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

[Andreas Dilger]

On Feb 21, 2020, at 12:20 PM, Roman Gushchin <guro@xxxxxx> 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.
> 
> Fixes: 85c8f176a611 ("ext4: preload block group descriptors")
> Signed-off-by: Roman Gushchin <guro@xxxxxx>

Makes sense.

Reviewed-by: Andreas Dilger <adilger@xxxxxxxxx>

> Cc: Andrew Perepechko <andrew.perepechko@xxxxxxxxxxx>
> Cc: Theodore Ts'o <tytso@xxxxxxx>
> Cc: Gioh Kim <gioh.kim@xxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> ---
> fs/buffer.c                 | 11 +++++++++++
> fs/ext4/super.c             |  2 +-
> include/linux/buffer_head.h |  8 ++++++++
> 3 files changed, 20 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 4299e100a05b..25462edd920e 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -1414,6 +1414,17 @@ void __breadahead(struct block_device *bdev, sector_t block, unsigned size)
> }
> EXPORT_SYMBOL(__breadahead);
> 
> +void __breadahead_gfp(struct block_device *bdev, sector_t block, unsigned size,
> +		      gfp_t gfp)
> +{
> +	struct buffer_head *bh = __getblk_gfp(bdev, block, size, gfp);
> +	if (likely(bh)) {
> +		ll_rw_block(REQ_OP_READ, REQ_RAHEAD, 1, &bh);
> +		brelse(bh);
> +	}
> +}
> +EXPORT_SYMBOL(__breadahead_gfp);
> +
> /**
>  *  __bread_gfp() - reads a specified block and returns the bh
>  *  @bdev: the block_device to read from
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 3a401f930bca..6a10f7d44719 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -4321,7 +4321,7 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
> 	/* Pre-read the descriptors into the buffer cache */
> 	for (i = 0; i < db_count; i++) {
> 		block = descriptor_loc(sb, logical_sb_block, i);
> -		sb_breadahead(sb, block);
> +		sb_breadahead_unmovable(sb, block);
> 	}
> 
> 	for (i = 0; i < db_count; i++) {
> diff --git a/include/linux/buffer_head.h b/include/linux/buffer_head.h
> index 7b73ef7f902d..b56cc825f64d 100644
> --- a/include/linux/buffer_head.h
> +++ b/include/linux/buffer_head.h
> @@ -189,6 +189,8 @@ struct buffer_head *__getblk_gfp(struct block_device *bdev, sector_t block,
> void __brelse(struct buffer_head *);
> void __bforget(struct buffer_head *);
> void __breadahead(struct block_device *, sector_t block, unsigned int size);
> +void __breadahead_gfp(struct block_device *, sector_t block, unsigned int size,
> +		  gfp_t gfp);
> struct buffer_head *__bread_gfp(struct block_device *,
> 				sector_t block, unsigned size, gfp_t gfp);
> void invalidate_bh_lrus(void);
> @@ -319,6 +321,12 @@ sb_breadahead(struct super_block *sb, sector_t block)
> 	__breadahead(sb->s_bdev, block, sb->s_blocksize);
> }
> 
> +static inline void
> +sb_breadahead_unmovable(struct super_block *sb, sector_t block)
> +{
> +	__breadahead_gfp(sb->s_bdev, block, sb->s_blocksize, 0);
> +}
> +
> static inline struct buffer_head *
> sb_getblk(struct super_block *sb, sector_t block)
> {
> --
> 2.24.1
> 


Cheers, Andreas

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