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Date:   Fri, 20 Nov 2020 14:23:16 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        linux-mm@...ck.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Dave Chinner <david@...morbit.com>, Qian Cai <cai@....pw>,
        Thomas Hellström <thomas_os@...pmail.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-rdma@...r.kernel.org,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Christian König <christian.koenig@....com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH 1/3] mm: Track mmu notifiers in fs_reclaim_acquire/release

On Fri, Nov 20, 2020 at 10:54:42AM +0100, Daniel Vetter wrote:
> fs_reclaim_acquire/release nicely catch recursion issues when
> allocating GFP_KERNEL memory against shrinkers (which gpu drivers tend
> to use to keep the excessive caches in check). For mmu notifier
> recursions we do have lockdep annotations since 23b68395c7c7
> ("mm/mmu_notifiers: add a lockdep map for invalidate_range_start/end").
> 
> But these only fire if a path actually results in some pte
> invalidation - for most small allocations that's very rarely the case.
> The other trouble is that pte invalidation can happen any time when
> __GFP_RECLAIM is set. Which means only really GFP_ATOMIC is a safe
> choice, GFP_NOIO isn't good enough to avoid potential mmu notifier
> recursion.
> 
> I was pondering whether we should just do the general annotation, but
> there's always the risk for false positives. Plus I'm assuming that
> the core fs and io code is a lot better reviewed and tested than
> random mmu notifier code in drivers. Hence why I decide to only
> annotate for that specific case.
> 
> Furthermore even if we'd create a lockdep map for direct reclaim, we'd
> still need to explicit pull in the mmu notifier map - there's a lot
> more places that do pte invalidation than just direct reclaim, these
> two contexts arent the same.
> 
> Note that the mmu notifiers needing their own independent lockdep map
> is also the reason we can't hold them from fs_reclaim_acquire to
> fs_reclaim_release - it would nest with the acquistion in the pte
> invalidation code, causing a lockdep splat. And we can't remove the
> annotations from pte invalidation and all the other places since
> they're called from many other places than page reclaim. Hence we can
> only do the equivalent of might_lock, but on the raw lockdep map.
> 
> With this we can also remove the lockdep priming added in 66204f1d2d1b
> ("mm/mmu_notifiers: prime lockdep") since the new annotations are
> strictly more powerful.
> 
> v2: Review from Thomas Hellstrom:
> - unbotch the fs_reclaim context check, I accidentally inverted it,
>   but it didn't blow up because I inverted it immediately
> - fix compiling for !CONFIG_MMU_NOTIFIER
> 
> v3: Unbreak the PF_MEMALLOC_ context flags. Thanks to Qian for the
> report and Dave for explaining what I failed to see.
> 
> Cc: linux-fsdevel@...r.kernel.org
> Cc: Dave Chinner <david@...morbit.com>
> Cc: Qian Cai <cai@....pw>
> Cc: linux-xfs@...r.kernel.org
> Cc: Thomas Hellström (Intel) <thomas_os@...pmail.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Jason Gunthorpe <jgg@...lanox.com>
> Cc: linux-mm@...ck.org
> Cc: linux-rdma@...r.kernel.org
> Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>
> Cc: Christian König <christian.koenig@....com>
> Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> Signed-off-by: Daniel Vetter <daniel.vetter@...el.com>
> ---
>  mm/mmu_notifier.c |  7 -------
>  mm/page_alloc.c   | 31 ++++++++++++++++++++-----------
>  2 files changed, 20 insertions(+), 18 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason

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