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

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Date:   Fri, 10 Apr 2020 09:12:18 -0700
From:   Roman Gushchin <guro@...com>
To:     "Theodore Y. Ts'o" <tytso@....edu>
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 Thu, Apr 09, 2020 at 11:23:44PM -0400, Theodore Y. Ts'o wrote:
> 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

Thank you!

Roman