Messages in this thread | | | Date | Mon, 7 Mar 2016 10:03:36 +1100 | From | Dave Chinner <> | Subject | Re: THP-enabled filesystem vs. FALLOC_FL_PUNCH_HOLE |
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On Sun, Mar 06, 2016 at 03:30:34AM +0300, Kirill A. Shutemov wrote: > On Sun, Mar 06, 2016 at 09:38:11AM +1100, Dave Chinner wrote: > > On Sat, Mar 05, 2016 at 02:24:12AM +0300, Kirill A. Shutemov wrote: > > > Would it be acceptable for fallocate(FALLOC_FL_PUNCH_HOLE) to return > > > -EBUSY (or other errno on your choice), if we cannot split the page > > > right away? > > > > Which means THP are not transparent any more. What does an > > application do when it gets an EBUSY, anyway? > > I guess it's reasonable to expect from an application to handle EOPNOTSUPP > as FALLOC_FL_PUNCH_HOLE is not supported by some filesystems.
Yes, but this is usually done as a check at the program initialisation to determine whether to issue hole punches at all. It's not suppose to be a dynamic error.
> Although, non-consistent result from the same fd can be confusing.
Exactly.
> > And it's not just hole punching that has this problem. Direct IO is > > going to have the same issue with invalidation of the mapped ranges > > over the IO being done. XFS already WARNs when page cache > > invalidation fails with EBUSY in direct IO, because that is > > indicative of an application with a potential data corruption vector > > and there's nothing we can do in the kernel code to prevent it. > > My current understanding is that for filesystems with persistent storage, > in order to make THP any useful, we would need to implement writeback > without splitting the huge page.
Algorithmically it is no different to filesytem block size < page size writeback.
> At the moment, I have no idea how hard it would be..
THP support would effectively require us to remove PAGE_CACHE_SIZE assumptions from all of the filesystem and buffer code. That's a large chunk of work e.g. fs/buffer.c and any filesystem that uses bufferheads for tracking filesystem block state through the page cache.
Cheers,
Dave. -- Dave Chinner david@fromorbit.com
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