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SubjectRe: [RFC PATCH V2 10/12] fs/xfs: Fix truncate up
On Mon, Jan 13, 2020 at 05:14:07PM -0800, Darrick J. Wong wrote:
> On Mon, Jan 13, 2020 at 04:40:47PM -0800, Ira Weiny wrote:
> > On Mon, Jan 13, 2020 at 02:27:55PM -0800, Darrick J. Wong wrote:
> > > On Fri, Jan 10, 2020 at 11:29:40AM -0800, ira.weiny@intel.com wrote:
> > > > From: Ira Weiny <ira.weiny@intel.com>
> > > >
> > > > When zeroing the end of a file we must account for bytes contained in
> > > > the final page which are past EOF.
> > > >
> > > > Extend the range passed to iomap_zero_range() to reach LLONG_MAX which
> > > > will include all bytes of the final page.
> > > >
> > > > Signed-off-by: Ira Weiny <ira.weiny@intel.com>
> > > > ---
> > > > fs/xfs/xfs_iops.c | 2 +-
> > > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/fs/xfs/xfs_iops.c b/fs/xfs/xfs_iops.c
> > > > index a2f2604c3187..a34b04e8ac9c 100644
> > > > --- a/fs/xfs/xfs_iops.c
> > > > +++ b/fs/xfs/xfs_iops.c
> > > > @@ -910,7 +910,7 @@ xfs_setattr_size(
> > > > */
> > > > if (newsize > oldsize) {
> > > > trace_xfs_zero_eof(ip, oldsize, newsize - oldsize);
> > > > - error = iomap_zero_range(inode, oldsize, newsize - oldsize,
> > > > + error = iomap_zero_range(inode, oldsize, LLONG_MAX - oldsize,
> > >
> > > Huh? Won't this cause the file size to be set to LLONG_MAX?
> >
> > Not as I understand the code.
>
> iomap_zero_range uses the standard iomap_write_{begin,end} functions,
> which means that if you pass it an (offset, length) that extend beyond
> EOF it will change isize to offset+length.

I don't see that but I'll take your word for it... That is unfortunate because
I would have thought that the full page would have been zero'ed by something
already.

I found code in xfs_free_file_space() with this comment:

/*
* If we zeroed right up to EOF and EOF straddles a page boundary we
* must make sure that the post-EOF area is also zeroed because the
* page could be mmap'd and iomap_zero_range doesn't do that for us.
* Writeback of the eof page will do this, albeit clumsily.
*/

But that just calls filemap_write_and_wait_range()... :-/

>
> > But as I said in the cover I am not 100% sure of
> > this fix.
>
> > From what I can tell xfs_ioctl_setattr_dax_invalidate() should invalidate the
> > mappings and the page cache and the traces I have indicate that the DAX mode
> > is not changing or was properly held off.
>
> Hmm, that implies the invalidation didn't work. Can you find a way to
> report the contents of the page cache after the dax mode change
> invalidation fails? I wonder if this is something dorky like rounding
> down such that the EOF page doesn't actually get invalidated?
>
> Hmm, no, xfs_ioctl_setattr_dax_invalidate should be nuking all the
> pages... do you have a quick reproducer?

I thought I did...

What I have done is take this patch:

https://www.spinics.net/lists/fstests/msg13313.html

and put [run_fsx ""] in a loop... (diff below) And without this truncate fix
patch it was failing in about 5 - 10 iterations. But I'm running it right now
and it has gone for 30+... :-(

I am 90% confident that this series works for 100% of the use cases we have. I
think this is an existing bug which I've just managed to find. And again I'm
not comfortable with this patch either. So I'm not trying to argue for it but
I just don't know what could be wrong...

Ira

diff --git a/tests/generic/999 b/tests/generic/999
index 6dd5529dbc65..929c20c6db04 100755
--- a/tests/generic/999
+++ b/tests/generic/999
@@ -274,7 +274,9 @@ function run_fsx {
pid=""
}

-run_fsx ""
+while [ 1 ]; do
+ run_fsx ""
+done
run_fsx "-A"
run_fsx "-Z -r 4096 -w 4096"
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