When cramfs_physmem is used then we have the opportunity to map files
directly from ROM, directly into user space, saving on RAM usage.
This gives us Execute-In-Place (XIP) support.
For a file to be mmap()-able, the map area has to correspond to a range
of uncompressed and contiguous blocks, and in the MMU case it also has
to be page aligned. A version of mkcramfs with appropriate support is
necessary to create such a filesystem image.
In the MMU case it may happen for a vma structure to extend beyond the
actual file size. This is notably the case in binfmt_elf.c:elf_map().
Or the file's last block is shared with other files and cannot be mapped
as is. Rather than refusing to mmap it, we do a partial map and set up a
special vm_ops fault handler that splits the vma in two: the direct mapping
vma and the memory-backed vma populated by the readpage method.
In the non-MMU case it is the get_unmapped_area method that is responsible
for providing the address where the actual data can be found. No mapping
is necessary of course.
Signed-off-by: Nicolas Pitre <nico@xxxxxxxxxx>
---
fs/cramfs/inode.c | 270 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 270 insertions(+)
diff --git a/fs/cramfs/inode.c b/fs/cramfs/inode.c
index b825ae162c..e3884c607b 100644
--- a/fs/cramfs/inode.c
+++ b/fs/cramfs/inode.c
@@ -16,6 +16,7 @@
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/pagemap.h>
+#include <linux/ramfs.h>
#include <linux/init.h>
#include <linux/string.h>
#include <linux/blkdev.h>
@@ -49,6 +50,7 @@ static inline struct cramfs_sb_info *CRAMFS_SB(struct super_block *sb)
static const struct super_operations cramfs_ops;
static const struct inode_operations cramfs_dir_inode_operations;
static const struct file_operations cramfs_directory_operations;
+static const struct file_operations cramfs_physmem_fops;
static const struct address_space_operations cramfs_aops;
static DEFINE_MUTEX(read_mutex);
@@ -96,6 +98,10 @@ static struct inode *get_cramfs_inode(struct super_block *sb,
case S_IFREG:
inode->i_fop = &generic_ro_fops;
inode->i_data.a_ops = &cramfs_aops;
+ if (IS_ENABLED(CONFIG_CRAMFS_PHYSMEM) &&
+ CRAMFS_SB(sb)->flags & CRAMFS_FLAG_EXT_BLOCK_POINTERS &&
+ CRAMFS_SB(sb)->linear_phys_addr)
+ inode->i_fop = &cramfs_physmem_fops;
break;
case S_IFDIR:
inode->i_op = &cramfs_dir_inode_operations;
@@ -277,6 +283,270 @@ static void *cramfs_read(struct super_block *sb, unsigned int offset,
return NULL;
}
+/*
+ * For a mapping to be possible, we need a range of uncompressed and
+ * contiguous blocks. Return the offset for the first block and number of
+ * valid blocks for which that is true, or zero otherwise.
+ */
+static u32 cramfs_get_block_range(struct inode *inode, u32 pgoff, u32 *pages)
+{
+ struct super_block *sb = inode->i_sb;
+ struct cramfs_sb_info *sbi = CRAMFS_SB(sb);
+ int i;
+ u32 *blockptrs, blockaddr;
+
+ /*
+ * We can dereference memory directly here as this code may be
+ * reached only when there is a direct filesystem image mapping
+ * available in memory.
+ */
+ blockptrs = (u32 *)(sbi->linear_virt_addr + OFFSET(inode) + pgoff*4);
+ blockaddr = blockptrs[0] & ~CRAMFS_BLK_FLAGS;
+ i = 0;
+ do {
+ u32 expect = blockaddr + i * (PAGE_SIZE >> 2);
+ expect |= CRAMFS_BLK_FLAG_DIRECT_PTR|CRAMFS_BLK_FLAG_UNCOMPRESSED;
+ if (blockptrs[i] != expect) {
+ pr_debug("range: block %d/%d got %#x expects %#x\n",
+ pgoff+i, pgoff+*pages-1, blockptrs[i], expect);
+ if (i == 0)
+ return 0;
+ break;
+ }
+ } while (++i < *pages);
+
+ *pages = i;
+
+ /* stored "direct" block ptrs are shifted down by 2 bits */
+ return blockaddr << 2;
+}
+
+/*
+ * It is possible for cramfs_physmem_mmap() to partially populate the mapping
+ * causing page faults in the unmapped area. When that happens, we need to
+ * split the vma so that the unmapped area gets its own vma that can be backed
+ * with actual memory pages and loaded normally. This is necessary because
+ * remap_pfn_range() overwrites vma->vm_pgoff with the pfn and filemap_fault()
+ * no longer works with it. Furthermore this makes /proc/x/maps right.
