linux-ext4 - Re: [PATCH] e2fsck: do not allow initialized blocks pass i

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Message-Id: <D7A7B2DA-DC6C-480C-A759-342D0964F84B@dilger.ca>
Date:   Thu, 28 Jun 2018 21:05:04 -0600
From:   Andreas Dilger <adilger@...ger.ca>
To:     "Theodore Y. Ts'o" <tytso@....EDU>
Cc:     Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH] e2fsck: do not allow initialized blocks pass i_size

> On Jun 28, 2018, at 7:45 PM, Theodore Y. Ts'o <tytso@....EDU> wrote:
> 
> On Thu, Jun 28, 2018 at 06:07:47PM -0600, Andreas Dilger wrote:
>>> But does an RDMA operation actually do a block allocation?  Really?
>>> And if it is willing to do a block allocation, why is it not willing
>>> to bump i_size?
>> 
>> It's not that the RDMA does block allocation, but rather that the RDMA
>> always transfers and writes the full PAGE_SIZE of data, even if i_size
>> is less than the end of the last block.  This simplifies the RDMA code
>> so that it can always write the data instead of having to stop at i_size.
> 
> Right.  So there are two choices:
> 
> 1) Keep the blocks beyond i_size marked as uninitialized.  You
> transfer and write the full PAGE_SIZE of data, but it simply will
> never be available to the user.
> 
> 2)  Zero the page, write it out to the file, and then extend i_size and
> mark the extents as uninitialized.

The end of the page would already be zeroed before write.

> Why is it that Lustre is choosing to keep i_size where it is, but to
> mark the blocks beyond it as initialized?

This isn't about initialized vs. uninitialized extents.  It is only about
allocated vs. unallocated blocks, possibly with block-mapped files.  There
is no way to have uninitialized blocks with a block-mapped file.

The code is checking whether there are any blocks allocated beyond i_size,
and if there are, without the patch it considers i_size broken and extends
it to the end of the last allocated block.  The patch allows a small number
of blocks to be allocated beyond i_size without triggering this heuristic.

The only difference vs. the previous code is that it correctly calculates
what the PAGE_SIZE aligned block number is (the old code assumed that lblock
was the base-1 block number instead of the base-0 block number that it is).

Cheers, Andreas

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