[LSF/MM/BPF TOPIC] Removing GFP_NOFS

[Viacheslav Dubeyko]

> On Jan 5, 2024, at 12:17 AM, Matthew Wilcox <willy at infradead.org> wrote:
> 
> This is primarily a _FILESYSTEM_ track topic.  All the work has already
> been done on the MM side; the FS people need to do their part.  It could
> be a joint session, but I'm not sure there's much for the MM people
> to say.
> 
> There are situations where we need to allocate memory, but cannot call
> into the filesystem to free memory.  Generally this is because we're
> holding a lock or we've started a transaction, and attempting to write
> out dirty folios to reclaim memory would result in a deadlock.
> 
> The old way to solve this problem is to specify GFP_NOFS when allocating
> memory.  This conveys little information about what is being protected
> against, and so it is hard to know when it might be safe to remove.
> It's also a reflex -- many filesystem authors use GFP_NOFS by default
> even when they could use GFP_KERNEL because there's no risk of deadlock.
> 
> The new way is to use the scoped APIs -- memalloc_nofs_save() and
> memalloc_nofs_restore().  These should be called when we start a
> transaction or take a lock that would cause a GFP_KERNEL allocation to
> deadlock.  Then just use GFP_KERNEL as normal.  The memory allocators
> can see the nofs situation is in effect and will not call back into
> the filesystem.
> 
> This results in better code within your filesystem as you don't need to
> pass around gfp flags as much, and can lead to better performance from
> the memory allocators as GFP_NOFS will not be used unnecessarily.
> 
> The memalloc_nofs APIs were introduced in May 2017, but we still have
> over 1000 uses of GFP_NOFS in fs/ today (and 200 outside fs/, which is
> really sad).  This session is for filesystem developers to talk about
> what they need to do to fix up their own filesystem, or share stories
> about how they made their filesystem better by adopting the new APIs.
> 

Many file systems are still heavily using GFP_NOFS for kmalloc and
kmem_cache_alloc family methods even if  memalloc_nofs_save() and
memalloc_nofs_restore() pair is used too. But I can see that GFP_NOFS
is used in radix_tree_preload(), bio_alloc(), posix_acl_clone(),
sb_issue_zeroout, sb_issue_discard(), alloc_inode_sb(), blkdev_issue_zeroout(),
blkdev_issue_secure_erase(), blkdev_zone_mgmt(), etc.

Would it be safe to switch on memalloc_nofs_save()/memalloc_nofs_restore() for
all possible cases? Any potential issues or downsides?

Thanks,
Slava.