linux-ext4 - Re: Checks in ext4_ext_fiemap

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Date:	Tue, 26 Jul 2011 11:48:44 -0700
From:	Aditya Kali <adityakali@...gle.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Yongqiang Yang <xiaoqiangnk@...il.com>, "Ted Ts'o" <tytso@....edu>,
	linux-ext4@...r.kernel.org, Andreas Dilger <adilger@...ger.ca>
Subject: Re: Checks in ext4_ext_fiemap_cb() broken

On Tue, Jul 26, 2011 at 5:12 AM, Jan Kara <jack@...e.cz> wrote:
>  Hi Yongqiang,
>
> On Tue 26-07-11 09:20:28, Yongqiang Yang wrote:
>> I have been thinking if we can handle fiemap much simpler for a while.
>>  Current code is very ugly due to page cache look up.  I have a
>> thought on simplifying these code.  The reason leading us to looking
>> up page cache is that delayed extents are not in extents tree.  I
>> think we can add an in-memory delayed extents list in inode, and we
>> can delete entries in the list after we allocate blocks for them.
>> There is no limit on length of extents in the list, this way can an
>> entry contain as many blocks as they are contiguous logically.
>>
>> What's your opinion?
>  Yes, that should be doable and shouldn't have too big overhead. It's just
> stupid we'll do all this stuff only for fiemap call which is relatively
> rare.
>
Delayed extents lookup will also help resolve another race that we
currently have in bigalloc code path. Here, we need to figure out if a
cluster is already under delayed allocation or not (to determine
whether we need to reserve quota for this cluster). But, determining
this races against the writeback of delayed allocated pages.
ext4_find_delalloc_range() function has a comment about this race. If
there is a delayed extents list and the extents are removed from his
list when they are actually mapped, then ext4_find_delalloc_range()
can simply check against this list.


>                                                                Honza
>
>> On Mon, Jul 25, 2011 at 11:58 PM, Jan Kara <jack@...e.cz> wrote:
>> >  Hello,
>> >
>> >  I just had a look at the code checking delayed allocated buffers in
>> > ext4_ext_fiemap_cb(). I believe the checks there could use some elimiation
>> > of common patterns but that's just a minor thing. The main problem is that
>> > the code can easily crash the kernel when it races with page reclaim. You
>> > just cannot access most of the page contents (and for buffers it is
>> > especially true) without locking the page. Getting a reference via
>> > find_get_pages_tag() guarantees you the structure cannot go away but mm is
>> > still free to detach the page from the mapping at any moment. So you must
>> > always lock a page and check that it still belongs to the desired mapping
>> > before you check 'page_has_buffers()'.
>> >
>> >                                                                Honza
>> > --
>> > Jan Kara <jack@...e.cz>
>> > SUSE Labs, CR
>> >
>>
>>
>>
>> --
>> Best Wishes
>> Yongqiang Yang
> --
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
> --
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