linux-kernel - Re: [patch] new aop loop fix

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Message-ID: <20070613172939.GE13815@localhost.sw.ru>
Date:	Wed, 13 Jun 2007 21:29:39 +0400
From:	Dmitriy Monakhov <dmonakhov@...ru>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	linux-kernel@...r.kernel.org, npiggin@...e.de,
	mark.fasheh@...cle.com, linux-ext4@...r.kernel.org
Subject: Re: [patch] new aop loop fix

On 14:36 Срд 13 Июн     , Hugh Dickins wrote:
> On Wed, 13 Jun 2007, Dmitriy Monakhov wrote:
> 
> > 	loop.c code itself is not perfect. In fact before Nick's patch
> > 	partial write was't possible. Assumption what write chunks are
> > 	always page aligned is realy weird ( see "index++" line).
> > 
> > Signed-off-by: Dmitriy Monakhov <dmonakhov@...nvz.org>
> 
> I'm interested, because I'm trying to chase down an -mm bug which
> occasionally leaves me with 1k of zeroes where it shouldn't (in a
> 1k bsize ext2 looped over tmpfs).  The length of time for this to
> happen varies a lot, so bisection has been misleading: maybe the
> problem lies in Nick's patches, maybe it does not.
> 
> But I don't understand your fix below at all.  _If_ you need to
> change the handling of index, then you need to change the handling
> of offset too, don't you?
> 
> But what's wrong with how inded was handled anyway?  Yes, it might
> be being incremented at the bottom of the loop when we haven't
> reached the end of this page, but in that case we're not going
> round the loop again anyway: len will now be 0.  So no problem.
> 
> One of us is missing something: please enlighten me - thanks.
Yepp. You absolutely right, wrong patch was attached :(
Btw: Nick's patches broke  LO_CRYPT_XOR mode, but it is ok because
this feature was absolete and not used by anyone, am i right here?
> 
> Hugh
> 
> > 
> > diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> > index 4bab9b1..8726da5 100644
> > --- a/drivers/block/loop.c
> > +++ b/drivers/block/loop.c
> > @@ -215,7 +215,6 @@ static int do_lo_send_aops(struct loop_device *lo, struct bio_vec *bvec,
> >  	int len, ret;
> >  
> >  	mutex_lock(&mapping->host->i_mutex);
> > -	index = pos >> PAGE_CACHE_SHIFT;
> >  	offset = pos & ((pgoff_t)PAGE_CACHE_SIZE - 1);
> >  	bv_offs = bvec->bv_offset;
> >  	len = bvec->bv_len;
> > @@ -226,6 +225,7 @@ static int do_lo_send_aops(struct loop_device *lo, struct bio_vec *bvec,
> >  		struct page *page;
> >  		void *fsdata;
> >  
> > +		index = pos >> PAGE_CACHE_SHIFT;
> >  		IV = ((sector_t)index << (PAGE_CACHE_SHIFT - 9))+(offset >> 9);
> >  		size = PAGE_CACHE_SIZE - offset;
> >  		if (size > len)
> > @@ -255,7 +255,6 @@ static int do_lo_send_aops(struct loop_device *lo, struct bio_vec *bvec,
> >  		bv_offs += copied;
> >  		len -= copied;
> >  		offset = 0;
> > -		index++;
> >  		pos += copied;
> >  	}
> >  	ret = 0;
> -
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