Re: Why PAGEOUT_IO_SYNC stalls for a long time

[Andrew Morton]

On Wed, 28 Jul 2010 20:40:21 +0900 (JST) KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> wrote:

> 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.

The idea is that vmscan doesn't call ->writepage if the underlying
queue is congested.  may_write_to_queue()->bdi_queue_congested() should
return false and we skip the write.

If that logic is broken then that would explain a few things...
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