Re: [PATCH v4 2/5] mm/vmscan: extract folio_free() from shrink_folio_list()

["David Hildenbrand (Arm)" <david@kernel.org>]

On 5/25/26 16:57, Zhang Peng wrote:
> shrink_folio_list() contains a self-contained folio-freeing section:
> buffer release, lazyfree, __remove_mapping, and folio_batch drain.
> Extract it into folio_free() to reduce the size of shrink_folio_list()
> and make the freeing step independently readable.
> 
> No functional change.

As you are touching the code, I'll suggest some simple improvements as part of
the move.

> 
> Signed-off-by: Zhang Peng <bruzzhang@tencent.com>
> ---
>  mm/vmscan.c | 168 +++++++++++++++++++++++++++++++++---------------------------
>  1 file changed, 92 insertions(+), 76 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 886d8b4843aa..b31f67801836 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1072,6 +1072,95 @@ static void folio_activate_locked(struct folio *folio,
>  	}
>  }
>  
> +static bool folio_free(struct folio *folio, struct folio_batch *free_folios,

I don't quite like the function name, as it sounds like something very generic,
when it's really specific to memory reclaim code and does things like removing
the folio from the pagecache.

Any way we can easily make that clearer? I think it should contain something
about release and reclaim.

folio_release_for_reclaim() or just "folio_reclaim()" ? Not sure.



> +		struct scan_control *sc, struct reclaim_stat *stat,
> +		unsigned int *nr_reclaimed)

Instead of returning a bool, can we just return 0 / -EBUSY? IMHO, a bool mostly
only makes sense if the function name implies some sort of yes/no test. (there
are exceptions).

> +{
> +	unsigned int nr_pages = folio_nr_pages(folio);

Can be const.

> +	struct address_space *mapping = folio_mapping(folio);
> +
> +	/*
> +	 * If the folio has buffers, try to free the buffer
> +	 * mappings associated with this folio. If we succeed
> +	 * we try to free the folio as well.
> +	 *
> +	 * We do this even if the folio is dirty.
> +	 * filemap_release_folio() does not perform I/O, but it
> +	 * is possible for a folio to have the dirty flag set,
> +	 * but it is actually clean (all its buffers are clean).
> +	 * This happens if the buffers were written out directly,
> +	 * with submit_bh(). ext3 will do this, as well as
> +	 * the blockdev mapping.  filemap_release_folio() will
> +	 * discover that cleanness and will drop the buffers
> +	 * and mark the folio clean - it can be freed.
> +	 *
> +	 * Rarely, folios can have buffers and no ->mapping.
> +	 * These are the folios which were not successfully
> +	 * invalidated in truncate_cleanup_folio().  We try to
> +	 * drop those buffers here and if that worked, and the
> +	 * folio is no longer mapped into process address space
> +	 * (refcount == 1) it can be freed.  Otherwise, leave
> +	 * the folio on the LRU so it is swappable.
> +	 */

I assume you can now adjust the comments to use more space and less LOC.

> +	if (folio_needs_release(folio)) {
> +		if (!filemap_release_folio(folio, sc->gfp_mask)) {
> +			folio_activate_locked(folio, stat, nr_pages);
> +			return false;
> +		}
> +
> +		if (!mapping && folio_ref_count(folio) == 1) {
> +			folio_unlock(folio);
> +			if (folio_put_testzero(folio))
> +				goto free_it;
> +			else {> +				/*
> +				 * rare race with speculative reference.
> +				 * the speculative reference will free
> +				 * this folio shortly, so we may
> +				 * increment nr_reclaimed here (and
> +				 * leave it off the LRU).
> +				 */
> +				*nr_reclaimed += nr_pages;
> +				return true;
> +			}
> +		}
> +	}
> +
> +	if (folio_test_lazyfree(folio)) {
> +		/* follow __remove_mapping for reference */
> +		if (!folio_ref_freeze(folio, 1))
> +			return false;
> +		/*
> +		 * The folio has only one reference left, which is
> +		 * from the isolation. After the caller puts the
> +		 * folio back on the lru and drops the reference, the
> +		 * folio will be freed anyway. It doesn't matter
> +		 * which lru it goes on. So we don't bother checking
> +		 * the dirty flag here.
> +		 */
> +		count_vm_events(PGLAZYFREED, nr_pages);
> +		count_memcg_folio_events(folio, PGLAZYFREED, nr_pages);
> +	} else if (!mapping || !__remove_mapping(mapping, folio, true,
> +							sc->target_mem_cgroup))
> +		return false;
> +
> +	folio_unlock(folio);
> +free_it:

Okay, at this point we should have a refcount of 0. We could make that
more obvious through

VM_WARN_ON_FOLIO_ONCE(folio_ref_count(folio), folio);

> +	/*
> +	 * Folio may get swapped out as a whole, need to account
> +	 * all pages in it.
> +	 */

Likely can shorten to

/* Only full folios get reclaimed. */

Or just drop the comment completely, I don't quite see the point.

