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From: Dave Chinner <david@fromorbit.com>
To: Andreas Dilger <adilger@dilger.ca>
Cc: Matthew Wilcox <willy@infradead.org>,
	linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	"Darrick J . Wong" <darrick.wong@oracle.com>,
	linux-nvdimm@lists.01.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 9/9] iomap: Change calling convention for zeroing
Date: Tue, 25 Aug 2020 14:27:11 +1000	[thread overview]
Message-ID: <20200825042711.GL12131@dread.disaster.area> (raw)
In-Reply-To: <E47B2C68-43F2-496F-AA91-A83EB3D91F28@dilger.ca>

On Mon, Aug 24, 2020 at 09:35:59PM -0600, Andreas Dilger wrote:
> On Aug 24, 2020, at 9:26 PM, Matthew Wilcox <willy@infradead.org> wrote:
> > 
> > On Tue, Aug 25, 2020 at 10:27:35AM +1000, Dave Chinner wrote:
> >>> 	do {
> >>> -		unsigned offset, bytes;
> >>> -
> >>> -		offset = offset_in_page(pos);
> >>> -		bytes = min_t(loff_t, PAGE_SIZE - offset, count);
> >>> +		loff_t bytes;
> >>> 
> >>> 		if (IS_DAX(inode))
> >>> -			status = dax_iomap_zero(pos, offset, bytes, iomap);
> >>> +			bytes = dax_iomap_zero(pos, length, iomap);
> >> 
> >> Hmmm. everything is loff_t here, but the callers are defining length
> >> as u64, not loff_t. Is there a potential sign conversion problem
> >> here? (sure 64 bit is way beyond anything we'll pass here, but...)
> > 
> > I've gone back and forth on the correct type for 'length' a few times.
> > size_t is too small (not for zeroing, but for seek()).  An unsigned type
> > seems right -- a length can't be negative, and we don't want to give
> > the impression that it can.  But the return value from these functions
> > definitely needs to be signed so we can represent an error.  So a u64
> > length with an loff_t return type feels like the best solution.  And
> > the upper layers have to promise not to pass in a length that's more
> > than 2^63-1.
> 
> The problem with allowing a u64 as the length is that it leads to the
> possibility of an argument value that cannot be returned.  Checking
> length < 0 is not worse than checking length > 0x7ffffffffffffff,
> and has the benefit of consistency with the other argument types and
> signs...

I think the problem here is that we have no guaranteed 64 bit size
type. when that was the case with off_t, we created loff_t to always
represent a 64 bit offset value. However, we never created one for
the count/size that is passed alongside loff_t in many places - it
was said that "syscalls are limited to 32 bit sizes" and
"size_t is 64 bit on 64 bit platforms" and so on and so we still
don't have a clean way to pass 64 bit sizes through the IO path.

We've been living with this shitty situation for a long time now, so
perhaps it's time for us to define lsize_t for 64 bit lengths and
start using that everywhere that needs a 64 bit clean path
through the code, regardless of whether the arch is 32 or 64 bit...

Thoughts?

-Dave.

-- 
Dave Chinner
david@fromorbit.com

  reply	other threads:[~2020-08-25  4:27 UTC|newest]

Thread overview: 42+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-24 14:55 [PATCH 0/9] THP iomap patches for 5.10 Matthew Wilcox (Oracle)
2020-08-24 14:55 ` [PATCH 1/9] iomap: Fix misplaced page flushing Matthew Wilcox (Oracle)
2020-08-24 23:51   ` Dave Chinner
2020-08-25 20:47   ` Darrick J. Wong
2020-08-27  8:24   ` Christoph Hellwig
2020-08-24 14:55 ` [PATCH 2/9] fs: Introduce i_blocks_per_page Matthew Wilcox (Oracle)
2020-08-24 23:55   ` Dave Chinner
2020-08-25 20:49   ` Darrick J. Wong
2020-08-25 22:21     ` Dave Chinner
2020-08-24 14:55 ` [PATCH 3/9] iomap: Use kzalloc to allocate iomap_page Matthew Wilcox (Oracle)
2020-08-24 23:56   ` Dave Chinner
2020-08-25 20:49   ` Darrick J. Wong
2020-08-24 14:55 ` [PATCH 4/9] iomap: Use bitmap ops to set uptodate bits Matthew Wilcox (Oracle)
2020-08-24 23:56   ` Dave Chinner
2020-08-25 20:50   ` Darrick J. Wong
2020-08-24 14:55 ` [PATCH 5/9] iomap: Support arbitrarily many blocks per page Matthew Wilcox (Oracle)
2020-08-24 23:59   ` Dave Chinner
2020-08-25  0:22     ` Matthew Wilcox
2020-08-25 21:02   ` Darrick J. Wong
2020-08-26  2:26     ` Matthew Wilcox
2020-08-26  3:32       ` Darrick J. Wong
2020-08-27  8:26   ` Christoph Hellwig
2020-08-24 14:55 ` [PATCH 6/9] iomap: Convert read_count to byte count Matthew Wilcox (Oracle)
2020-08-25  0:09   ` Dave Chinner
2020-08-25 22:14     ` Darrick J. Wong
2020-08-27  8:35   ` Christoph Hellwig
2020-08-24 14:55 ` [PATCH 7/9] iomap: Convert write_count " Matthew Wilcox (Oracle)
2020-08-27  8:36   ` Christoph Hellwig
2020-08-24 14:55 ` [PATCH 8/9] iomap: Convert iomap_write_end types Matthew Wilcox (Oracle)
2020-08-25  0:12   ` Dave Chinner
2020-08-25  1:06     ` Matthew Wilcox
2020-08-25  1:33       ` Dave Chinner
2020-08-27  8:41   ` Christoph Hellwig
2020-08-24 14:55 ` [PATCH 9/9] iomap: Change calling convention for zeroing Matthew Wilcox (Oracle)
2020-08-25  0:27   ` Dave Chinner
2020-08-25  3:26     ` Matthew Wilcox
2020-08-25  3:35       ` Andreas Dilger
2020-08-25  4:27         ` Dave Chinner [this message]
2020-08-25 12:40           ` Matthew Wilcox
2020-08-25 22:05             ` Dave Chinner
2020-08-25 22:23   ` Darrick J. Wong
2020-08-27  8:39     ` Christoph Hellwig

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