[PATCH] mm: Track mmu notifiers in fs_reclaim_acquire/release
daniel at ffwll.ch
Sun Jun 21 20:01:03 UTC 2020
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 at lca.pw> 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 at shipmail.org>
> > > Cc: Andrew Morton <akpm at linux-foundation.org>
> > > Cc: Jason Gunthorpe <jgg at mellanox.com>
> > > Cc: linux-mm at kvack.org
> > > Cc: linux-rdma at vger.kernel.org
> > > Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> > > Cc: Christian König <christian.koenig at amd.com>
> > > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > Replying the right patch here...
> > Reverting this commit  fixed the lockdep warning below while applying
> > some memory pressure.
> >  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
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.
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d807587c9ae6..27ea763c6155 100644
@@ -4191,11 +4191,6 @@ void fs_reclaim_acquire(gfp_t gfp_mask)
if (gfp_mask & __GFP_FS)
Software Engineer, Intel Corporation
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