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

On Thu, May 13, 2021 at 09:01:14PM +0200, Jan Kara wrote:

On Wed 12-05-21 15:40:21, Matthew Wilcox wrote:

quoted

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?

quoted

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.