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

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Message-ID: <CA+55aFz1Wv2S+3XVmZE1cfiN0TZ4gNA8yWbNgufDLTMc8sKf2A@mail.gmail.com>
Date:	Wed, 28 Nov 2012 18:04:06 -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>
Subject: Re: [PATCH] Introduce a method to catch mmap_region (was: Recent
 kernel "mount" slow)

[ Sorry, I was offline for a while driving kids around ]

On Wed, Nov 28, 2012 at 4:38 PM, Mikulas Patocka <mpatocka@...hat.com> wrote:
>
> It can happen. Take your patch (the one that moves bd_block_size_semaphore
> into blkdev_readpage, blkdev_writepage and blkdev_write_begin).

Interesting. The code *has* the block size (it's in "bh->b_size"), but
it actually then uses the inode blocksize instead, and verifies the
two against each other. It could just have used the block size
directly (and then used the inode i_blkbits only when no buffers
existed), avoiding that dependency entirely..

It actually does the same thing (with the same verification) in
__block_write_full_page() and (_without_ the verification) in
__block_commit_write().

Ho humm. All of those places actually do hold the rwsem for reading,
it's just that I don't want to hold it for writing over the sync..

Need to think on this,

                Linus
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