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SubjectRe: [PATCH v5 4/5] cramfs: add mmap support
On Fri, 6 Oct 2017, Christoph Hellwig wrote:

> > + /* Don't map the last page if it contains some other data */
> > + if (unlikely(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;
> > + if (memchr_inv(data, 0, PAGE_SIZE - partial) != NULL) {
> > + pr_debug("mmap: %s: last page is shared\n",
> > + file_dentry(file)->d_name.name);
> > + pages--;
> > + }
> > + }
> > + }
>
> Why is pgoff + pages == max_pages marked unlikely? Mapping the whole
> file seems like a perfectly normal and likely case to me..

In practice it is not. This is typically used for executables where a
mmap() call covers the executable and read-only segments located at the
beginning of a file. The end of the file is covered by a r/w mapping
where .data is normally located and we can't direct-mmap that.

Whatever. I have no strong opinion here as this is hardly
performance critical so it is removed.

> Also if this was my code I'd really prefer to move this into a helper:
>
> static bool cramfs_mmap_last_page_is_shared(struct inode *inode, int offset)
> {
> unsigned int partial = offset_in_page(inode->i_size);
> char *data = CRAMFS_SB(inode->i_sb)->linear_virt_addr + offset +
> (inode->i_size & PAGE_MASK);
>
> return memchr_inv(data + partial, 0, PAGE_SIZE - partial);
> }
>
> if (pgoff + pages == max_pages && offset_in_page(inode->i_size) &&
> cramfs_mmap_last_page_is_shared(inode, offset))
> pages--;
>
> as that's much more readable and the function name provides a good
> documentation of what is going on.

OK. The above got some details wrong but the idea is clear. Please see
the new patch bleow.

> > + if (pages != vma_pages(vma)) {
>
> here is how I would turn this around:
>
> if (!pages)
> goto done;
>
> if (pages == vma_pages(vma)) {
> remap_pfn_range();
> goto done;
> }
>
> ...
> for (i = 0; i < pages; i++) {
> ...
> vm_insert_mixed();
> nr_mapped++;
> }
>
>
> done:
> pr_debug("mapped %d out ouf %d\n", ..);
> if (pages != vma_pages(vma))
> vma->vm_ops = &generic_file_vm_ops;
> return 0;
> }
>
> In fact we probably could just set the vm_ops unconditionally, they
> just wouldn't be called, but that might be more confusing then helpful.

Well... For one thing generic_file_vm_ops is not exported to modules so
now I simply call generic_file_readonly_mmap() at the beginning and let
it validate the vma flags. That allowed for some simplifications at the
end.

Personally I think the if-else form is clearer over an additional goto
label, but I moved the more common case first as you did above. At this
point I hope you'll indulge me.

Here's the latest version of patch 4/5:

diff --git a/fs/cramfs/inode.c b/fs/cramfs/inode.c
index 6aa1d94ed8..e1b9192f23 100644
--- a/fs/cramfs/inode.c
+++ b/fs/cramfs/inode.c
@@ -15,7 +15,10 @@

