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Re: [PATCH 2/4] [PATCH 2/4] xfs: remove wrappers for read/write file ope

To: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Subject: Re: [PATCH 2/4] [PATCH 2/4] xfs: remove wrappers for read/write file operations
From: Alex Elder <aelder@xxxxxxx>
Date: Thu, 25 Feb 2010 14:33:12 -0600
Cc: Dave Chinner <david@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <20100217083105.GB19943@xxxxxxxxxxxxx>
References: <20100215094445.528696829@xxxxxxxxxxxxxxxxxxxxxx> <20100215094604.318261333@xxxxxxxxxxxxxxxxxxxxxx> <20100217035511.GJ28392@xxxxxxxxxxxxxxxx> <20100217083105.GB19943@xxxxxxxxxxxxx>
Reply-to: aelder@xxxxxxx
On Wed, 2010-02-17 at 03:31 -0500, Christoph Hellwig wrote:
> On Wed, Feb 17, 2010 at 02:55:11PM +1100, Dave Chinner wrote:
> > You've changed a local variable "pos" which had the value
> > iocb->ki_pos to a function parameter of the same name which has a
> > different value.  Given this, all the existing uses of "pos" in this
> > function need to be converted to "iocb->ki_pos" as the old
> > xfs_write() never saw the original "pos" variable passed to
> > xfs_file_aio_write().

I think the simplest explanation is that previously, the value
of "offset" passed in is simply &iocb->ki_pos, and Christoph's
changes are then simply replacing "*offset" with "iocb->ki_pos".

In any case, I'm convinced the old is equivalent to the new...

                                        -Alex

> Oh, I should have explained this in more detail.  The aio_read/aio_write
> ABI both has a pos argument and the file position in iocb->ki_pos.
> They were added for allowing aio that does partial I/O in each method
> call using retries, but we don't actually use the anywhere.  Thus these
> two are always these same and we even enforce that with a
> 
>       BUG_ON(iocb->ki_pos != pos);
> 
> in the code (both old and new).  Now how does the old pos local variable
> come in play?  The old code didn't want to pass the kiocb to the
> low-level xfs-write function, but as want the offset it passes a pointer
> to iocb->ki_pos, which is called offset.  We take a local copy of it
> before we might start modifying it, which we call pos.  pos gets updated
> early in generic_write_checks if this is an O_APPEND write, but
> otherwise stays immutable and marks the position where this write
> started, while iocb->ki_pos (aka the old offset) gets updated by
> generic_file_direct_write / generic_file_buffered_write to the new
> file position after the I/O was done.
> 
> > Such as here. Actually, I'm surprised the compiler let you take
> > the address of a function parameter considering parameters may be
> > passed in registers....
> 
> Taking the address of arguments is perfectly valid in C, the only thing
> you can't take addresses of are "register" variables.  This code is
> the same as mm/filemap.c:__generic_file_aio_write, btw.
> 
> > > -         trace_xfs_file_direct_write(xip, count, *offset, ioflags);
> > > +         trace_xfs_file_direct_write(ip, count, iocb->ki_pos, ioflags);
> > >           ret = generic_file_direct_write(iocb, iovp,
> > > -                         &segs, pos, offset, count, ocount);
> > > +                         &nr_segs, pos, &iocb->ki_pos, count, ocount);
> > 
> > But you did convert some ;)
> 
> I did carefull convert all offset references to ki->ki_pos, but all
> uses of pos stay the same.
> 
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