[PATCH] mm: fix unproperly folio_put by changing API in read_pages

[PATCH] mm: fix unproperly folio_put by changing API in read_pages

[zhaoyang.huang]

From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>

According to the comments of readahead_page[3], the refcnt which
represents page cache dropped in [1] makes sense for two reasons, '1.
The folio is going to do IO and is locked until IO done;2. The refcnt
will be added back when found again from the page cache and then serve
for PTE or vfs' while it doesn't make sense in [2] as the refcnt of
page cache will be dropped in filemap_remove_folio.

  read_pages()
  {
    aops->readahead
    {
[1]
        folio = readahead_folio();
    }
...
[2]
    while (folio = readahead_folio())
        filemap_remove_folio(folio);
  }

[3]
 * Context: The page is locked and has an elevated refcount.  The caller
 * should decreases the refcount once the page has been submitted for I/O
 * and unlock the page once all I/O to that page has completed.
 * Return: A pointer to the next page, or %NULL if we are done.

Signed-off-by: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
---
 mm/readahead.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index 130c0e7df99f..657736200a92 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -163,7 +163,7 @@ static void read_pages(struct readahead_control *rac)
 		 * may be used to size the next readahead, so make sure
 		 * they accurately reflect what happened.
 		 */
-		while ((folio = readahead_folio(rac)) != NULL) {
+		while ((folio = __readahead_folio(rac)) != NULL) {
 			unsigned long nr = folio_nr_pages(folio);
 
 			folio_get(folio);
-- 
2.25.1

Re: [PATCH] mm: fix unproperly folio_put by changing API in read_pages

[Matthew Wilcox]

On Wed, Mar 27, 2024 at 01:54:06PM +0800, zhaoyang.huang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> 
> According to the comments of readahead_page[3], the refcnt which
> represents page cache dropped in [1] makes sense for two reasons, '1.
> The folio is going to do IO and is locked until IO done;2. The refcnt
> will be added back when found again from the page cache and then serve
> for PTE or vfs' while it doesn't make sense in [2] as the refcnt of
> page cache will be dropped in filemap_remove_folio.

NAK, per previous email.