Re: [PATCH] fix mapping_writably_mapped()

[Lee Schermerhorn]

On Wed, 2008-12-10 at 20:48 +0000, Hugh Dickins wrote:
> Lee Schermerhorn noticed yesterday that I broke the mapping_writably_mapped
> test in 2.6.7!  Bad bad bug, good good find.
> 
> The i_mmap_writable count must be incremented for VM_SHARED (just as
> i_writecount is for VM_DENYWRITE, but while holding the i_mmap_lock)
> when dup_mmap() copies the vma for fork: it has its own more optimal
> version of __vma_link_file(), and I missed this out.  So the count
> was later going down to 0 (dangerous) when one end unmapped, then
> wrapping negative (inefficient) when the other end unmapped.
> 
> The only impact on x86 would have been that setting a mandatory lock on
> a file which has at some time been opened O_RDWR and mapped MAP_SHARED
> (but not necessarily PROT_WRITE) across a fork, might fail with -EAGAIN
> when it should succeed, or succeed when it should fail.
> 
> But those architectures which rely on flush_dcache_page() to flush
> userspace modifications back into the page before the kernel reads it,
> may in some cases have skipped the flush after such a fork - though any
> repetitive test will soon wrap the count negative, in which case it will
> flush_dcache_page() unnecessarily.
> 
> Fix would be a two-liner, but mapping variable added, and comment moved.
> 
> Reported-by: Lee Schermerhorn <Lee.Schermerhorn@xxxxxx>
> Signed-off-by: Hugh Dickins <hugh@xxxxxxxxxxx>

I tested the version you sent out earlier atop 28-rc7-mmotm-081208 and
it appears to be working as expected.  Not very stressful testing, tho'.
Just a few ad hoc shared file mmap()ing, fork()ing, unmap()ing, ... with
some printk()s to verify the values of i_mmap_writable.

Lee

> ---
> 
>  kernel/fork.c |   15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> --- 2.6.28-rc7/kernel/fork.c	2008-11-15 23:09:30.000000000 +0000
> +++ linux/kernel/fork.c	2008-12-10 12:49:13.000000000 +0000
> @@ -315,17 +315,20 @@ static int dup_mmap(struct mm_struct *mm
>  		file = tmp->vm_file;
>  		if (file) {
>  			struct inode *inode = file->f_path.dentry->d_inode;
> +			struct address_space *mapping = file->f_mapping;
> +
>  			get_file(file);
>  			if (tmp->vm_flags & VM_DENYWRITE)
>  				atomic_dec(&inode->i_writecount);
> -
> -			/* insert tmp into the share list, just after mpnt */
> -			spin_lock(&file->f_mapping->i_mmap_lock);
> +			spin_lock(&mapping->i_mmap_lock);
> +			if (tmp->vm_flags & VM_SHARED)
> +				mapping->i_mmap_writable++;
>  			tmp->vm_truncate_count = mpnt->vm_truncate_count;
> -			flush_dcache_mmap_lock(file->f_mapping);
> +			flush_dcache_mmap_lock(mapping);
> +			/* insert tmp into the share list, just after mpnt */
>  			vma_prio_tree_add(tmp, mpnt);
> -			flush_dcache_mmap_unlock(file->f_mapping);
> -			spin_unlock(&file->f_mapping->i_mmap_lock);
> +			flush_dcache_mmap_unlock(mapping);
> +			spin_unlock(&mapping->i_mmap_lock);
>  		}
>  
>  		/*

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