Re: [PATCH v4] Btrfs: fix deadlock with memory reclaim during scrub

[David Sterba <dsterba@suse.cz>]

On Fri, Nov 23, 2018 at 06:25:40PM +0000, fdmanana@kernel.org wrote:
> From: Filipe Manana <fdmanana@suse.com>
> 
> When a transaction commit starts, it attempts to pause scrub and it blocks
> until the scrub is paused. So while the transaction is blocked waiting for
> scrub to pause, we can not do memory allocation with GFP_KERNEL from scrub,
> otherwise we risk getting into a deadlock with reclaim.
> 
> Checking for scrub pause requests is done early at the beginning of the
> while loop of scrub_stripe() and later in the loop, scrub_extent() and
> scrub_raid56_parity() are called, which in turn call scrub_pages() and
> scrub_pages_for_parity() respectively. These last two functions do memory
> allocations using GFP_KERNEL. Same problem could happen while scrubbing
> the super blocks, since it calls scrub_pages().
> 
> So make sure GFP_NOFS is used for the memory allocations because at any
> time a scrub pause request can happen from another task that started to
> commit a transaction.
> 
> Fixes: 58c4e173847a ("btrfs: scrub: use GFP_KERNEL on the submission path")
> Signed-off-by: Filipe Manana <fdmanana@suse.com>
> ---
> 
> V2: Make using GFP_NOFS unconditionial. Previous version was racy, as pausing
> requests migth happen just after we checked for them.
> 
> V3: Use memalloc_nofs_save() just like V1 did.
> 
> V4: Similar problem happened for raid56, which was previously missed, so
>     deal with it as well as the case for scrub_supers().

Enclosing the whole scrub to 'nofs' seems like the best option and
future proof. What I missed in 58c4e173847a was the "don't hold big lock
under GFP_KERNEL allocation" pattern.

>  fs/btrfs/scrub.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/fs/btrfs/scrub.c b/fs/btrfs/scrub.c
> index 3be1456b5116..e08b7502d1f0 100644
> --- a/fs/btrfs/scrub.c
> +++ b/fs/btrfs/scrub.c
> @@ -3779,6 +3779,7 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
>  	struct scrub_ctx *sctx;
>  	int ret;
>  	struct btrfs_device *dev;
> +	unsigned int nofs_flag;
>  
>  	if (btrfs_fs_closing(fs_info))
>  		return -EINVAL;
> @@ -3882,6 +3883,16 @@ int btrfs_scrub_dev(struct btrfs_fs_info *fs_info, u64 devid, u64 start,
>  	atomic_inc(&fs_info->scrubs_running);
>  	mutex_unlock(&fs_info->scrub_lock);
>  
> +	/*
> +	 * In order to avoid deadlock with reclaim when there is a transaction
> +	 * trying to pause scrub, make sure we use GFP_NOFS for all the
> +	 * allocations done at btrfs_scrub_pages() and scrub_pages_for_parity()
> +	 * invoked by our callees. The pausing request is done when the
> +	 * transaction commit starts, and it blocks the transaction until scrub
> +	 * is paused (done at specific points at scrub_stripe() or right above
> +	 * before incrementing fs_info->scrubs_running).

This hilights why there's perhaps no point in trying to make the nofs
section smaller, handling all the interactions between scrub and
transaction would be too complex.

Reviewed-by: David Sterba <dsterba@suse.com>