David Howells <dhowells@redhat.com>]

Hi Pankaj,

I can reproduce the problem with:

xfs_io -t -f -c "pwrite -S 0x58 0 40" -c "fsync" -c "truncate 4" -c "truncate 4096" /xfstest.test/wubble; od -x /xfstest.test/wubble

borrowed from generic/393.  I've distilled it down to the attached C program.

Turning on tracing and adding a bit more, I can see the problem happening.
Here's an excerpt of the tracing (I've added some non-upstream tracepoints).
Firstly, you can see the second pwrite at fpos 0, 40 bytes (ie. 0x28):

 pankaj-5833: netfs_write_iter: WRITE-ITER i=9e s=0 l=28 f=0
 pankaj-5833: netfs_folio: pfn=116fec i=0009e ix=00000-00001 mod-streamw

Then first ftruncate() is called to reduce the file size to 4:

 pankaj-5833: netfs_truncate: ni=9e isz=2028 rsz=2028 zp=4000 to=4
 pankaj-5833: netfs_inval_folio: pfn=116fec i=0009e ix=00000-00001 o=4 l=1ffc d=78787878
 pankaj-5833: netfs_folio: pfn=116fec i=0009e ix=00000-00001 inval-part
 pankaj-5833: netfs_set_size: ni=9e resize-file isz=4 rsz=4 zp=4

You can see the invalidate_folio call, with the offset at 0x4 an the length as
0x1ffc.  The data at the beginning of the page is 0x78787878.  This looks
correct.

Then second ftruncate() is called to increase the file size to 4096
(ie. 0x1000):

 pankaj-5833: netfs_truncate: ni=9e isz=4 rsz=4 zp=4 to=1000
 pankaj-5833: netfs_inval_folio: pfn=116fec i=0009e ix=00000-00001 o=1000 l=1000 d=78787878
 pankaj-5833: netfs_folio: pfn=116fec i=0009e ix=00000-00001 inval-part
 pankaj-5833: netfs_set_size: ni=9e resize-file isz=1000 rsz=1000 zp=4

And here's the problem: in the invalidate_folio() call, the offset is 0x1000
and the length is 0x1000 (o= and l=).  But that's the wrong half of the folio!
I'm guessing that the caller thereafter clears the other half of the folio -
the bit that should be kept.

David
---
/* Distillation of the generic/393 xfstest */
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>

#define ERR(x, y) do { if ((long)(x) == -1) { perror(y); exit(1); } } while(0)

static const char xxx[40] = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx";
static const char yyy[40] = "yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy";
static const char dropfile[] = "/proc/sys/vm/drop_caches";
static const char droptype[] = "3";
static const char file[] = "/xfstest.test/wubble";

int main(int argc, char *argv[])
{
        int fd, drop;

	/* Fill in the second 8K block of the file... */
        fd = open(file, O_CREAT|O_TRUNC|O_WRONLY, 0666);
        ERR(fd, "open");
        ERR(ftruncate(fd, 0), "pre-trunc $file");
        ERR(pwrite(fd, yyy, sizeof(yyy), 0x2000), "write-2000");
        ERR(close(fd), "close");

	/* ... and drop the pagecache so that we get a streaming
	 * write, attaching some private data to the folio.
	 */
        drop = open(dropfile, O_WRONLY);
        ERR(drop, dropfile);
        ERR(write(drop, droptype, sizeof(droptype) - 1), "write-drop");
        ERR(close(drop), "close-drop");

        fd = open(file, O_WRONLY, 0666);
        ERR(fd, "reopen");
	/* Make a streaming write on the first 8K block (needs O_WRONLY). */
        ERR(pwrite(fd, xxx, sizeof(xxx), 0), "write-0");
	/* Now use truncate to shrink and reexpand. */
        ERR(ftruncate(fd, 4), "trunc-4");
        ERR(ftruncate(fd, 4096), "trunc-4096");
        ERR(close(fd), "close-2");
        exit(0);
}