Re: [PATCH v2 1/2] mm: vmscan: remove folio_test_private() check in pageout()
From: Baolin Wang
Date: Mon Sep 22 2025 - 02:02:32 EST
On 2025/9/22 13:32, Hugh Dickins wrote:
On Fri, 19 Sep 2025, Baolin Wang wrote:
On 2025/9/19 09:06, Shakeel Butt wrote:...
On Thu, Sep 18, 2025 at 05:36:17PM +0800, Baolin Wang wrote:
On 2025/9/18 14:00, David Hildenbrand wrote:
On 18.09.25 05:46, Baolin Wang wrote:
diff --git a/mm/vmscan.c b/mm/vmscan.c
index f1fc36729ddd..930add6d90ab 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -701,16 +701,10 @@ static pageout_t pageout(struct folio *folio,
struct address_space *mapping,
return PAGE_KEEP;
if (!mapping) {
/*
- * Some data journaling orphaned folios can have
- * folio->mapping == NULL while being dirty with clean buffers.
+ * Is it still possible to have a dirty folio with
+ * a NULL mapping? I think not.
*/
I would rephrase slightly (removing the "I think not"):
/*
* We should no longer have dirty folios with clean buffers and a NULL
* mapping. However, let's be careful for now.
*/
LGTM.
Andrew, could you help squash these comments into this patch? Thanks.
(Myself, I would delete the comment entirely: it's justifying the change,
which is good history to go into the commit message, but not so good in
the final source. And we don't fully understand what to say here anyway.)
- if (folio_test_private(folio)) {
- if (try_to_free_buffers(folio)) {
- folio_clear_dirty(folio);
- pr_info("%s: orphaned folio\n", __func__);
- return PAGE_CLEAN;
- }
- }
+ VM_WARN_ON_FOLIO(true, folio);
Unexpected but better to use VM_WARN_ON_ONCE_FOLIO here.
Um, I don't think it makes much difference, because we should no longer hit
this.
We do hit it. Observe, there was no WARNING before in the !mapping
case, just a pr_info in a particular instance of the !mapping case.
We believe that instance went away with reiserfs, but that does not
imply that there are no more !mapping cases left.
We do hit that WARNING: not often, and I've not seen it repeatedly
(as ONCE-advisors fear), but a few cutdown examples from my testing
appended below.
Thanks for reporting. That surprises me how that happened.
I'm sure you'd like me to explain how it comes about: I did spend a
while looking to do so, but then found better things to do. By all
means go in search of the explanation if it's worth your time: it
might indicate a bug somewhere; but more likely it's just a race
against code elsewhere cleaning up the folio.
Interesting. I'll try to reproduce this issue.
The fact that it does not appear repeatedly suggests that the folio
is successfully dealt with afterwards. I didn't think to check at
first, but in later runs did check back on such folios after, and
verified that they had moved on to being freed and reused, not leaked.
No leaks, good.
The examples I've seen have all been swapbacked, though that may
just reflect my tmpfs swapping load. With mapping NULLed, there's
not much to go on: but index 0x7ff5ce103 is likely to have been
anon, and index 0xcff more likely to have been tmpfs.
My vote is simply to delete the warning (and the comment).
Thanks for your examples and sound reasonable to me.
Andrew, could you help squash the following changes (if you need a new version, please let me know)? Thanks.
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 4907e255857a..aadbee50a851 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -689,16 +689,8 @@ static pageout_t pageout(struct folio *folio, struct address_space *mapping,
* A freeable shmem or swapcache folio is referenced only by the
* caller that isolated the folio and the page cache.
*/
- if (folio_ref_count(folio) != 1 + folio_nr_pages(folio))
+ if (folio_ref_count(folio) != 1 + folio_nr_pages(folio) || !mapping)
return PAGE_KEEP;
- if (!mapping) {
- /*
- * We should no longer have dirty folios with clean buffers and
- * a NULL mapping. However, let's be careful for now.
- */
- VM_WARN_ON_FOLIO(true, folio);
- return PAGE_KEEP;
- }
if (!shmem_mapping(mapping) && !folio_test_anon(folio))
return PAGE_ACTIVATE;