Re: fs/buffer.c: WARNING: alloc_page_buffers while mke2fs

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Mar 03, 2020 at 12:40:33PM -0800, Shakeel Butt wrote:
> On Tue, Mar 3, 2020 at 12:26 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > From e0e5ace069af5a36e41eafe3bf21a67966127c04 Mon Sep 17 00:00:00 2001
> > From: Johannes Weiner <hannes@cmpxchg.org>
> > Date: Tue, 3 Mar 2020 15:15:39 -0500
> > Subject: [PATCH] mm: support nesting memalloc_use_memcg()
> >
> > The memalloc_use_memcg() function to override the default memcg
> > accounting context currently doesn't nest. But the patches to make the
> > loop driver cgroup-aware will end up nesting:
> >
> > [   98.137605]  alloc_page_buffers+0x210/0x288
> > [   98.141799]  __getblk_gfp+0x1d4/0x400
> > [   98.145475]  ext4_read_block_bitmap_nowait+0x148/0xbc8
> > [   98.150628]  ext4_mb_init_cache+0x25c/0x9b0
> > [   98.154821]  ext4_mb_init_group+0x270/0x390
> > [   98.159014]  ext4_mb_good_group+0x264/0x270
> > [   98.163208]  ext4_mb_regular_allocator+0x480/0x798
> > [   98.168011]  ext4_mb_new_blocks+0x958/0x10f8
> > [   98.172294]  ext4_ext_map_blocks+0xec8/0x1618
> > [   98.176660]  ext4_map_blocks+0x1b8/0x8a0
> > [   98.180592]  ext4_writepages+0x830/0xf10
> > [   98.184523]  do_writepages+0xb4/0x198
> > [   98.188195]  __filemap_fdatawrite_range+0x170/0x1c8
> > [   98.193086]  filemap_write_and_wait_range+0x40/0xb0
> > [   98.197974]  ext4_punch_hole+0x4a4/0x660
> > [   98.201907]  ext4_fallocate+0x294/0x1190
> > [   98.205839]  loop_process_work+0x690/0x1100
> > [   98.210032]  loop_workfn+0x2c/0x110
> > [   98.213529]  process_one_work+0x3e0/0x648
> > [   98.217546]  worker_thread+0x70/0x670
> > [   98.221217]  kthread+0x1b8/0x1c0
> > [   98.224452]  ret_from_fork+0x10/0x18
> >
> > where loop_process_work() sets the memcg override to the memcg that
> > submitted the IO request, and alloc_page_buffers() sets the override
> > to the memcg that instantiated the cache page, which may differ.
> >
> > Make memalloc_use_memcg() return the old memcg and convert existing
> > users to a stacking model. Delete the unused memalloc_unuse_memcg().
> >
> > Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
> 
> Reviewed-by: Shakeel Butt <shakeelb@google.com>

Thanks Shakeel

> > @@ -316,31 +316,26 @@ static inline void memalloc_nocma_restore(unsigned int flags)
> >   * __GFP_ACCOUNT allocations till the end of the scope will be charged to the
> >   * given memcg.
> >   *
> > - * NOTE: This function is not nesting safe.
> > - */
> > -static inline void memalloc_use_memcg(struct mem_cgroup *memcg)
> > -{
> > -       WARN_ON_ONCE(current->active_memcg);
> > -       current->active_memcg = memcg;
> > -}
> > -
> > -/**
> > - * memalloc_unuse_memcg - Ends the remote memcg charging scope.
> > + * NOTE: This function can nest. Users must save the return value and
> > + * reset the previous value after their own charging scope is over:
> 
> Should we mention that this is still not irq safe? At the moment other
> than skmem we don't do memcg charging in the interrupt and skmem has a
> memcg already associated with it. Maybe in future there might be a
> case where we want a specific memcg be charged for kmem in the
> interrupt context. Until then we can mark that this function should
> not be used in interrupt.

Is it actually unsafe? It's not an RMW operation, being interrupted
doesn't corrupt its state.

I.e. this is safe:

  process:                            interrupt:
  old = current->active_memcg
                                      old = current->active_memcg
                                      current->active_memcg = new
                                      allocate
                                      current->active_memcg = old
  current->active_memcg = new
  return old

This is safe as well:

  process:                            interrupt:
  old = current->active_memcg
  current->active_memcg = new
                                      old = current->active_memcg
                                      current->active_memcg = new
                                      allocate
                                      current->active_memcg = old
  return old