+ * Q: is there a way to do split vma at mmap() time?
+ */
+static const struct vm_operations_struct cramfs_vmasplit_ops;
+static int cramfs_vmasplit_fault(struct vm_fault *vmf)
+{
+ struct mm_struct *mm = vmf->vma->vm_mm;
+ struct vm_area_struct *vma, *new_vma;
+ unsigned long split_val, split_addr;
+ unsigned int split_pgoff, split_page;
+ int ret;
+
+ /* Retrieve the vma split address and validate it */
+ vma = vmf->vma;
+ split_val = (unsigned long)vma->vm_private_data;
+ split_pgoff = split_val & 0xffff;
+ split_page = split_val >> 16;
+ split_addr = vma->vm_start + split_page * PAGE_SIZE;
+ pr_debug("fault: addr=%#lx vma=%#lx-%#lx split=%#lx\n",
+ vmf->address, vma->vm_start, vma->vm_end, split_addr);
+ if (!split_val || split_addr >= vma->vm_end || vmf->address < split_addr)
+ return VM_FAULT_SIGSEGV;
+
+ /* We have some vma surgery to do and need the write lock. */
+ up_read(&mm->mmap_sem);
+ if (down_write_killable(&mm->mmap_sem))
+ return VM_FAULT_RETRY;
+
+ /* Make sure the vma didn't change between the locks */
+ vma = find_vma(mm, vmf->address);
+ if (vma->vm_ops != &cramfs_vmasplit_ops) {
+ /*
+ * Someone else raced with us and could have handled the fault.
+ * Let it go back to user space and fault again if necessary.
+ */
+ downgrade_write(&mm->mmap_sem);
+ return VM_FAULT_NOPAGE;
+ }
+
+ /* Split the vma between the directly mapped area and the rest */
+ ret = split_vma(mm, vma, split_addr, 0);
+ if (ret) {
+ downgrade_write(&mm->mmap_sem);
+ return VM_FAULT_OOM;
+ }
+
+ /* The direct vma should no longer ever fault */
+ vma->vm_ops = NULL;
+
+ /* Retrieve the new vma covering the unmapped area */
+ new_vma = find_vma(mm, split_addr);
+ BUG_ON(new_vma == vma);
+ if (!new_vma) {
+ downgrade_write(&mm->mmap_sem);
+ return VM_FAULT_SIGSEGV;
+ }
+
+ /*
+ * Readjust the new vma with the actual file based pgoff and
+ * process the fault normally on it.
+ */
+ new_vma->vm_pgoff = split_pgoff;
+ new_vma->vm_ops = &generic_file_vm_ops;
+ vmf->vma = new_vma;
+ vmf->pgoff = split_pgoff;
+ vmf->pgoff += (vmf->address - new_vma->vm_start) >> PAGE_SHIFT;
+ downgrade_write(&mm->mmap_sem);
+ return filemap_fault(vmf);
+}
+
+static const struct vm_operations_struct cramfs_vmasplit_ops = {
+ .fault = cramfs_vmasplit_fault,
+};
+
+static int cramfs_physmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+ struct inode *inode = file_inode(file);
+ struct super_block *sb = inode->i_sb;
+ struct cramfs_sb_info *sbi = CRAMFS_SB(sb);
+ unsigned int pages, vma_pages, max_pages, offset;
+ unsigned long address;
+ char *fail_reason;
+ int ret;
+
+ if (!IS_ENABLED(CONFIG_MMU))
+ return vma->vm_flags & (VM_SHARED | VM_MAYSHARE) ? 0 : -ENOSYS;
+
+ if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_MAYWRITE))
+ return -EINVAL;
+
+ /* Could COW work here? */
+ fail_reason = "vma is writable";
+ if (vma->vm_flags & VM_WRITE)
+ goto fail;
+
+ vma_pages = (vma->vm_end - vma->vm_start + PAGE_SIZE - 1) >> PAGE_SHIFT;
+ max_pages = (inode->i_size + PAGE_SIZE - 1) >> PAGE_SHIFT;
+ fail_reason = "beyond file limit";
+ if (vma->vm_pgoff >= max_pages)
+ goto fail;
+ pages = vma_pages;
+ if (pages > max_pages - vma->vm_pgoff)
+ pages = max_pages - vma->vm_pgoff;
+
+ offset = cramfs_get_block_range(inode, vma->vm_pgoff, &pages);
+ fail_reason = "unsuitable block layout";
+ if (!offset)
+ goto fail;
+ address = sbi->linear_phys_addr + offset;
+ fail_reason = "data is not page aligned";
+ if (!PAGE_ALIGNED(address))
+ goto fail;
+
+ /* Don't map the last page if it contains some other data */
+ if (unlikely(vma->vm_pgoff + pages == max_pages)) {
+ unsigned int partial = offset_in_page(inode->i_size);
+ if (partial) {
+ char *data = sbi->linear_virt_addr + offset;
+ data += (max_pages - 1) * PAGE_SIZE + partial;
+ while ((unsigned long)data & 7)
+ if (*data++ != 0)
+ goto nonzero;
+ while (offset_in_page(data)) {
+ if (*(u64 *)data != 0) {
+ nonzero:
+ pr_debug("mmap: %s: last page is shared\n",
+ file_dentry(file)->d_name.name);
+ pages--;
+ break;
+ }
+ data += 8;
+ }
+ }
+ }
+
+ if (pages) {
+ /*
+ * If we can't map it all, page faults will occur if the
+ * unmapped area is accessed. Let's handle them to split the
+ * vma and let the normal paging machinery take care of the
+ * rest through cramfs_readpage(). Because remap_pfn_range()
+ * repurposes vma->vm_pgoff, we have to save it somewhere.
