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@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@shipmail.org Cc: Andrew Morton akpm@linux-foundation.org Cc: Jason Gunthorpe jgg@mellanox.com Cc: linux-mm@kvack.org Cc: linux-rdma@vger.kernel.org Cc: Maarten Lankhorst maarten.lankhorst@linux.intel.com Cc: Christian König christian.koenig@amd.com Signed-off-by: Daniel Vetter daniel.vetter@intel.com
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.