Re: Why PAGEOUT_IO_SYNC stalls for a long time

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> > (1) and (8) might be solved
> > by sleeping awhile, but it's unrelated on io-congestion. but might not be. It only works
> > by lucky. So I don't like to depned on luck. 
> 
> In this case, waiting a while really in the right thing to do. It stalls
> the caller, but it's a high-order allocation. The alternative is for it
> to keep scanning which when under memory pressure could result in far
> too many pages being evicted. How long to wait is a tricky one to answer
> but I would recommend making this a low priority.

For case (1), just lock_page() instead trylock is brilliant way than random sleep. 
Is there any good reason to give up synchrounous lumpy reclaim when trylock_page() failed?
IOW, briefly lock_page() and wait_on_page_writeback() have the same latency. why should
we only avoid former?

side note: page lock contention is very common case.

For case (8), I don't think sleeping is right way. get_page() is used in really various place of
our kernel. so we can't assume it's only temporary reference count increasing. In the other
hand, this contention is not so common because shrink_page_list() is excluded from IO
activity by page-lock and wait_on_page_writeback(). so I think giving up this case don't
makes too many pages eviction.
If you disagree, can you please explain your expected bad scinario?



> > > > 3. pageout() is intended anynchronous api. but doesn't works so.
> > > > 
> > > > pageout() call ->writepage with wbc->nonblocking=1. because if the system have
> > > > default vm.dirty_ratio (i.e. 20), we have 80% clean memory. so, getting stuck
> > > > on one page is stupid, we should scan much pages as soon as possible.
> > > > 
> > > > HOWEVER, block layer ignore this argument. if slow usb memory device connect
> > > > to the system, ->writepage() will sleep long time. because submit_bio() call
> > > > get_request_wait() unconditionally and it doesn't have any PF_MEMALLOC task
> > > > bonus.
> > > 
> > > Is this not a problem in the writeback layer rather than pageout()
> > > specifically?
> > 
> > Well, outside pageout(), probably only XFS makes PF_MEMALLOC + writeout. 
> > because PF_MEMALLOC is enabled only very limited situation. but I don't know
> > XFS detail at all. I can't tell this area...
> > 
> 
> All direct reclaimers have PF_MEMALLOC set so it's not that limited a
> situation. See here

Yes, all direct reclaimers have PF_MEMALLOC. but usually all direct reclaimers don't call
any IO related function except pageout(). As far as I know, current shrink_icache() and 
shrink_dcache() doesn't make IO. Am I missing something?



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