Re: [PATCH 2/3] xfs: add kmem_alloc_io()

[Vlastimil Babka <vbabka@suse.cz>]

On 8/22/19 2:07 PM, Dave Chinner wrote:
> On Thu, Aug 22, 2019 at 01:14:30PM +0200, Vlastimil Babka wrote:
> 
> No, the problem is this (using kmalloc as a general term for
> allocation, whether it be kmalloc, kmem_cache_alloc, alloc_page, etc)
> 
>    some random kernel code
>     kmalloc(GFP_KERNEL)
>      reclaim
>      PF_MEMALLOC
>      shrink_slab
>       xfs_inode_shrink
>        XFS_ILOCK
>         xfs_buf_allocate_memory()
>          kmalloc(GFP_KERNEL)
> 
> And so locks on inodes in reclaim are seen below reclaim. Then
> somewhere else we have:
> 
>    some high level read-only xfs code like readdir
>     XFS_ILOCK
>      xfs_buf_allocate_memory()
>       kmalloc(GFP_KERNEL)
>        reclaim
> 
> And this one throws false positive lockdep warnings because we
> called into reclaim with XFS_ILOCK held and GFP_KERNEL alloc

OK, and what exactly makes this positive a false one? Why can't it continue like
the first example where reclaim leads to another XFS_ILOCK, thus deadlock?

> context. So the only solution we had at the tiem to shut it up was:
> 
>    some high level read-only xfs code like readdir
>     XFS_ILOCK
>      xfs_buf_allocate_memory()
>       kmalloc(GFP_NOFS)
> 
> So that lockdep sees it's not going to recurse into reclaim and
> doesn't throw a warning...

AFAICS that GFP_NOFS would fix not only a warning but also a real deadlock
(depending on the answer to my previous question).

> Cheers,
> 
> Dave.
>