#include <linux/module.h>
#include <linux/fs.h>
+#include <linux/file.h>
#include <linux/pagemap.h>
+#include <linux/pfn_t.h>
+#include <linux/ramfs.h>
#include <linux/init.h>
#include <linux/string.h>
#include <linux/blkdev.h>
@@ -49,6 +52,7 @@ 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 +100,10 @@ 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 +285,207 @@ 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 cramfs_sb_info *sbi = CRAMFS_SB(inode->i_sb);
+ int i;
+ u32 *blockptrs, first_block_addr;
+
+ /*
+ * 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);
+ first_block_addr = blockptrs[0] & ~CRAMFS_BLK_FLAGS;
+ i = 0;
+ do {
+ u32 block_off = i * (PAGE_SIZE >> CRAMFS_BLK_DIRECT_PTR_SHIFT);
+ u32 expect = (first_block_addr + block_off) |
+ 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;
+ return first_block_addr << CRAMFS_BLK_DIRECT_PTR_SHIFT;
+}
+
+#ifdef CONFIG_MMU
+
+/*
+ * Return true if the last page of a file in the filesystem image contains
+ * some other data that doesn't belong to that file. It is assumed that the
+ * last block is CRAMFS_BLK_FLAG_DIRECT_PTR | CRAMFS_BLK_FLAG_UNCOMPRESSED
+ * (verified by cramfs_get_block_range() and directly accessible in memory.
+ */
+static bool cramfs_last_page_is_shared(struct inode *inode)
+{
+ struct cramfs_sb_info *sbi = CRAMFS_SB(inode->i_sb);
+ u32 partial, last_page, blockaddr, *blockptrs;
+ char *tail_data;
+
+ partial = offset_in_page(inode->i_size);
+ if (!partial)
+ return false;
+ last_page = inode->i_size >> PAGE_SHIFT;
+ blockptrs = (u32 *)(sbi->linear_virt_addr + OFFSET(inode));
+ blockaddr = blockptrs[last_page] & ~CRAMFS_BLK_FLAGS;
+ blockaddr <<= CRAMFS_BLK_DIRECT_PTR_SHIFT;
+ tail_data = sbi->linear_virt_addr + blockaddr + partial;
+ return memchr_inv(tail_data, 0, PAGE_SIZE - partial) ? true : false;
+}
+
+static int cramfs_physmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+ struct inode *inode = file_inode(file);
+ struct cramfs_sb_info *sbi = CRAMFS_SB(inode->i_sb);
+ unsigned int pages, max_pages, offset;
+ unsigned long address, pgoff = vma->vm_pgoff;
+ char *bailout_reason;
+ int ret;
+
+ ret = generic_file_readonly_mmap(file, vma);
+ if (ret)
+ return ret;
+
+ /*
+ * Now try to pre-populate ptes for this vma with a direct
+ * mapping avoiding memory allocation when possible.
+ */
+
+ /* Could COW work here? */
+ bailout_reason = "vma is writable";
+ if (vma->vm_flags & VM_WRITE)
+ goto bailout;
+
+ max_pages = (inode->i_size + PAGE_SIZE - 1) >> PAGE_SHIFT;
+ bailout_reason = "beyond file limit";
+ if (pgoff >= max_pages)
+ goto bailout;
+ pages = min(vma_pages(vma), max_pages - pgoff);
+
+ offset = cramfs_get_block_range(inode, pgoff, &pages);
+ bailout_reason = "unsuitable block layout";
+ if (!offset)
+ goto bailout;
+ address = sbi->linear_phys_addr + offset;
+ bailout_reason = "data is not page aligned";
+ if (!PAGE_ALIGNED(address))
+ goto bailout;
+
+ /* Don't map the last page if it contains some other data */
+ if (pgoff + pages == max_pages && cramfs_last_page_is_shared(inode)) {
+ pr_debug("mmap: %s: last page is shared\n",
+ file_dentry(file)->d_name.name);
+ pages--;
+ }
+
+ if (!pages) {
+ bailout_reason = "no suitable block remaining";
+ goto bailout;
+ }
+
+ if (pages == vma_pages(vma)) {
+ /*
+ * The entire vma is mappable. remap_pfn_range() will
+ * make it distinguishable from a non-direct mapping
+ * in /proc/<pid>/maps by substituting the file offset
+ * with the actual physical address.
+ */
+ ret = remap_pfn_range(vma, vma->vm_start, address >> PAGE_SHIFT,
+ pages * PAGE_SIZE, vma->vm_page_prot);
+ } else {
+ /*
+ * Let's create a mixed map if we can't map it all.
+ * The normal paging machinery will take care of the
+ * unpopulated ptes via cramfs_readpage().
+ */
+ int i;
+ vma->vm_flags |= VM_MIXEDMAP;
+ for (i = 0; i < pages && !ret; i++) {
+ unsigned long off = i * PAGE_SIZE;
+ pfn_t pfn = phys_to_pfn_t(address + off, PFN_DEV);
+ ret = vm_insert_mixed(vma, vma->vm_start + off, pfn);
+ }
+ }
+
+ if (!ret)
+ pr_debug("mapped %s[%lu] at 0x%08lx (%u/%lu pages) "
+ "to vma 0x%08lx, page_prot 0x%llx\n",
+ file_dentry(file)->d_name.name, pgoff,
+ address, pages, vma_pages(vma), vma->vm_start,
+ (unsigned long long)pgprot_val(vma->vm_page_prot));
+ return ret;
+
+bailout:
+ pr_debug("%s[%lu]: direct mmap impossible: %s\n",
+ file_dentry(file)->d_name.name, pgoff, bailout_reason);
+ /* Didn't manage any direct map, but normal paging is still possible */
+ return 0;
+}
+
+#else /* CONFIG_MMU */
+
+static int cramfs_physmem_mmap(struct file *file, struct vm_area_struct *vma)
+{
+ return vma->vm_flags & (VM_SHARED | VM_MAYSHARE) ? 0 : -ENOSYS;
+}
+
+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 int cramfs_physmem_mmap_capabilities(struct file *file)
+{
+ return NOMMU_MAP_COPY | NOMMU_MAP_DIRECT |
+ NOMMU_MAP_READ | NOMMU_MAP_EXEC;
+}
+
+#endif /* CONFIG_MMU */
+
+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);
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