Re: [patch 2/2] only allow nonlinear vmas for ram backed filesystems

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Thu, 2007-04-05 at 11:30 +0200, Miklos Szeredi wrote:
> From: Miklos Szeredi <mszeredi@suse.cz>
> 
> page_mkclean() doesn't re-protect ptes for non-linear mappings, so a
> later re-dirty through such a mapping will not generate a fault,
> PG_dirty will not reflect the dirty state and the dirty count will be
> skewed.  This implies that msync() is also currently broken for
> nonlinear mappings.
> 
> Peter Zijlstra writes:
> > In order to make page_mkclean() work for nonlinear vmas we need to do a
> > full pte scan for each invocation (we could perhaps only scan 1 in n
> > times to try and limit the damage) and that hurts. This will basically
> > render it useless.
> > 
> > The other solution is adding rmap information to nonlinear vmas but
> > doubling the memory overhead for nonlinear mappings was not deemed a
> > good idea.
> 
> The easiest solution is to emulate remap_file_pages on non-linear
> mappings with simple mmap() for non ram-backed filesystems.
> Applications continue to work (albeit slower), as long as the number
> of remappings remain below the maximum vma count.
> 
> However all currently known real uses of non-linear mappings are for
> ram backed filesystems, which this patch doesn't affect.
> 
> William Lee Irwin III writes:
> > It's used for > 3GB files on tmpfs and also ramfs, sometimes
> > substantially larger than 3GB.
> > 
> > It's not used for the database proper. It's used for the buffer pool,
> > which is the in-core destination and source of direct I/O, the on-disk
> > source and destination of the I/O being the database.
> 
> Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
> 
> Index: linux/mm/fremap.c
> ===================================================================
> --- linux.orig/mm/fremap.c	2007-04-05 11:18:21.000000000 +0200
> +++ linux/mm/fremap.c	2007-04-05 11:18:25.000000000 +0200
> @@ -181,6 +181,24 @@ asmlinkage long sys_remap_file_pages(uns
>  			goto retry;
>  		}
>  		mapping = vma->vm_file->f_mapping;
> +		/*
> +		 * page_mkclean doesn't work on nonlinear vmas, so if dirty
> +		 * pages need to be accounted, emulate with linear vmas.
> +		 */
> +		if (mapping_cap_account_dirty(mapping)) {

Perhaps this should read:

		if (vma_wants_writenotify(vma)) {

That way we would even allow read only non-linear mappings of 'real'
filesystem files.

> +			unsigned long addr;
> +
> +			flags &= MAP_NONBLOCK;
> +			addr = mmap_region(vma->vm_file, start, size, flags,
> +					   vma->vm_flags, pgoff, 1);
> +			if (IS_ERR_VALUE(addr))
> +				err = addr;
> +			else {
> +				BUG_ON(addr != start);
> +				err = 0;
> +			}
> +			goto out;
> +		}
>  		spin_lock(&mapping->i_mmap_lock);
>  		flush_dcache_mmap_lock(mapping);
>  		vma->vm_flags |= VM_NONLINEAR;

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