[PATCH] tmpfs: support loopback

Patch from Hugh Dickins

Added shmem_readpage and shmem_prepare_write so tmpfs files can be used
by the loop driver (together with simple_commit_write).  shmem_getpage
extended to accept file page passed in, which may have to be copied
over from swap page.

Use bdget and sb_set_blocksize so loop can see our preferred blocksize
PAGE_CACHE_SIZE.  Use copy_highpage, removed from highmem.h in 2.4.17:
restore it but with kmap_atomics.  Restore (a simple) copy_page to
asm-sparc64/page.h, which alone of all arches discarded it.

Documentation/filesystems/tmpfs.txt

@@ -47,10 +47,9 @@ tmpfs has the following uses:
    shared memory)
 
 3) Some people (including me) find it very convenient to mount it
-   e.g. on /tmp and /var/tmp and have a big swap partition. But be
-   aware: loop mounts of tmpfs files do not work due to the internal
-   design. So mkinitrd shipped by most distributions will fail with a
-   tmpfs /tmp.
+   e.g. on /tmp and /var/tmp and have a big swap partition. And now
+   loop mounts of tmpfs files do work, so mkinitrd shipped by most
+   distributions should succeed with a tmpfs /tmp.
 
 4) And probably a lot more I do not know about :-)
 
@@ -90,13 +89,9 @@ TODOs:
    size=50% the tmpfs instance should be able to grow to 50 percent of
    RAM + swap. So the instance should adapt automatically if you add
    or remove swap space.
-2) loop mounts: This is difficult since loop.c relies on the readpage
-   operation. This operation gets a page from the caller to be filled
-   with the content of the file at that position. But tmpfs always has
-   the page and thus cannot copy the content to the given page. So it
-   cannot provide this operation. The VM had to be changed seriously
-   to achieve this.
-3) Show the number of tmpfs RAM pages. (As shared?)
+2) Show the number of tmpfs RAM pages. (As shared?)
 
 Author:
    Christoph Rohland <cr@sap.com>, 1.12.01
+Updated:
+   Hugh Dickins <hugh@veritas.com>, 17 Oct 2002

include/asm-sparc64/page.h

@@ -39,6 +39,7 @@ extern void _clear_page(void *page);
 #define clear_page(X)	_clear_page((void *)(X))
 struct page;
 extern void clear_user_page(void *addr, unsigned long vaddr, struct page *page);
+#define copy_page(X,Y)	__memcpy((void *)(X), (void *)(Y), PAGE_SIZE)
 extern void copy_user_page(void *to, void *from, unsigned long vaddr, struct page *topage);
 
 /* GROSS, defining this makes gcc pass these types as aggregates,

include/linux/highmem.h

@@ -73,4 +73,15 @@ static inline void copy_user_highpage(struct page *to, struct page *from, unsign
 	kunmap_atomic(vto, KM_USER1);
 }
 
+static inline void copy_highpage(struct page *to, struct page *from)
+{
+	char *vfrom, *vto;
+
+	vfrom = kmap_atomic(from, KM_USER0);
+	vto = kmap_atomic(to, KM_USER1);
+	copy_page(vto, vfrom);
+	kunmap_atomic(vfrom, KM_USER0);
+	kunmap_atomic(vto, KM_USER1);
+}
+
 #endif /* _LINUX_HIGHMEM_H */