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Date:   Sat, 5 Sep 2020 13:02:33 -0400 (EDT)
From:   Mikulas Patocka <mpatocka@redhat.com>
To:     "Darrick J. Wong" <darrick.wong@oracle.com>
cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Jan Kara <jack@suse.cz>, Dave Chinner <dchinner@redhat.com>,
        Jann Horn <jannh@google.com>, Christoph Hellwig <hch@lst.de>,
        Oleg Nesterov <oleg@redhat.com>,
        Kirill Shutemov <kirill@shutemov.name>,
        "Theodore Ts'o" <tytso@mit.edu>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dan Williams <dan.j.williams@intel.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-nvdimm@lists.01.org,
        linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org
Subject: Re: [PATCH 2/2] xfs: don't update mtime on COW faults
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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@redhat.com>
> > Cc: stable@vger.kernel.org
> > 
> > ---
> >  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.

Mikulas


#include <stdio.h>
#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;
}