On Monday 13 August 2007 05:04, Evgeniy Polyakov wrote: > On Mon, Aug 13, 2007 at 04:04:26AM -0700, Daniel Phillips (phillips@xxxxxxxxx) wrote: > > On Monday 13 August 2007 01:14, Evgeniy Polyakov wrote: > > > > Oops, and there is also: > > > > > > > > 3) The bio throttle, which is supposed to prevent deadlock, can > > > > itself deadlock. Let me see if I can remember how it goes. > > > > > > > > * generic_make_request puts a bio in flight > > > > * the bio gets past the throttle and initiates network IO > > > > * net calls sk_alloc->alloc_pages->shrink_caches > > > > * shrink_caches submits a bio recursively to our block device > > > > * this bio blocks on the throttle > > > > * net may never get the memory it needs, and we are wedged > > > > > > If system is in such condition, it is already broken - throttle > > > limit must be lowered (next time) not to allow such situation. > > > > Agreed that the system is broken, however lowering the throttle > > limit gives no improvement in this case. > > How is it ever possible? The whole idea of throttling is to remove > such situation, and now you say it can not be solved. It was solved, by not throttling writeout that comes from shrink_caches. Ugly. > If limit is for > 1gb of pending block io, and system has for example 2gbs of ram (or > any other resonable parameters), then there is no way we can deadlock > in allocation, since it will not force page reclaim mechanism. The problem is that sk_alloc (called from our block driver via socket->write) would recurse into shrink_pages, which recursively submits IO to our block driver and blocks on the throttle. Subtle indeed, and yet another demonstration of why vm recursion is a Bad Thing. I will find a traceback for you tomorrow, which makes this deadlock much clearer. Regards - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
