Re: [PATCH V4 2/2] jfs: implement migrate_folio for jfs_metapage_aops

[David Hildenbrand <david@redhat.com>]

On 22.04.25 13:40, Shivank Garg wrote:
> Add the missing migrate_folio operation to jfs_metapage_aops to fix
> warnings during memory compaction. These warnings were introduced by
> commit 7ee3647243e5 ("migrate: Remove call to ->writepage") which
> added explicit warnings when filesystems don't implement migrate_folio.
> 
> System reports following warnings:
>    jfs_metapage_aops does not implement migrate_folio
>    WARNING: CPU: 0 PID: 6870 at mm/migrate.c:955 fallback_migrate_folio mm/migrate.c:953 [inline]
>    WARNING: CPU: 0 PID: 6870 at mm/migrate.c:955 move_to_new_folio+0x70e/0x840 mm/migrate.c:1007
> 
> Implement metapage_migrate_folio which handles both single and multiple
> metapages per page configurations.
> 
> Fixes: 35474d52c605 ("jfs: Convert metapage_writepage to metapage_write_folio")
> Reported-by: syzbot+8bb6fd945af4e0ad9299@syzkaller.appspotmail.com
> Closes: https://lore.kernel.org/all/67faff52.050a0220.379d84.001b.GAE@google.com
> Signed-off-by: Shivank Garg <shivankg@amd.com>
> ---
>   fs/jfs/jfs_metapage.c | 94 +++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 94 insertions(+)
> 
> diff --git a/fs/jfs/jfs_metapage.c b/fs/jfs/jfs_metapage.c
> index df575a873ec6..a12fbd92cc69 100644
> --- a/fs/jfs/jfs_metapage.c
> +++ b/fs/jfs/jfs_metapage.c
> @@ -15,6 +15,7 @@
>   #include <linux/mempool.h>
>   #include <linux/seq_file.h>
>   #include <linux/writeback.h>
> +#include <linux/migrate.h>
>   #include "jfs_incore.h"
>   #include "jfs_superblock.h"
>   #include "jfs_filsys.h"
> @@ -151,6 +152,54 @@ static inline void dec_io(struct folio *folio, blk_status_t status,
>   		handler(folio, anchor->status);
>   }
>   
> +static int __metapage_migrate_folio(struct address_space *mapping, struct folio *dst,
> +				    struct folio *src, enum migrate_mode mode)
> +{
> +	struct meta_anchor *src_anchor = src->private;
> +	struct metapage *mps[MPS_PER_PAGE] = {0};
> +	struct metapage *mp;
> +	int i, rc;
> +
> +	for (i = 0; i < MPS_PER_PAGE; i++) {
> +		mp = src_anchor->mp[i];
> +		if (mp && metapage_locked(mp))
> +			return -EAGAIN;
> +	}
> +
> +	rc = filemap_migrate_folio(mapping, dst, src, mode);
> +	if (rc != MIGRATEPAGE_SUCCESS)
> +		return rc;
> +
> +	for (i = 0; i < MPS_PER_PAGE; i++) {
> +		mp = src_anchor->mp[i];
> +		if (!mp)
> +			continue;
> +		if (unlikely(insert_metapage(dst, mp))) {
> +			/* If error, roll-back previosly inserted pages */
> +			for (int j = 0 ; j < i; j++) {
> +				if (mps[j])
> +					remove_metapage(dst, mps[j]);
> +			}
> +			return -EAGAIN;
> +		}
> +		mps[i] = mp;
> +	}
> +
> +	/* Update the metapage and remove it from src */
> +	for (i = 0; i < MPS_PER_PAGE; i++) {
> +		mp = mps[i];
> +		if (mp) {
> +			int page_offset = mp->data - folio_address(src);
> +
> +			mp->data = folio_address(dst) + page_offset;
> +			mp->folio = dst;
> +			remove_metapage(src, mp);
> +		}
> +	}
> +
> +	return MIGRATEPAGE_SUCCESS;
> +}
> +
>   #else
>   static inline struct metapage *folio_to_mp(struct folio *folio, int offset)
>   {
> @@ -175,6 +224,32 @@ static inline void remove_metapage(struct folio *folio, struct metapage *mp)
>   #define inc_io(folio) do {} while(0)
>   #define dec_io(folio, status, handler) handler(folio, status)
>   
> +static int __metapage_migrate_folio(struct address_space *mapping, struct folio *dst,
> +				    struct folio *src, enum migrate_mode mode)
> +{
> +	struct metapage *mp;
> +	int page_offset;
> +	int rc;
> +
> +	mp = folio_to_mp(src, 0);
> +	if (mp && metapage_locked(mp))
> +		return -EAGAIN;
> +
> +	rc = filemap_migrate_folio(mapping, dst, src, mode);
> +	if (rc != MIGRATEPAGE_SUCCESS)
> +		return rc;
> +
> +	if (unlikely(insert_metapage(dst, mp)))
> +		return -EAGAIN;
> +
> +	page_offset = mp->data - folio_address(src);
> +	mp->data = folio_address(dst) + page_offset;
> +	mp->folio = dst;
> +	remove_metapage(src, mp);
> +
> +	return MIGRATEPAGE_SUCCESS;
> +}
> +
>   #endif
>   
>   static inline struct metapage *alloc_metapage(gfp_t gfp_mask)
> @@ -554,6 +629,24 @@ static bool metapage_release_folio(struct folio *folio, gfp_t gfp_mask)
>   	return ret;
>   }
>   
> +/**
> + * metapage_migrate_folio - Migration function for JFS metapages
> + */
> +static int metapage_migrate_folio(struct address_space *mapping, struct folio *dst,
> +				  struct folio *src, enum migrate_mode mode)
> +{
> +	int expected_count;
> +
> +	if (!src->private)
> +		return filemap_migrate_folio(mapping, dst, src, mode);
> +
> +	/* Check whether page does not have extra refs before we do more work */
> +	expected_count = folio_migration_expected_refs(mapping, src);
> +	if (folio_ref_count(src) != expected_count)

Probably no need for the temporary variable.

Hm, makes me wonder if it should be called 
folio_migration_expected_ref_count() ... :)

Bit it's even longer, whatever you think is best.

-- 
Cheers,

David / dhildenb