linux-kernel - Re: regression introduced by "block: Add support for DAX reads/writes to block devices"

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Message-ID: <x49bneiygpl.fsf@segfault.boston.devel.redhat.com>
Date:	Fri, 07 Aug 2015 16:41:10 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	"Wilcox\, Matthew R" <matthew.r.wilcox@...el.com>
Cc:	"linda.knippers\@hp.com" <linda.knippers@...com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel\@vger.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: regression introduced by "block: Add support for DAX reads/writes to block devices"

"Wilcox, Matthew R" <matthew.r.wilcox@...el.com> writes:

> So ... what you're doing here is if somebody requests the last 512
> bytes, you're asking for the last page.  That's going to work as long
> as the partition is a multiple of PAGE_SIZE, but not if the partition
> happens to be misaligned.  It's an improvement, although I'd be
> tempted to do this as:
>
> 		if (pos == max) {
>  			unsigned blkbits = inode->i_blkbits;
> - 			sector_t block = pos >> blkbits;
> +			long page = pos >> PAGE_SHIFT;
> +			sector_t block = page << (PAGE_SHIFT - blkbits);
> 			unsigned first = pos - (block << blkbits);
> 			long size;

Yeah, that's easier to read.

> We need to cope with the case where the end of a partition isn't on a
> page boundary though.

Well, that's usually done by falling back to buffered I/O.  I gave that
a try and panicked the box.  :)  I'll keep looking into it, but probably
won't have another patch until next week.

Cheers,
Jeff
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