Re: [PATCH] mm/compaction: add check mechanism to avoid cma alloc fail

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jan 22, 2024 at 10:23:17AM +0800, Haiqiang Gong wrote:
> cma alloc may fail when we doing cma alloc/free test on kernel 5.10/5.15.
> 
> We found that the next memory cannot be migrated because of the alloc of
> fs as next backtrace:
> __alloc_pages_nodemask
> pagecache_get_page
> grow_dev_page
> __getblk_gfp
> ext4_sb_breadahead_unmovable
> __ext4_get_inode_loc
> __ext4_iget
> ext4_lookup
> __lookup_slow
> walk_component
> path_lookupat
> filename_lookup
> vfs_statx
> This kind of unmovable memory is not placed in the cma buffer when kernel
> memory alloc but is migrated in by kcompactd when the kernel migration.
> It will cause memory can't be migrate when cma alloc.

I don't understand.  You say that the memory isn't movable, but then you
say that it's migrated in.  So it was movable, but it's no longer
movable after being moved once?  

> Add check mechanism in the compaction_alloc() where kcompaced alloc for
> memory. Will return NULL and give up this memory migration if the
> allocated memory is in the cma buffer and the memory is unmovable.

> +static bool forbid_move_to_cma_range(struct folio *src, struct folio *dst)
> +{
> +	if (folio_mapping(src) && is_in_cma_range(dst))
> +		return true;
> +
> +	return false;
> +}

Why would folio_mapping() be the right way to determine if memory is
unmovable?  The vast majority of filesystem data is movable.