Re: [PATCH v3 2/4] mm/util: introduce snapshot_page()

[Luiz Capitulino]

On 2025-07-16 21:52, Luiz Capitulino wrote:
> On 2025-07-16 18:19, Andrew Morton wrote:
> > On Wed, 16 Jul 2025 20:18:00 +0200 David Hildenbrand <david@xxxxxxxxxx> wrote:
> >
> > > On 16.07.25 19:36, Luiz Capitulino wrote:
> > > > Yes, good catch. This was from an earlier version.
> > > >
> > > > Is it fine if I fix only this with a follow up patch for Andrew in this
> > > > thread or would you prefer that I post v4 with all the other changes as
> > > > well?
> > >
> > > I think the series was part of mm-new, but now I only spot it in
> > > mm-everything, weird. Maybe because of a conflict with the other
> > > stable_page_flags() change?
> >
> > I think I removed v2 from mm-unstable and added v3 to mm-new.  Maybe.
> >
> > > So whatever Andrew prefers, really :)
> >
> > Either is OK at this stage.  I do prefer little fixes so that I and
> > others can see what changed - it depends how tricky and large they are.
>
> I liked some of the other suggestions from David so I'll send v4.

I changed my mind again (sorry). I think the current version should be
good as is except for the doc bug, so here's a patch to fix it on top:

diff --git a/mm/util.c b/mm/util.c
index f270bf42465b..6e5b6ff1c90d 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1198,10 +1198,12 @@ static void set_ps_flags(struct page_snapshot *ps, const struct folio *folio,
  * Create a snapshot of the page and store both its struct page and struct
  * folio representations in @ps.
  *
- * Note that creating a faithful snapshot may fail if the compound
- * state of the page keeps changing (e.g., due to a folio split). In
- * this case, ps->faithful is set to false, and the snapshot assumes
- * that @page refers to a single page.
+ * A snapshot is marked as "faithful" if the compound state of @page was
+ * stable and allowed safe reconstruction of the folio representation. In
+ * rare cases where this is not possible (e.g. due to folio splitting),
+ * snapshot_page() falls back to treating @page as a single page and the
+ * snapshot is marked as "unfaithful". The snapshot_page_is_faithful()
+ * helper can be used to check for this condition.
  */
 void snapshot_page(struct page_snapshot *ps, const struct page *page)
 {