mm: add FGP_ENTRY

The functionality of find_lock_entry() and find_get_entry() can be
provided by pagecache_get_page(), which lets us delete find_lock_entry()
and make find_get_entry() static.

Link: https://lkml.kernel.org/r/20201112212641.27837-5-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Dave Chinner <dchinner@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: William Kucharski <william.kucharski@oracle.com>
Cc: Yang Shi <yang.shi@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/pagemap.h

@@ -315,6 +315,7 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping,
 #define FGP_NOWAIT		0x00000020
 #define FGP_FOR_MMAP		0x00000040
 #define FGP_HEAD		0x00000080
+#define FGP_ENTRY		0x00000100
 
 struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset,
 		int fgp_flags, gfp_t cache_gfp_mask);

mm/filemap.c

@@ -1658,7 +1658,7 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping,
 }
 EXPORT_SYMBOL(page_cache_prev_miss);
 
-/**
+/*
  * find_get_entry - find and get a page cache entry
  * @mapping: the address_space to search
  * @index: The page cache index.
@@ -1671,7 +1671,7 @@ EXPORT_SYMBOL(page_cache_prev_miss);
  *
  * Return: The head page or shadow entry, %NULL if nothing is found.
  */
-struct page *find_get_entry(struct address_space *mapping, pgoff_t index)
+static struct page *find_get_entry(struct address_space *mapping, pgoff_t index)
 {
 	XA_STATE(xas, &mapping->i_pages, index);
 	struct page *page;
@@ -1707,39 +1707,6 @@ out:
 	return page;
 }
 
-/**
- * find_lock_entry - Locate and lock a page cache entry.
- * @mapping: The address_space to search.
- * @index: The page cache index.
- *
- * Looks up the page at @mapping & @index.  If there is a page in the
- * cache, the head page is returned locked and with an increased refcount.
- *
- * If the slot holds a shadow entry of a previously evicted page, or a
- * swap entry from shmem/tmpfs, it is returned.
- *
- * Context: May sleep.
- * Return: The head page or shadow entry, %NULL if nothing is found.
- */
-struct page *find_lock_entry(struct address_space *mapping, pgoff_t index)
-{
-	struct page *page;
-
-repeat:
-	page = find_get_entry(mapping, index);
-	if (page && !xa_is_value(page)) {
-		lock_page(page);
-		/* Has the page been truncated? */
-		if (unlikely(page->mapping != mapping)) {
-			unlock_page(page);
-			put_page(page);
-			goto repeat;
-		}
-		VM_BUG_ON_PAGE(!thp_contains(page, index), page);
-	}
-	return page;
-}
-
 /**
  * pagecache_get_page - Find and get a reference to a page.
  * @mapping: The address_space to search.
@@ -1755,6 +1722,8 @@ repeat:
  * * %FGP_LOCK - The page is returned locked.
  * * %FGP_HEAD - If the page is present and a THP, return the head page
  *   rather than the exact page specified by the index.
+ * * %FGP_ENTRY - If there is a shadow / swap / DAX entry, return it
+ *   instead of allocating a new page to replace it.
  * * %FGP_CREAT - If no page is present then a new page is allocated using
  *   @gfp_mask and added to the page cache and the VM's LRU list.
  *   The page is returned locked and with an increased refcount.
@@ -1779,8 +1748,11 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t index,
 
 repeat:
 	page = find_get_entry(mapping, index);
-	if (xa_is_value(page))
+	if (xa_is_value(page)) {
+		if (fgp_flags & FGP_ENTRY)
+			return page;
 		page = NULL;
+	}
 	if (!page)
 		goto no_page;
 

mm/internal.h

@@ -60,9 +60,6 @@ static inline void force_page_cache_readahead(struct address_space *mapping,
 	force_page_cache_ra(&ractl, &file->f_ra, nr_to_read);
 }
 
-struct page *find_get_entry(struct address_space *mapping, pgoff_t index);
-struct page *find_lock_entry(struct address_space *mapping, pgoff_t index);
-
 /**
  * page_evictable - test whether a page is evictable
  * @page: the page to test

mm/shmem.c

@@ -1812,7 +1812,8 @@ repeat:
 	sbinfo = SHMEM_SB(inode->i_sb);
 	charge_mm = vma ? vma->vm_mm : current->mm;
 
-	page = find_lock_entry(mapping, index);
+	page = pagecache_get_page(mapping, index,
+					FGP_ENTRY | FGP_HEAD | FGP_LOCK, 0);
 	if (xa_is_value(page)) {
 		error = shmem_swapin_page(inode, index, &page,
 					  sgp, gfp, vma, fault_type);

mm/swap_state.c

@@ -403,7 +403,8 @@ struct page *find_get_incore_page(struct address_space *mapping, pgoff_t index)
 {
 	swp_entry_t swp;
 	struct swap_info_struct *si;
-	struct page *page = find_get_entry(mapping, index);
+	struct page *page = pagecache_get_page(mapping, index,
+						FGP_ENTRY | FGP_HEAD, 0);
 
 	if (!page)
 		return page;