Re: [PATCH 3/3] mlock: avoid dirtying pages and triggering writeback

[Theodore Tso <tytso@google.com>]

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On Fri, Nov 19, 2010 at 2:23 AM, Michel Lespinasse <walken@google.com>wrote:

> Approaching the problem the other way - would there be any objection to
> adding code to do an fallocate() equivalent at the start of mlock ?
> This would be a no-op when the file is fully allocated on disk, and would
> allow mlock to return an error if the file can't get fully allocated
> (no idea what errno should be for such case, though).
>

My vote would be against.   If you if you mmap a sparse file and then try
writing to it willy-nilly, bad things will happen.  This is true without a
mlock().   Where is it written that mlock() has anything to do with
improving this situation?

If userspace wants to call fallocate() before it calls mlock(), it should do
that.  And in fact, in most cases, userspace should be encouraged to do
that.   But having mlock() call fallocate() and then return ENOSPC if
there's no room?

That just makes me feel icky as all heck.

Look, it was an accident / bug of the implementation that mlock() magically
dirtied all these pages.  It might have made some situations better, but I
very much doubt applications depended upon it, and I'd really rather not
perpetuate this particular magic side effect of the buggy implementation of
mlock().

-- Ted

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