linux-ext4 - Re: [PATCH v2] ext4: use non-movable memory for superblock readahead

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Date:   Thu, 9 Apr 2020 23:23:44 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Roman Gushchin <guro@...com>
Cc:     linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Andrew Perepechko <andrew.perepechko@...gate.com>,
        Gioh Kim <gioh.kim@....com>, Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v2] ext4: use non-movable memory for superblock readahead

On Fri, Feb 28, 2020 at 04:14:11PM -0800, Roman Gushchin wrote:
> Since commit a8ac900b8163 ("ext4: use non-movable memory for the
> superblock") buffers for ext4 superblock were allocated using
> the sb_bread_unmovable() helper which allocated buffer heads
> out of non-movable memory blocks. It was necessarily to not block
> page migrations and do not cause cma allocation failures.
> 
> However commit 85c8f176a611 ("ext4: preload block group descriptors")
> broke this by introducing pre-reading of the ext4 superblock.
> The problem is that __breadahead() is using __getblk() underneath,
> which allocates buffer heads out of movable memory.
> 
> It resulted in page migration failures I've seen on a machine
> with an ext4 partition and a preallocated cma area.
> 
> Fix this by introducing sb_breadahead_unmovable() and
> __breadahead_gfp() helpers which use non-movable memory for buffer
> head allocations and use them for the ext4 superblock readahead.

Applied, thanks.  Apologies for not picking this up earlier.

	 	  	    	    	    - Ted