linux-kernel - Re: [PATCH] Introduce a method to catch mmap_region (was: Recent kernel "mount" slow)

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Date:	Wed, 28 Nov 2012 22:16:21 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Jeff Chua <jeff.chua.linux@...il.com>,
	Lai Jiangshan <laijs@...fujitsu.com>, Jan Kara <jack@...e.cz>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] Introduce a method to catch mmap_region (was: Recent
 kernel "mount" slow)

On Wed, Nov 28, 2012 at 6:58 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> But the fact that the code wants to do things like
>
>         block = (sector_t)page->index << (PAGE_CACHE_SHIFT - bbits);
>
> seriously seems to be the main thing that keeps us using
> 'inode->i_blkbits'. Calculating bbits from bh->b_size is just costly
> enough to hurt (not everywhere, but on some machines).
>
> Very annoying.

Hmm. Here's a patch that does that anyway. I'm not 100% happy with the
whole ilog2 thing, but at the same time, in other cases it actually
seems to improve code generation (ie gets rid of the whole unnecessary
two dereferences through page->mapping->host just to get the block
size, when we have it in the buffer-head that we have to touch
*anyway*).

Comments? Again, untested.

And I notice that Al Viro hasn't been cc'd, which is sad, since he's
been involved in much of fs/block_dev.c.

Al - this is an independent patch to fs/buffer.c to make
fs/block_dev.c able to change the block size of a block device while
there is IO in progress that may still use the old block size. The
discussion has been on fsdevel and lkml, but you may have missed it...

                Linus

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