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

[Dave Chinner <david@fromorbit.com>]

On Sun, Jun 21, 2020 at 01:42:05PM -0400, Qian Cai 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")
> 
> [  190.455003][  T369] WARNING: possible circular locking dependency detected
> [  190.487291][  T369] 5.8.0-rc1-next-20200621 #1 Not tainted
> [  190.512363][  T369] ------------------------------------------------------
> [  190.543354][  T369] kswapd3/369 is trying to acquire lock:
> [  190.568523][  T369] ffff889fcf694528 (&xfs_nondir_ilock_class){++++}-{3:3}, at: xfs_reclaim_inode+0xdf/0x860
> spin_lock at include/linux/spinlock.h:353
> (inlined by) xfs_iflags_test_and_set at fs/xfs/xfs_inode.h:166
> (inlined by) xfs_iflock_nowait at fs/xfs/xfs_inode.h:249
> (inlined by) xfs_reclaim_inode at fs/xfs/xfs_icache.c:1127
> [  190.614359][  T369]
> [  190.614359][  T369] but task is already holding lock:
> [  190.647763][  T369] ffffffffb50ced00 (fs_reclaim){+.+.}-{0:0}, at: __fs_reclaim_acquire+0x0/0x30
> __fs_reclaim_acquire at mm/page_alloc.c:4200
> [  190.687845][  T369]
> [  190.687845][  T369] which lock already depends on the new lock.
> [  190.687845][  T369]
> [  190.734890][  T369]
> [  190.734890][  T369] the existing dependency chain (in reverse order) is:
> [  190.775991][  T369]
> [  190.775991][  T369] -> #1 (fs_reclaim){+.+.}-{0:0}:
> [  190.808150][  T369]        fs_reclaim_acquire+0x77/0x80
> [  190.832152][  T369]        slab_pre_alloc_hook.constprop.52+0x20/0x120
> slab_pre_alloc_hook at mm/slab.h:507
> [  190.862173][  T369]        kmem_cache_alloc+0x43/0x2a0
> [  190.885602][  T369]        kmem_zone_alloc+0x113/0x3ef
> kmem_zone_alloc at fs/xfs/kmem.c:129
> [  190.908702][  T369]        xfs_inode_item_init+0x1d/0xa0
> xfs_inode_item_init at fs/xfs/xfs_inode_item.c:639
> [  190.934461][  T369]        xfs_trans_ijoin+0x96/0x100
> xfs_trans_ijoin at fs/xfs/libxfs/xfs_trans_inode.c:34
> [  190.961530][  T369]        xfs_setattr_nonsize+0x1a6/0xcd0

OK, this patch has royally screwed something up if this path thinks
it can enter memory reclaim. This path is inside a transaction, so
it is running under PF_MEMALLOC_NOFS context, so should *never*
enter memory reclaim.

I'd suggest that whatever mods were made to fs_reclaim_acquire by
this patch broke it's basic functionality....

> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index 13cc653122b7..7536faaaa0fd 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -57,6 +57,7 @@
> >  #include <trace/events/oom.h>
> >  #include <linux/prefetch.h>
> >  #include <linux/mm_inline.h>
> > +#include <linux/mmu_notifier.h>
> >  #include <linux/migrate.h>
> >  #include <linux/hugetlb.h>
> >  #include <linux/sched/rt.h>
> > @@ -4124,7 +4125,7 @@ should_compact_retry(struct alloc_context *ac, unsigned int order, int alloc_fla
> >  static struct lockdep_map __fs_reclaim_map =
> >  	STATIC_LOCKDEP_MAP_INIT("fs_reclaim", &__fs_reclaim_map);
> >  
> > -static bool __need_fs_reclaim(gfp_t gfp_mask)
> > +static bool __need_reclaim(gfp_t gfp_mask)
> >  {
> >  	gfp_mask = current_gfp_context(gfp_mask);

This is applies the per-task memory allocation context flags to the
mask that is checked here.

> > @@ -4136,10 +4137,6 @@ static bool __need_fs_reclaim(gfp_t gfp_mask)
> >  	if (current->flags & PF_MEMALLOC)
> >  		return false;
> >  
> > -	/* We're only interested __GFP_FS allocations for now */
> > -	if (!(gfp_mask & __GFP_FS))
> > -		return false;
> > -
> >  	if (gfp_mask & __GFP_NOLOCKDEP)
> >  		return false;
> >  
> > @@ -4158,15 +4155,25 @@ void __fs_reclaim_release(void)
> >  
> >  void fs_reclaim_acquire(gfp_t gfp_mask)
> >  {
> > -	if (__need_fs_reclaim(gfp_mask))
> > -		__fs_reclaim_acquire();
> > +	if (__need_reclaim(gfp_mask)) {
> > +		if (gfp_mask & __GFP_FS)
> > +			__fs_reclaim_acquire();

.... and they have not been applied in this path. There's your
breakage.

For future reference, please post anything that changes NOFS
allocation contexts or behaviours to linux-fsdevel, as filesystem
developers need to know about proposed changes to infrastructure
that is critical to the correct functioning of filesystems...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com