Re: [PATCH] Memory management livelock

[Nick Piggin]

On Friday 03 October 2008 21:26, Mikulas Patocka wrote:
> > > *What* is, forever? Data integrity syncs should have pages operated on
> > > in-order, until we get to the end of the range. Circular writeback
> > > could go through again, possibly, but no more than once.
> >
> > OK, I have been able to reproduce it somewhat. It is not a livelock,
> > but what is happening is that direct IO read basically does an fsync
> > on the file before performing the IO. The fsync gets stuck behind the
> > dd that is dirtying the pages, and ends up following behind it and
> > doing all its IO for it.
> >
> > The following patch avoids the issue for direct IO, by using the range
> > syncs rather than trying to sync the whole file.
> >
> > The underlying problem I guess is unchanged. Is it really a problem,
> > though? The way I'd love to solve it is actually by adding another bit
> > or two to the pagecache radix tree,  that can be used to transiently tag
> > the tree for future operations. That way we could record the dirty and
> > writeback pages up front, and then only bother with operating on them.
> >
> > That's *if* it really is a problem. I don't have much pity for someone
> > doing buffered IO and direct IO to the same pages of the same file :)
>
> LVM does (that is where the bug was discovered). Basically, it scans all
> the block devices with direct IO and if someone else does buffered IO on
> any device simultaneously, it locks up.

Scans all block devices with direct IO? Hmm, why, I wonder? Should
really consider using buffered (posix_fadvise to readahead/dropbehind).


> That fsync-vs-write livelock is quite improbably (why would some
> application do it?) --- although it could be used as a DoS --- getting
> unkillable process.

I'm not sure.
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