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

[Qian Cai]

On Sun, Jun 21, 2020 at 10:01:03PM +0200, Daniel Vetter wrote:
> On Sun, Jun 21, 2020 at 08:07:08PM +0200, Daniel Vetter wrote:
> > On Sun, Jun 21, 2020 at 7:42 PM Qian Cai <cai@xxxxxx> wrote:
> > >
> > > On Wed, Jun 10, 2020 at 09:41:01PM +0200, 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
> > > >
> > > > 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>
> > > > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> > >
> > > Replying the right patch here...
> > >
> > > Reverting this commit [1] fixed the lockdep warning below while applying
> > > some memory pressure.
> > >
> > > [1] linux-next cbf7c9d86d75 ("mm: track mmu notifiers in fs_reclaim_acquire/release")
> > 
> > Hm, then I'm confused because
> > - there's not mmut notifier lockdep map in the splat at a..
> > - the patch is supposed to not change anything for fs_reclaim (but the
> > interim version got that wrong)
> > - looking at the paths it's kmalloc vs kswapd, both places I totally
> > expect fs_reflaim to be used.
> > 
> > But you're claiming reverting this prevents the lockdep splat. If
> > that's right, then my reasoning above is broken somewhere. Someone
> > less blind than me having an idea?
> > 
> > Aside this is the first email I've typed, until I realized the first
> > report was against the broken patch and that looked like a much more
> > reasonable explanation (but didn't quite match up with the code
> > paths).
> 
> Below diff should undo the functional change in my patch. Can you pls test
> whether the lockdep splat is really gone with that? Might need a lot of
> testing and memory pressure to be sure, since all these reclaim paths
> aren't very deterministic.

Well, I am running even heavy memory pressure workloads on linux-next
like every day, and never saw this splat until today where your patch
first show up.

Since I am rather busy tracking another regression, here is the steps to
reproduce (super easy to reproduce on multiple machines here.):

# git clone https://github.com/cailca/linux-mm.git
# cd linux-mm; make
# ./random 0

The .config is in there as well if ever matters.