Re: New bug in patch and existing Linux code - race with install_page() (was: Re: [PATCH] 2.6.14 patch for supporting madvise(MADV_REMOVE))

[Badari Pulavarty <pbadari@us.ibm.com>]

On Wed, 2005-11-02 at 20:54 +0100, Blaisorblade wrote:
> On Wednesday 02 November 2005 17:12, Badari Pulavarty wrote:
> > Hi Andrew & Andrea,
> >
> > Here is the updated patch with name change again :(
> > Hopefully this would be final. (MADV_REMOVE).
> >
> > BTW, I am not sure if we need to hold i_sem and i_allocsem
> > all the way ? I wanted to be safe - but this may be overkill ?
> While looking into this, I probably found another problem, a race with 
> install_page(), which doesn't use the seqlock-style check we use for 
> everything else (aka do_no_page) but simply assumes a page is valid if its 
> index is below the current file size.
> 
> This is clearly "truncate" specific, and is already racy. Suppose I truncate a 
> file and reduce its size, and then re-extend it, the page which I previously 
> fetched from the cache is invalid. The current install_page code generates 
> corruption.
> 
> In fact the page is fetched from the caller of install_page and passed to it.
> 
> This affects anybody using MAP_POPULATE or using remap_file_pages.
> 
> > +       /* XXX - Do we need both i_sem and i_allocsem all the way ? */
> > +       down(&inode->i_sem);
> > +       down_write(&inode->i_alloc_sem);
> > +       unmap_mapping_range(mapping, offset, (end - offset), 1);
> In my opinion, as already said, unmap_mapping_range can be called without 
> these two locks, as it operates only on mappings for the file.
> 
> However currently it's called with these locks held in vmtruncate, but I think 
> the locks are held in that case only because we need to truncate the file, 
> and are hold in excess also across this call.

I agree, I can push down the locking only for ->truncate_range - if
no one has objections. (But again, it so special case - no one really
cares about the performance of this interface ?).

Thanks,
Badari

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