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

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Message-ID: <CAAJw_ZuMGk+r73LBUik=bz8YD2=dd4WTwzmf8ru0oCFv+GSSCA@mail.gmail.com>
Date:	Thu, 29 Nov 2012 18:57:44 +0800
From:	Jeff Chua <jeff.chua.linux@...il.com>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mikulas Patocka <mpatocka@...hat.com>,
	Jens Axboe <axboe@...nel.dk>,
	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)

On Thu, Nov 29, 2012 at 2:45 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
> On Wed, Nov 28, 2012 at 10:37:27PM -0800, Linus Torvalds wrote:
>> On Wed, Nov 28, 2012 at 10:30 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>> >
>> > Note that sync_blockdev() a few lines prior to that is good only if we
>> > have no other processes doing write(2) (or dirtying the mmapped pages,
>> > for that matter).  The window isn't too wide, but...
>>
>> So with Mikulas' patches, the write actually would block (at write
>> level) due to the locking. The mmap'ed patches may be around and
>> flushed, but the logic to not allow currently *active* mmaps (with the
>> rather nasty random -EBUSY return value) should mean that there is no
>> race.
>>
>> Or rather, there's a race, but it results in that EBUSY thing.
>
> Same as with fs mounted on it, or the sucker having been claimed for
> RAID array, etc.  Frankly, I'm more than slightly tempted to make
> bdev mmap() just claim the sodding device exclusive for as long as
> it's mmapped...
>
> In principle, I agree, but...  I still have nightmares from mmap/truncate
> races way back.  You are stepping into what used to be a really nasty
> minefield.  I'll look into that, but it's *definitely* not -rc8 fodder.

Just let me know which relevant patch(es) you want me to test or break.

Thanks,
Jeff
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