Re: [PATCH v2 6/8] fsstress: implement the clonerange/deduperange ioctls

[Luis Henriques <lhenriques@suse.com>]

On Thu, Feb 22, 2018 at 09:27:41AM -0800, Darrick J. Wong wrote:
> On Thu, Feb 22, 2018 at 04:06:14PM +0000, Luis Henriques wrote:
> > On Thu, Dec 14, 2017 at 06:07:31PM -0800, Darrick J. Wong wrote:
> > 
> > <snip>
> > 
> > > +void
> > > +clonerange_f(
> > > +	int			opno,
> > > +	long			r)
> > > +{
> > 
> > <snip>
> > 
> > > +	/* Calculate offsets */
> > > +	len = (random() % FILELEN_MAX) + 1;
> > > +	len &= ~(stat1.st_blksize - 1);
> > > +	if (len == 0)
> > > +		len = stat1.st_blksize;
> > > +	if (len > stat1.st_size)
> > > +		len = stat1.st_size;
> > > +
> > > +	lr = ((__int64_t)random() << 32) + random();
> > > +	if (stat1.st_size == len)
> > > +		off1 = 0;
> > > +	else
> > > +		off1 = (off64_t)(lr % MIN(stat1.st_size - len, MAXFSIZE));
> > > +	off1 %= maxfsize;
> > > +	off1 &= ~(stat1.st_blksize - 1);
> > > +
> > > +	/*
> > > +	 * If srcfile == destfile, randomly generate destination ranges
> > > +	 * until we find one that doesn't overlap the source range.
> > > +	 */
> > > +	do {
> > > +		lr = ((__int64_t)random() << 32) + random();
> > > +		off2 = (off64_t)(lr % MIN(stat2.st_size + (1024 * 1024), MAXFSIZE));
> > > +		off2 %= maxfsize;
> > > +		off2 &= ~(stat2.st_blksize - 1);
> > > +	} while (stat1.st_ino == stat2.st_ino && llabs(off2 - off1) < len);
> > 
> > I started seeing hangs in generic/013 on cephfs.  After spending some
> > time looking, I found that this loops forever.  And the reason seems to
> > be that stat1.st_blksize is too big for this filesystem (4M) -- when
> > doing:
> 
> "Too big for this filesystem"?
> 
> Uh... maybe you'd better start by giving me more stat buffer info --
> what's st_size?
> 
> > 	off1 &= ~(stat1.st_blksize - 1);
> 
> These bits round the start offset down to block granularity, since clone
> range implementations generally require that the ranges align to block
> boundaries.
> 
> (Though AFAICT ceph doesn't support clone range anyway...)
> 
> So reading between the lines, is the problem here that ceph advertises a
> blocksize of 4M and fsstress calls clonerange_f with files that are
> smaller than 4M in size, so the only possible offsets with a 4M
> blocksize are zero and that's why we end up looping forever?

Brilliantly described!  That is *exactly* what I'm seeing and failed to
describe.  I guess I could use FSSTRESS_AVOID to work around this issue,
but there are probably better options.

Cheers,
--
Luís

> 
> --D
> 
> > 
> > off1 (and off2) will both end up with 0.  Does this make sense?  Would
> > something like:
> > 
> > -	off1 &= ~(stat1.st_blksize - 1);
> > +	if (stat1.st_blksize <= stat1.st_size)
> > +		off1 &= ~(stat1.st_blksize - 1);
> > 
> > be acceptable?  (and a similar change for off2, of course.)
> 
> > Cheers,
> > --
> > Luís
> > --
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