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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