Re: [PATCH] mm: prevent concurrent unmap_mapping_range() on the same inode

[Hugh Dickins]

On Mon, Jan 24, 2011 at 11:47 AM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> On Fri, 21 Jan 2011, Hugh Dickins wrote:
>> On Thu, 20 Jan 2011, Miklos Szeredi wrote:
>> > On Thu, 20 Jan 2011, Christoph Hellwig wrote:
>> > > On Thu, Jan 20, 2011 at 01:30:58PM +0100, Miklos Szeredi wrote:
>> > > >
>> > > > Truncate and hole punching already serialize with i_mutex. ÂOther
>> > > > callers of unmap_mapping_range() do not, and it's difficult to get
>> > > > i_mutex protection for all callers. ÂIn particular ->d_revalidate(),
>> > > > which calls invalidate_inode_pages2_range() in fuse, may be called
>> > > > with or without i_mutex.
>> > >
>> > >
>> > > Which I think is mostly a fuse problem. ÂI really hate bloating the
>> > > generic inode (into which the address_space is embedded) with another
>> > > mutex for deficits in rather special case filesystems.
>> >
>> > As Hugh pointed out unmap_mapping_range() has grown a varied set of
>> > callers, which are difficult to fix up wrt i_mutex. ÂFuse was just an
>> > example.
>> >
>> > I don't like the bloat either, but this is the best I could come up
>> > with for fixing this problem generally. ÂIf you have a better idea,
>> > please share it.
>>
>> If we start from the point that this is mostly a fuse problem (I expect
>> that a thorough audit will show up a few other filesystems too, but
>> let's start from this point): you cite ->d_revalidate as a particular
>> problem, but can we fix up its call sites so that it is always called
>> either with, or much preferably without, i_mutex held? ÂThough actually
>> I couldn't find where ->d_revalidate() is called while holding i_mutex.
>
> lookup_one_len
> lookup_hash
> Â__lookup_hash
> Â Âdo_revalidate
>  Â Âd_revalidate

Right, thanks.

>
> I don't see an easy way to get rid of i_mutex for lookup_one_len() and
> lookup_hash().
>
>> Failing that, can fuse down_write i_alloc_sem before calling
>> invalidate_inode_pages2(_range), to achieve the same exclusion?
>> The setattr truncation path takes i_alloc_sem as well as i_mutex,
>> though I'm not certain of its full coverage.
>
> Yeah, fuse could use i_alloc_sem or a private mutex, but that would
> leave the other uses of unmap_mapping_range() to sort this out for
> themsevels.

I had wanted to propose that for now you modify just fuse to use
i_alloc_sem for serialization there, and I provide a patch to
unmap_mapping_range() to give safety to whatever other cases there are
(I'm now sure there are other cases, but also sure that I cannot
safely identify them all and fix them correctly at source myself -
even if I found time to do the patches, they'd need at least a release
cycle to bed in with BUG_ONs).

I've spent quite a while on it, but not succeeded: even if I could get
around the restart_addr issue, we're stuck with the deadly embrace
when two are in unmap_mapping_range(), each repeatedly yielding to the
other, each having to start over again.  Anything I came up with was
inferior to the two alternatives you have proposed: your original
wait_on_bit patch, or your current unmap_mutex patch.

Your wait_on_bit patch doesn't bloat (and may be attractive to
enterprise distros seeking binary compatibility), but several of us
agreed with Andrew's comments:

> I do think this was premature optimisation.  The open-coded lock is
> hidden from lockdep so we won't find out if this introduces potential
> deadlocks.  It would be better to add a new mutex at least temporarily,
> then look at replacing it with a MiklosLock later on, when the code is
> bedded in.
>
> At which time, replacing mutexes with MiklosLocks becomes part of a
> general "shrink the address_space" exercise in which there's no reason
> to exclusively concentrate on that new mutex!

It really does seem a mutex too far; but we may let Peter do away with
all that lock breaking when/if his preemptibility patches go in, and
could cut it out at that time.  I don't see a good alternative.

Hugh
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