Re: [lkml]Re: VM problems still in 2.2.18

[Andrea Arcangeli <andrea@suse.de>]

On Fri, Dec 15, 2000 at 05:57:18PM +0000, Alan Cox wrote:
> How hard is it to seperate losing kpiod (optimisation) from the MAP_SHARED 
> changes ? I am assuming they are two seperate issues, possibly wrongly

Losing kpiod isn't an optimization ;(. Losing kpiod is the MAP_SHARED bugfix.

The problem was:

o	swap_out
o	wants to flush a MAP_SHARED dirty page to disk
o	so allocate kpiod-struct
o	sumbit the page-flush request to kpiod
o	don't wait I/O completion to avoid deadlocking on the i_sem
o	swap_out returns 1 and memory balancing code so thinks we did progress
	in freeing memory and goes to allocate memory from the freelist
	without waiting I/O completion
o	repeat N times the above

o	in the meantime kpiod has a big queue but it's blocked slowly writing
	those pages to disk
o	while it writes a few pages swap_out floods again the queue
	without waiting and it empties the freelist (task killed)

The problem was the lack of write throttling due the kpiod async-only nature.

> Providing no inode semaphore is upped from a different task , which seems
> currently quite a valid legal thing to do (ditto doing the up on completion of
> something in bh or irq context)

Yes, the same `current' context must run the down/up pair of calls and as you
said it is legal to rely on it on all the places it's used.

Andrea
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