Re: [PATCH 1/3] mm: Track mmu notifiers in fs_reclaim_acquire/release

[Jason Gunthorpe]

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@xxxxxxxxxxxxxxx
> Cc: Dave Chinner <david@xxxxxxxxxxxxx>
> Cc: Qian Cai <cai@xxxxxx>
> Cc: linux-xfs@xxxxxxxxxxxxxxx
> Cc: Thomas Hellström (Intel) <thomas_os@xxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> Cc: linux-mm@xxxxxxxxx
> Cc: linux-rdma@xxxxxxxxxxxxxxx
> Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> Cc: Christian König <christian.koenig@xxxxxxx>
> Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> ---
>  mm/mmu_notifier.c |  7 -------
>  mm/page_alloc.c   | 31 ++++++++++++++++++++-----------
>  2 files changed, 20 insertions(+), 18 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>

Jason