Re: [linux-next:master 7893/8235] fs/jfs/jfs_metapage.c:245 __metapage_migrate_folio() error: we previously assumed 'mp' could be null (see line 235)

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 7 May 2025 17:19:52 +0530 Shivank Garg <shivankg@amd.com> wrote:

> This is a false positive.
> 
> In metapage_migrate_folio(), it checks if (!src->private) and only calls
> __metapage_migrate_folio() if src->private is non-NULL
> 
> static int metapage_migrate_folio(struct address_space *mapping, struct folio *dst,
> 				  struct folio *src, enum migrate_mode mode)
> {
> ...
> 	if (!src->private)
> 		return filemap_migrate_folio(mapping, dst, src, mode);
> ...
> ...
> 	return __metapage_migrate_folio(mapping, dst, src, mode);
> }
> 
> Then __metapage_migrate_folio() calls mp = folio_to_mp(src, 0), which returns src->private.
> Which should not be NULL as previously checked.
> 
> I think we can skip the mp != NULL check from 
> > 39ed4d1a0e03ce Shivank Garg  2025-04-30 @235  	if (mp && metapage_locked(mp))
> It seem redundant here.

Yes.  Right now the code simply looks wrong:

	mp = folio_to_mp(src, 0);
	if (mp && metapage_locked(mp))
		return -EAGAIN;

	...

	page_offset = mp->data - folio_address(src);

any reader of this will be worried.  A code comment (at least) would
save them concern and time.  Reworking the code to eliminate doubt
would be better.