Re: [PATCH 2/2] xfs: don't update mtime on COW faults

[Darrick J. Wong]

On Sat, Sep 05, 2020 at 01:02:33PM -0400, Mikulas Patocka wrote:
> 
> 
> On Sat, 5 Sep 2020, Darrick J. Wong wrote:
> 
> > On Sat, Sep 05, 2020 at 08:13:02AM -0400, Mikulas Patocka wrote:
> > > When running in a dax mode, if the user maps a page with MAP_PRIVATE and
> > > PROT_WRITE, the xfs filesystem would incorrectly update ctime and mtime
> > > when the user hits a COW fault.
> > > 
> > > This breaks building of the Linux kernel.
> > > How to reproduce:
> > > 1. extract the Linux kernel tree on dax-mounted xfs filesystem
> > > 2. run make clean
> > > 3. run make -j12
> > > 4. run make -j12
> > > - at step 4, make would incorrectly rebuild the whole kernel (although it
> > >   was already built in step 3).
> > > 
> > > The reason for the breakage is that almost all object files depend on
> > > objtool. When we run objtool, it takes COW page fault on its .data
> > > section, and these faults will incorrectly update the timestamp of the
> > > objtool binary. The updated timestamp causes make to rebuild the whole
> > > tree.
> > > 
> > > Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > 
> > > ---
> > >  fs/xfs/xfs_file.c |   11 +++++++++--
> > >  1 file changed, 9 insertions(+), 2 deletions(-)
> > > 
> > > Index: linux-2.6/fs/xfs/xfs_file.c
> > > ===================================================================
> > > --- linux-2.6.orig/fs/xfs/xfs_file.c	2020-09-05 10:01:42.000000000 +0200
> > > +++ linux-2.6/fs/xfs/xfs_file.c	2020-09-05 13:59:12.000000000 +0200
> > > @@ -1223,6 +1223,13 @@ __xfs_filemap_fault(
> > >  	return ret;
> > >  }
> > >  
> > > +static bool
> > > +xfs_is_write_fault(
> > 
> > Call this xfs_is_shared_dax_write_fault, and throw in the IS_DAX() test?
> > 
> > You might as well make it a static inline.
> 
> Yes, it is possible. I'll send a second version.
> 
> > > +	struct vm_fault		*vmf)
> > > +{
> > > +	return vmf->flags & FAULT_FLAG_WRITE && vmf->vma->vm_flags & VM_SHARED;
> > 
> > Also, is "shortcutting the normal fault path" the reason for ext2 and
> > xfs both being broken?
> > 
> > /me puzzles over why write_fault is always true for page_mkwrite and
> > pfn_mkwrite, but not for fault and huge_fault...
> > 
> > Also: Can you please turn this (checking for timestamp update behavior
> > wrt shared and private mapping write faults) into an fstest so we don't
> > mess this up again?
> 
> I've written this program that tests it - you can integrate it into your 
> testsuite.

I don't get it.  You're a filesystem maintainer too, which means you're
a regular contributor.  Do you:

(a) not use fstests?  If you don't, I really hope you use something else
to QA hpfs.

(b) really think that it's my problem to integrate and submit your
regression tests for you?

> Mikulas
> 
> 
> #include <stdio.h>

and (c) what do you want me to do with a piece of code that has no
signoff tag, no copyright, and no license?  This is your patch, and
therefore your responsibility to develop enough of an appropriate
regression test in a proper form that the rest of us can easily
determine we have the rights to contribute to it.

I don't have a problem with helping to tweak a properly licensed and
tagged test program into fstests, but this is a non-starter.

--D

> #include <stdlib.h>
> #include <unistd.h>
> #include <fcntl.h>
> #include <string.h>
> #include <sys/mman.h>
> #include <sys/stat.h>
> 
> #define FILE_NAME	"test.txt"
> 
> static struct stat st1, st2;
> 
> int main(void)
> {
> 	int h, r;
> 	char *map;
> 	unlink(FILE_NAME);
> 	h = creat(FILE_NAME, 0600);
> 	if (h == -1) perror("creat"), exit(1);
> 	r = write(h, "x", 1);
> 	if (r != 1) perror("write"), exit(1);
> 	if (close(h)) perror("close"), exit(1);
> 	h = open(FILE_NAME, O_RDWR);
> 	if (h == -1) perror("open"), exit(1);
> 
> 	map = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_PRIVATE, h, 0);
> 	if (map == MAP_FAILED) perror("mmap"), exit(1);
> 	if (fstat(h, &st1)) perror("fstat"), exit(1);
> 	sleep(2);
> 	*map = 'y';
> 	if (fstat(h, &st2)) perror("fstat"), exit(1);
> 	if (memcmp(&st1, &st2, sizeof(struct stat))) fprintf(stderr, "BUG: COW fault changed time!\n"), exit(1);
> 	if (munmap(map, 4096)) perror("munmap"), exit(1);
> 
> 	map = mmap(NULL, 4096, PROT_READ | PROT_WRITE, MAP_SHARED, h, 0);
> 	if (map == MAP_FAILED) perror("mmap"), exit(1);
> 	if (fstat(h, &st1)) perror("fstat"), exit(1);
> 	sleep(2);
> 	*map = 'z';
> 	if (fstat(h, &st2)) perror("fstat"), exit(1);
> 	if (st1.st_mtime == st2.st_mtime) fprintf(stderr, "BUG: Shared fault did not change mtime!\n"), exit(1);
> 	if (st1.st_ctime == st2.st_ctime) fprintf(stderr, "BUG: Shared fault did not change ctime!\n"), exit(1);
> 	if (munmap(map, 4096)) perror("munmap"), exit(1);
> 
> 	if (close(h)) perror("close"), exit(1);
> 	if (unlink(FILE_NAME)) perror("unlink"), exit(1);
> 	return 0;
> }
>