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SubjectRe: [PATCH] mm: fix missing wake-up event for FSDAX pages
On Mon, Jul 04, 2022 at 03:40:54PM +0800, Muchun Song wrote:
> FSDAX page refcounts are 1-based, rather than 0-based: if refcount is
> 1, then the page is freed. The FSDAX pages can be pinned through GUP,
> then they will be unpinned via unpin_user_page() using a folio variant
> to put the page, however, folio variants did not consider this special
> case, the result will be to miss a wakeup event (like the user of
> __fuse_dax_break_layouts()).

Argh, no. The 1-based refcounts are a blight on the entire kernel.
They need to go away, not be pushed into folios as well. I think
we're close to having that fixed, but until then, this should do
the trick?

diff --git a/include/linux/mm.h b/include/linux/mm.h
index cc98ab012a9b..4cef5e0f78b6 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1129,18 +1129,18 @@ static inline bool is_zone_movable_page(const struct page *page)
#if defined(CONFIG_ZONE_DEVICE) && defined(CONFIG_FS_DAX)
DECLARE_STATIC_KEY_FALSE(devmap_managed_key);

-bool __put_devmap_managed_page(struct page *page);
-static inline bool put_devmap_managed_page(struct page *page)
+bool __put_devmap_managed_page(struct page *page, int refs);
+static inline bool put_devmap_managed_page(struct page *page, int refs)
{
if (!static_branch_unlikely(&devmap_managed_key))
return false;
if (!is_zone_device_page(page))
return false;
- return __put_devmap_managed_page(page);
+ return __put_devmap_managed_page(page, refs);
}

#else /* CONFIG_ZONE_DEVICE && CONFIG_FS_DAX */
-static inline bool put_devmap_managed_page(struct page *page)
+static inline bool put_devmap_managed_page(struct page *page, int refs)
{
return false;
}
@@ -1246,7 +1246,7 @@ static inline void put_page(struct page *page)
* For some devmap managed pages we need to catch refcount transition
* from 2 to 1:
*/
- if (put_devmap_managed_page(&folio->page))
+ if (put_devmap_managed_page(&folio->page, 1))
return;
folio_put(folio);
}
diff --git a/mm/gup.c b/mm/gup.c
index d1132b39aa8f..28df02121c78 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -88,7 +88,8 @@ static inline struct folio *try_get_folio(struct page *page, int refs)
* belongs to this folio.
*/
if (unlikely(page_folio(page) != folio)) {
- folio_put_refs(folio, refs);
+ if (!put_devmap_managed_page(&folio->page, refs))
+ folio_put_refs(folio, refs);
goto retry;
}

@@ -177,6 +178,8 @@ static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
refs *= GUP_PIN_COUNTING_BIAS;
}

+ if (put_devmap_managed_page(&folio->page, refs))
+ return;
folio_put_refs(folio, refs);
}

diff --git a/mm/memremap.c b/mm/memremap.c
index b870a659eee6..b25e40e3a11e 100644
--- a/mm/memremap.c
+++ b/mm/memremap.c
@@ -499,7 +499,7 @@ void free_zone_device_page(struct page *page)
}

#ifdef CONFIG_FS_DAX
-bool __put_devmap_managed_page(struct page *page)
+bool __put_devmap_managed_page(struct page *page, int refs)
{
if (page->pgmap->type != MEMORY_DEVICE_FS_DAX)
return false;
@@ -509,7 +509,7 @@ bool __put_devmap_managed_page(struct page *page)
* refcount is 1, then the page is free and the refcount is
* stable because nobody holds a reference on the page.
*/
- if (page_ref_dec_return(page) == 1)
+ if (page_ref_sub_return(page, refs) == 1)
wake_up_var(&page->_refcount);
return true;
}
diff --git a/mm/swap.c b/mm/swap.c
index c6194cfa2af6..94e42a9bab92 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -960,7 +960,7 @@ void release_pages(struct page **pages, int nr)
unlock_page_lruvec_irqrestore(lruvec, flags);
lruvec = NULL;
}
- if (put_devmap_managed_page(&folio->page))
+ if (put_devmap_managed_page(&folio->page, 1))
continue;
if (folio_put_testzero(folio))
free_zone_device_page(&folio->page);
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