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.
