Subject: Re: [PATCH] mm-move-mremap-from-file_operations-to-vm_operations_struct-fix

From: Benjamin LaHaise <[email-masked]>
Date: Mon, 20 Jul 2015 15:39:12 -0400

On Mon, Jul 20, 2015 at 09:24:40PM +0200, Oleg Nesterov wrote:
> On 07/20, Oleg Nesterov wrote:
> >
> > But again, again, please ignore. This all is off-topic and my understanding
> > is very limited.
> 
> Yes, yes, but sorry for noise and let me repeat...
> 
> This memory lives in page-cache/lru, it is visible for shrinker which
> will unmap these pages for no reason on memory shortage. IOW, aio fools
> the kernel, this memory looks reclaimable but it is not. And we only do
> this for migration.

And we have the same problem with O_DIRECT.  Given the size of the LRU in 
a modern system, I highly doubt a handful of pages getting scanned is a 
major problem.  If you want to improve this, go ahead, but we need to 
retain support for page migration as people have run into the need for it.

> Even if this is not a problem, this does not look right. So perhaps at
> least mapping_set_unevictable() makes sense. But I simply do not know
> if migration will work with this change.

Nor do I know if that will work.

> And I should have changes the subject a long ago... So what do you think
> we should do with the build failure?

I honestly don't care what of the options you do -- please just don't go 
about adding BUG()s.

		-ben

> Oleg.

-- 
"Thought is the essence of where you are now."