linux-kernel - Re: [PATCH 1/2] mm/mmu_notifier: Mark up direct reclaim paths with MAYFAIL

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Message-ID: <20200624184804.GL6578@ziepe.ca>
Date:   Wed, 24 Jun 2020 15:48:04 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Chris Wilson <chris@...is-wilson.co.uk>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/2] mm/mmu_notifier: Mark up direct reclaim paths with
 MAYFAIL

On Wed, Jun 24, 2020 at 06:58:49PM +0100, Chris Wilson wrote:
> > I'm still a little unclear on what you are trying to fix - pinned
> > pages are definitely not freed, do you have some case where pages
> > which are pinned are being cleaned out from the MM despite being
> > pinned? Sounds a bit strange, maybe that is worth adressing directly?
> 
> It suffices to say that pin_user_pages does not prevent try_to_unmap_one
> from trying to revoke the page.

This doesn't sound right.. Maybe there are some odd ball cases, but in
common things like page out we absolutely cannot page out or discard
pages under DMA. This would be a significant bug.

What is the actual problem here?

Jason