Re: [PATCH] mm: fix missing wake-up event for FSDAX pages

[Matthew Wilcox]

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);