+ * Let's use vma->vm_private_data to hold both the pgoff and the actual address split point.
+ * Maximum file size is 16MB so we can pack both together.
+ */
+ if (pages != vma_pages) {
+ unsigned int split_pgoff = vma->vm_pgoff + pages;
+ unsigned long split_val = split_pgoff + (pages << 16);
+ vma->vm_private_data = (void *)split_val;
+ vma->vm_ops = &cramfs_vmasplit_ops;
+ /* to keep remap_pfn_range() happy */
+ vma->vm_end = vma->vm_start + pages * PAGE_SIZE;
+ }
+
+ ret = remap_pfn_range(vma, vma->vm_start, address >> PAGE_SHIFT,
+ pages * PAGE_SIZE, vma->vm_page_prot);
+ /* restore vm_end in case we cheated it above */
+ vma->vm_end = vma->vm_start + vma_pages * PAGE_SIZE;
+ if (ret)
+ return ret;
+
+ pr_debug("mapped %s at 0x%08lx (%u/%u pages) to vma 0x%08lx, "
+ "page_prot 0x%llx\n", file_dentry(file)->d_name.name,
+ address, pages, vma_pages, vma->vm_start,
+ (unsigned long long)pgprot_val(vma->vm_page_prot));
+ return 0;
+ }
+ fail_reason = "no suitable block remaining";
+
+fail:
+ pr_debug("%s: direct mmap failed: %s\n",
+ file_dentry(file)->d_name.name, fail_reason);
+
+ /* We failed to do a direct map, but normal paging will do it */
+ vma->vm_ops = &generic_file_vm_ops;
+ return 0;
+}
+
+#ifndef CONFIG_MMU
+
+static unsigned long cramfs_physmem_get_unmapped_area(struct file *file,
+ unsigned long addr, unsigned long len,
+ unsigned long pgoff, unsigned long flags)
+{
+ struct inode *inode = file_inode(file);
+ struct super_block *sb = inode->i_sb;
+ struct cramfs_sb_info *sbi = CRAMFS_SB(sb);
+ unsigned int pages, block_pages, max_pages, offset;
+
+ pages = (len + PAGE_SIZE - 1) >> PAGE_SHIFT;
+ max_pages = (inode->i_size + PAGE_SIZE - 1) >> PAGE_SHIFT;
+ if (pgoff >= max_pages || pages > max_pages - pgoff)
+ return -EINVAL;
+ block_pages = pages;
+ offset = cramfs_get_block_range(inode, pgoff, &block_pages);
+ if (!offset || block_pages != pages)
+ return -ENOSYS;
+ addr = sbi->linear_phys_addr + offset;
+ pr_debug("get_unmapped for %s ofs %#lx siz %lu at 0x%08lx\n",
+ file_dentry(file)->d_name.name, pgoff*PAGE_SIZE, len, addr);
+ return addr;
+}
+
+static unsigned cramfs_physmem_mmap_capabilities(struct file *file)
+{
+ return NOMMU_MAP_COPY | NOMMU_MAP_DIRECT | NOMMU_MAP_READ | NOMMU_MAP_EXEC;
+}
+#endif
+
+static const struct file_operations cramfs_physmem_fops = {
+ .llseek = generic_file_llseek,
+ .read_iter = generic_file_read_iter,
+ .splice_read = generic_file_splice_read,
+ .mmap = cramfs_physmem_mmap,
+#ifndef CONFIG_MMU
+ .get_unmapped_area = cramfs_physmem_get_unmapped_area,
+ .mmap_capabilities = cramfs_physmem_mmap_capabilities,
+#endif
+};
+
static void cramfs_blkdev_kill_sb(struct super_block *sb)
{
struct cramfs_sb_info *sbi = CRAMFS_SB(sb);
--
2.9.5
