Re: vm-pageout-throttling.patch: hanging in throttle_vm_writeout/blk_congestion_wait

[Hugh Dickins]

On Mon, 15 Nov 2004, Sami Farin wrote:
> 
> this time I had some swapspace on /dev/loop1 (file-backed, reiserfs,
> loop-AES-2.2d)...  I think (!) it caused this deadlock.

That's not at all surprising.  See the swap_extent work Andrew did
for 2.5 (in mm/swapfile.c), by which swap to a swapfile now avoids
the filesystem altogether (except while swapon prepares the map of
disk blocks).  By swapping to a loop device over a file, you're
sneaking past his work, and putting the filesystem back under swap.
It is begging for deadlocks: I'm not saying it couldn't be got to
work, and of course it would be nice to boast that there's no such
issue; but there are so many better places to invest such effort...

Hugh

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