Re: [PATCH 0/3] i_ino uniqueness: alternate approach -- hash the inodes

[Eric Sandeen <sandeen@redhat.com>]

Jeff Layton wrote:
> Resending this set of patches to the list per Al Viro's request. I've gotten
> no comments so far, so I'll presume that denotes consent and just ask Andrew
> to merge them if I don't hear anything after this ;-).
> 
> ------[snip]-----
> 
> Since Joern mentioned that he thought that hashing the inodes might be simpler
> and not have a drastic performance impact, I took the liberty of whipping up
> some patches that use that approach. They follow in the next set of emails.
> 
> To reiterate, the problems are:
> 
> 1) on filesystems w/o permanent inode numbers, i_ino values can be
> larger than 32 bits, which can cause problems for some 32 bit userspace
> programs on a 64 bit kernel. We can't do anything for filesystems that have
> actual >32-bit inode numbers, but on filesystems that generate i_ino
> values on the fly, we should try to have them fit in 32 bits. We could
> trivially fix this by making the static counters in new_inode and iunique
> 32 bits, but...
> 
> 2) many filesystems call new_inode and assume that the i_ino values they
> are given are unique. They are not guaranteed to be so, since the static
> counter can wrap. This problem is exacerbated by the fix for #1.
> 
> 3) after allocating a new inode, some filesystems call iunique to try to
> get a unique i_ino value, but they don't actually add their inodes to
> the hashtable, and so they're still not guaranteed to be unique if that
> counter wraps.
> 
> This patch set takes the simpler approach of simply using iunique and
> hashing the inodes afterward. Christoph H. previously mentioned that he
> thought that this approach may slow down lookups for filesystems that
> currently hash their inodes.
> 
> The questions are:
> 
> 1) how much would this slow down lookups for these filesystems?
> 2) is it enough to justify adding more infrastructure to avoid it?
> 
> What might be best is to start with this approach and then only move to using
> IDR or some other scheme if these extra inodes in the hashtable prove to be
> problematic.
> 
> I've done some cursory testing with this patch and the overhead of hashing
> and unhashing the inodes with pipefs is pretty low -- just a few seconds of
> system time added on to the creation and destruction of 10 million pipes (very
> similar to the overhead that the IDR approach would add).
> 
> The hard thing to measure is what effect this has on other filesystems. I'm
> open to ways to try and gauge this.
> 
> Again, I've only converted pipefs as an example. If this approach is
> acceptable then I'll start work on patches to convert other filesystems.
> 
> Comments and suggestions welcome...

The first two seem fine to me; I'm still thinking about how the
un-hashing works in the 3rd one.

Thanks,
-Eric
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