[PATCH] tmpfs preempt count panic

Just unearthed another of my warcrimes: reading a 17-page sparse file,
I mean holey file, hits the in_interrupt panic in do_exit on a current
highmem kernel (but 2.6.7 is okay).  Fix mismatched preempt count from
shmem_swp_alloc's swapindex hole case by mapping an empty_zero_page.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/shmem.c

@@ -337,7 +337,6 @@ static swp_entry_t *shmem_swp_alloc(struct shmem_inode_info *info, unsigned long
 	struct shmem_sb_info *sbinfo = SHMEM_SB(inode->i_sb);
 	struct page *page = NULL;
 	swp_entry_t *entry;
-	static const swp_entry_t unswapped = { 0 };
 
 	if (sgp != SGP_WRITE &&
 	    ((loff_t) index << PAGE_CACHE_SHIFT) >= i_size_read(inode))
@@ -345,7 +344,7 @@ static swp_entry_t *shmem_swp_alloc(struct shmem_inode_info *info, unsigned long
 
 	while (!(entry = shmem_swp_entry(info, index, &page))) {
 		if (sgp == SGP_READ)
-			return (swp_entry_t *) &unswapped;
+			return shmem_swp_map(ZERO_PAGE(0));
 		/*
 		 * Test free_blocks against 1 not 0, since we have 1 data
 		 * page (and perhaps indirect index pages) yet to allocate: