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

[Roman Gushchin <guro@fb.com>]

On Sat, Feb 29, 2020 at 12:49:13AM -0700, Andreas Dilger wrote:
> On Feb 28, 2020, at 5:14 PM, Roman Gushchin <guro@fb.com> 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.
> > 
> > v2: found a similar issue in __ext4_get_inode_loc()
> > 
> > Fixes: 85c8f176a611 ("ext4: preload block group descriptors")
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> 
> Reviewed-by: Andreas Dilger <adilger@dilger.ca>

Hello, Theodore!

Can you, please, pick this patch?

We've some changes on the mm side (more actively using a cma area for movable
allocations), which might bring a regression without this ext4 change.

Thank you!

Roman