Re: [PATCH 2/2] Updating ctime and mtime at syncing

Peter Zijlstra
2008-01-15T19:30:35Z

On Tue, 2008-01-15 at 20:18 +0300, Anton Salikhmetov wrote:
> 2008/1/15, Peter Zijlstra <a.p.zijlstra@chello.nl>:
> >
> > On Tue, 2008-01-15 at 19:02 +0300, Anton Salikhmetov wrote:
> >
> > > diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> > > index 3d3848f..53d0e34 100644
> > > --- a/mm/page-writeback.c
> > > +++ b/mm/page-writeback.c
> > > @@ -997,35 +997,39 @@ int __set_page_dirty_no_writeback(struct page *page)
> > > */
> > > int __set_page_dirty_nobuffers(struct page *page)
> > > {
> > > - if (!TestSetPageDirty(page)) {
> > > - struct address_space *mapping = page_mapping(page);
> > > - struct address_space *mapping2;
> > > + struct address_space *mapping = page_mapping(page);
> > > + struct address_space *mapping2;
> > >
> > > - if (!mapping)
> > > - return 1;
> > > + if (!mapping)
> > > + return 1;
> > >
> > > - write_lock_irq(&mapping->tree_lock);
> > > - mapping2 = page_mapping(page);
> > > - if (mapping2) { /* Race with truncate? */
> > > - BUG_ON(mapping2 != mapping);
> > > - WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));
> > > - if (mapping_cap_account_dirty(mapping)) {
> > > - __inc_zone_page_state(page, NR_FILE_DIRTY);
> > > - __inc_bdi_stat(mapping->backing_dev_info,
> > > - BDI_RECLAIMABLE);
> > > - task_io_account_write(PAGE_CACHE_SIZE);
> > > - }
> > > - radix_tree_tag_set(&mapping->page_tree,
> > > - page_index(page), PAGECACHE_TAG_DIRTY);
> > > - }
> > > - write_unlock_irq(&mapping->tree_lock);
> > > - if (mapping->host) {
> > > - /* !PageAnon && !swapper_space */
> > > - __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
> > > + mapping->mtime = CURRENT_TIME;
> > > + set_bit(AS_MCTIME, &mapping->flags);
> >
> > This seems vulnerable to the race we have against truncate, handled by
> > the mapping2 magic below. Do we care?
> >
> > > +
> > > + if (TestSetPageDirty(page))
> > > + return 0;
> > > +
> > > + write_lock_irq(&mapping->tree_lock);
> > > + mapping2 = page_mapping(page);
> > > + if (mapping2) {
> > > + /* Race with truncate? */
> > > + BUG_ON(mapping2 != mapping);
> > > + WARN_ON_ONCE(!PagePrivate(page) && !PageUptodate(page));
> > > + if (mapping_cap_account_dirty(mapping)) {
> > > + __inc_zone_page_state(page, NR_FILE_DIRTY);
> > > + __inc_bdi_stat(mapping->backing_dev_info,
> > > + BDI_RECLAIMABLE);
> > > + task_io_account_write(PAGE_CACHE_SIZE);
> > > }
> > > - return 1;
> > > + radix_tree_tag_set(&mapping->page_tree,
> > > + page_index(page), PAGECACHE_TAG_DIRTY);
> > > }
> > > - return 0;
> > > + write_unlock_irq(&mapping->tree_lock);
> > > +
> > > + if (mapping->host)
> > > + __mark_inode_dirty(mapping->host, I_DIRTY_PAGES);
>
> The inode gets marked dirty using the same "mapping" variable
> as my code does. So, AFAIU, my change does not introduce any new
> vulnerabilities. I would nevertherless be grateful to you for a scenario
> where the race would be triggered.

Ah, right, so that would be a resounding no to my previous question :-)