> +	*nr_reclaimed += nr_pages;
> +
> +	folio_unqueue_deferred_split(folio);
> +	if (folio_batch_add(free_folios, folio) == 0) {
> +		mem_cgroup_uncharge_folios(free_folios);
> +		try_to_unmap_flush();
> +		free_unref_folios(free_folios);
> +	}
> +	return true;
> +}
> +
>  /*
>   * shrink_folio_list() returns the number of reclaimed pages
>   */
> @@ -1459,83 +1548,10 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
>  			}
>  		}
>  
> -		/*
> -		 * If the folio has buffers, try to free the buffer
> -		 * mappings associated with this folio. If we succeed
> -		 * we try to free the folio as well.
> -		 *
> -		 * We do this even if the folio is dirty.
> -		 * filemap_release_folio() does not perform I/O, but it
> -		 * is possible for a folio to have the dirty flag set,
> -		 * but it is actually clean (all its buffers are clean).
> -		 * This happens if the buffers were written out directly,
> -		 * with submit_bh(). ext3 will do this, as well as
> -		 * the blockdev mapping.  filemap_release_folio() will
> -		 * discover that cleanness and will drop the buffers
> -		 * and mark the folio clean - it can be freed.
> -		 *
> -		 * Rarely, folios can have buffers and no ->mapping.
> -		 * These are the folios which were not successfully
> -		 * invalidated in truncate_cleanup_folio().  We try to
> -		 * drop those buffers here and if that worked, and the
> -		 * folio is no longer mapped into process address space
> -		 * (refcount == 1) it can be freed.  Otherwise, leave
> -		 * the folio on the LRU so it is swappable.
> -		 */
> -		if (folio_needs_release(folio)) {
> -			if (!filemap_release_folio(folio, sc->gfp_mask))
> -				goto activate_locked;
> -			if (!mapping && folio_ref_count(folio) == 1) {
> -				folio_unlock(folio);
> -				if (folio_put_testzero(folio))
> -					goto free_it;
> -				else {
> -					/*
> -					 * rare race with speculative reference.
> -					 * the speculative reference will free
> -					 * this folio shortly, so we may
> -					 * increment nr_reclaimed here (and
> -					 * leave it off the LRU).
> -					 */
> -					nr_reclaimed += nr_pages;
> -					continue;
> -				}
> -			}
> -		}
> -
> -		if (folio_test_lazyfree(folio)) {
> -			/* follow __remove_mapping for reference */
> -			if (!folio_ref_freeze(folio, 1))
> -				goto keep_locked;
> -			/*
> -			 * The folio has only one reference left, which is
> -			 * from the isolation. After the caller puts the
> -			 * folio back on the lru and drops the reference, the
> -			 * folio will be freed anyway. It doesn't matter
> -			 * which lru it goes on. So we don't bother checking
> -			 * the dirty flag here.
> -			 */
> -			count_vm_events(PGLAZYFREED, nr_pages);
> -			count_memcg_folio_events(folio, PGLAZYFREED, nr_pages);
> -		} else if (!mapping || !__remove_mapping(mapping, folio, true,
> -							 sc->target_mem_cgroup))
> +		if (!folio_free(folio, &free_folios, sc, stat, &nr_reclaimed))
>  			goto keep_locked;
> -
> -		folio_unlock(folio);
> -free_it:
> -		/*
> -		 * Folio may get swapped out as a whole, need to account
> -		 * all pages in it.
> -		 */
> -		nr_reclaimed += nr_pages;
> -
> -		folio_unqueue_deferred_split(folio);
> -		if (folio_batch_add(&free_folios, folio) == 0) {
> -			mem_cgroup_uncharge_folios(&free_folios);
> -			try_to_unmap_flush();
> -			free_unref_folios(&free_folios);
> -		}
> -		continue;
> +		else
> +			continue;

You can just drop the "else", no?

if (...)
	goto keep_locked;
continue;

>  
>  activate_locked_split:
>  		/*
> 


-- 
Cheers,

David