linux-ext4 - Re: [RFC PATCH] coredump: avoid ext4 auto_da

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Date:	Tue, 5 Jul 2016 12:57:15 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Josef Bacik <jbacik@...com>
Cc:	Omar Sandoval <osandov@...ndov.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	kernel-team@...com
Subject: Re: [RFC PATCH] coredump: avoid ext4 auto_da_alloc for core file

On Tue, Jul 05, 2016 at 11:01:40AM -0400, Josef Bacik wrote:
> > > Omar, this probably breaks the case where we do
> > > fallocate(FALLOC_FL_KEEP_SIZE), the i_size will be 0 but there will be
> > > blocks to truncate.  Probably want to check i_blocks or something.  Thanks,
> > 
> > Sure, but this is in the coredump code; do we care there?  What are
> > the odds that someone will have fallocated blocks beyond i_size in a
> > file named "core"?  And if so, it's not like it's going to make the
> > coredump invalid or non-useful in any way.
> 
> Wow I totally didn't notice this was in coredump.c, I thought it was in ext4
> code because you said it failed regression tests, which I assumed were your
> ext4 tests.  Ignore me.  Thanks,

Yeah, Omar's original patch was something he described as a "hack" to
the coredump code.  I actually don't think it's that bad, but it does
make sense to have ext4 not enable the "replace-via-truncate" code
when the truncate is a no-op, but it turns out this is a bit tricky
because the places where we set i_size and where we decide to truncate
beyond i_size are separated.  I tried to do something simple but it
didn't quite work right; I'll look into why it didn't work hopefully
later today.

						- Ted
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