linux-kernel - Re: [patch|rfc] block: don't mark buffers beyond end of disk as mapped

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Message-ID: <CAPa8GCChTmXTmjY5GvsFsvXpP_WSuiiWMRDu7Lvb71G9tW1PTw@mail.gmail.com>
Date:	Wed, 2 May 2012 00:26:55 +1000
From:	Nick Piggin <npiggin@...il.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	jaxboe@...ionio.com, Kyle McMartin <kmcmarti@...hat.com>
Subject: Re: [patch|rfc] block: don't mark buffers beyond end of disk as mapped

On 2 May 2012 00:08, Jeff Moyer <jmoyer@...hat.com> wrote:
> Nick Piggin <npiggin@...il.com> writes:
>
>> Not a bad fix. But it's kind of sad to have i_size checking logic also in
>> block_read_full_page, that does not cope with this.
>>
>> I have found there are parts of the kernel (readahead) that try to read
>> beyond EOF and seem to get angry if we return an error (by not
>> marking uptodate in readpage) in that case though :(
>>
>> But, either way, I think it's very reasonable to not mark buffers beyond
>> end of device as mapped. So I think your patch is fine.
>>
>> I guess for ext[234], it does not read metadata close to the end of the
>> device or you were using 4K sized blocks?
>
> Well, the test case just reads directly from the loop device, bypassing
> the file system, and I did use 1KB blocks when making the file system, so
> it is quite puzzling.

It's because buffer_head creation does not go through the same paths
for bdev file access versus getblk APIs.

blkdev_get_block does the right thing there

In fact, it's probably good to unify the checks here, i.e., use max_blocks()

Thanks,
Nick
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