The nommu do_mmap expects f_op->get_unmapped_area to either succeed or
return -ENOSYS for VM_MAYSHARE (e.g. private read-only) mappings.
Returning addr in the non-MAP_SHARED case was completely wrong, and
only happened to work because addr was 0. However, it prevented
VM_MAYSHARE mappings from sharing backing with the fs cache, and
forced such mappings (including shareable program text) to be copied
whenever the number of mappings transitioned from 0 to 1, impacting
performance and memory usage. Subsequent mappings beyond the first
still correctly shared memory with the first.
Instead, treat VM_MAYSHARE identically to VM_SHARED at the file ops
level; do_mmap already handles the semantic differences between them.
Signed-off-by: Rich Felker <dal...@libc.org>
---
fs/ramfs/file-nommu.c | 8 ++------
1 file changed, 2 insertions(+), 6 deletions(-)
diff --git a/fs/ramfs/file-nommu.c b/fs/ramfs/file-nommu.c
index a586467..be3ddd1 100644
--- a/fs/ramfs/file-nommu.c
+++ b/fs/ramfs/file-nommu.c
@@ -211,14 +211,11 @@ static unsigned long ramfs_nommu_get_unmapped_area(struct
file *file,
struct page **pages = NULL, **ptr, *page;
loff_t isize;
- if (!(flags & MAP_SHARED))
- return addr;
-
/* the mapping mustn't extend beyond the EOF */
lpages = (len + PAGE_SIZE - 1) >> PAGE_SHIFT;
isize = i_size_read(inode);
- ret = -EINVAL;
+ ret = -ENOSYS;
maxpages = (isize + PAGE_SIZE - 1) >> PAGE_SHIFT;
if (pgoff >= maxpages)
goto out;
@@ -227,7 +224,6 @@ static unsigned long ramfs_nommu_get_unmapped_area(struct
file *file,
goto out;
/* gang-find the pages */
- ret = -ENOMEM;
pages = kcalloc(lpages, sizeof(struct page *), GFP_KERNEL);
if (!pages)
goto out_free;
@@ -263,7 +259,7 @@ out:
*/
static int ramfs_nommu_mmap(struct file *file, struct vm_area_struct *vma)
{
- if (!(vma->vm_flags & VM_SHARED))
+ if (!(vma->vm_flags & (VM_SHARED | VM_MAYSHARE)))
return -ENOSYS;
file_accessed(file);
--
2.7.0
