linux-kernel - Re: [PATCHv2 1/1] ext4: don't put symlink in pagecache into highmem

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Date:   Tue, 6 Feb 2018 21:16:17 -0500
From:   Theodore Ts'o <tytso@....edu>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Jin Qian <jinqian@...roid.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jeff Layton <jlayton@...chiereds.net>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Jin Qian <jinqian@...gle.com>,
        stable@...r.kernel.org
Subject: Re: [PATCHv2 1/1] ext4: don't put symlink in pagecache into highmem

On Tue, Feb 06, 2018 at 03:38:09PM -0800, Eric Biggers wrote:
> I don't think backporting this change for other filesystems is particularly
> important, since if I understand correctly, the reasons that Al made the change
> originally were:
> 
> - to allow following symlinks in RCU mode, but that's not implemented in old
>   kernels

Yup.

> - to prevent a process from using up all kmaps and deadlocking the system, which
>   I'm not sure is a real problem (someone would need to try to put together a
>   reproducer), but if so it would probably just be a local device of service.

.. and *that's* only a problem on 32-bit systems.  And aside from
Android, it's unclear to me how much we need to support 32-bit systems
on upstream LTS kernels.  I suppose there might be Rasperry PI's which
are 32-bits and which might want to use btrfs.  Personally I'm not
sure we should care all that much, but others who care more about LTS
kernels and 32-bit systems might have a different opinion.

> Also if we actually backported the full commit there are follow-on fixes such as
> e8ecde25f5e that would be needed as well but might be missed.

Good point.

					- Ted