linux-kernel - Re: [PATCH] ext4: Fix deadlock on page reclaim

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Message-ID: <20190725115442.GA15733@infradead.org>
Date:   Thu, 25 Jul 2019 04:54:42 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Damien Le Moal <damien.lemoal@....com>
Cc:     Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, Christoph Hellwig <hch@....de>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Naohiro Aota <naohiro.aota@....com>,
        Masato Suzuki <masato.suzuki@....com>
Subject: Re: [PATCH] ext4: Fix deadlock on page reclaim

On Thu, Jul 25, 2019 at 06:33:58PM +0900, Damien Le Moal wrote:
> +	gfp_t gfp_mask;
> +
>  	switch (ext4_inode_journal_mode(inode)) {
>  	case EXT4_INODE_ORDERED_DATA_MODE:
>  	case EXT4_INODE_WRITEBACK_DATA_MODE:
> @@ -4019,6 +4019,14 @@ void ext4_set_aops(struct inode *inode)
>  		inode->i_mapping->a_ops = &ext4_da_aops;
>  	else
>  		inode->i_mapping->a_ops = &ext4_aops;
> +
> +	/*
> +	 * Ensure all page cache allocations are done from GFP_NOFS context to
> +	 * prevent direct reclaim recursion back into the filesystem and blowing
> +	 * stacks or deadlocking.
> +	 */
> +	gfp_mask = mapping_gfp_mask(inode->i_mapping);
> +	mapping_set_gfp_mask(inode->i_mapping, (gfp_mask & ~(__GFP_FS)));

This looks like something that could hit every file systems, so
shouldn't we fix this in common code?  We could also look into
just using memalloc_nofs_save for the page cache allocation path
instead of the per-mapping gfp_mask.

>  }
>  
>  static int __ext4_block_zero_page_range(handle_t *handle,
> -- 
> 2.21.0
> 
---end quoted text---