Re: [PATCH 03/11] mm: Protect operations adding pages to page cache with invalidate_lock

Matthew Wilcox
2021-05-13T19:39:45Z

On Thu, May 13, 2021 at 09:01:14PM +0200, Jan Kara wrote:
> On Wed 12-05-21 15:40:21, Matthew Wilcox wrote:
> > Remind me (or, rather, add to the documentation) why we have to hold the
> > invalidate_lock during the call to readpage / readahead, and we don't just
> > hold it around the call to add_to_page_cache / add_to_page_cache_locked
> > / add_to_page_cache_lru ? I appreciate that ->readpages is still going
> > to suck, but we're down to just three implementations of ->readpages now
> > (9p, cifs & nfs).
>
> There's a comment in filemap_create_page() trying to explain this. We need
> to protect against cases like: Filesystem with 1k blocksize, file F has
> page at index 0 with uptodate buffer at 0-1k, rest not uptodate. All blocks
> underlying page are allocated. Now let read at offset 1k race with hole
> punch at offset 1k, length 1k.
>
> read() hole punch
> ...
> filemap_read()
> filemap_get_pages()
> - page found in the page cache but !Uptodate
> filemap_update_page()
> locks everything
> truncate_inode_pages_range()
> lock_page(page)
> do_invalidatepage()
> unlock_page(page)
> locks page
> filemap_read_page()

Ah, this is the partial_start case, which means that page->mapping
is still valid. But that means that do_invalidatepage() was called
with (offset 1024, length 1024), immediately after we called
zero_user_segment(). So isn't this a bug in the fs do_invalidatepage()?
The range from 1k-2k _is_ uptodate. It's been zeroed in memory,
and if we were to run after the "free block" below, we'd get that
memory zeroed again.

> ->readpage()
> block underlying offset 1k
> still allocated -> map buffer
> free block under offset 1k
> submit IO -> corrupted data
>
> If you think I should expand it to explain more details, please tell.
> Or maybe I can put more detailed discussion like above into the changelog?

> > Why not:
> >
> > __init_rwsem(&mapping->invalidate_lock, "mapping.invalidate_lock",
> > &sb->s_type->invalidate_lock_key);
>
> I replicated what we do for i_rwsem but you're right, this is better.
> Updated.

Hmm, there's a few places we should use __init_rwsem() ... something
for my "when bored" pile of work.