On Tue, Jun 06, 2023 at 09:03:17AM -0700, Darrick J. Wong wrote: > On Tue, Jun 06, 2023 at 05:21:32AM +0530, Ritesh Harjani wrote: > > So, I do have a confusion in __folio_mark_dirty() function... > > > > i.e. __folio_mark_dirty checks whether folio->mapping is not NULL. > > That means for marking range of blocks dirty within iop from > > ->dirty_folio(), we can't use folio->mapping->host is it? > > We have to use inode from mapping->host (mapping is passed as a > > parameter in ->dirty_folio). It probably helps to read the commentary above filemap_dirty_folio(). * The caller must ensure this doesn't race with truncation. Most will * simply hold the folio lock, but e.g. zap_pte_range() calls with the * folio mapped and the pte lock held, which also locks out truncation. But __folio_mark_dirty() can't rely on that! Again, see the commentary: * This can also be called from mark_buffer_dirty(), which I * cannot prove is always protected against truncate. iomap doesn't do bottom-up dirtying, only top-down. So it absolutely can rely on the VFS having taken the appropriate locks. > Ah, yeah. folio->mapping can become NULL if truncate races with us in > removing the folio from the foliocache. > > For regular reads and writes this is a nonissue because those paths all > take i_rwsem and will block truncate. However, for page_mkwrite, xfs > doesn't take mmap_invalidate_lock until after the vm_fault has been > given a folio to play with. invalidate_lock isn't needed here. You take the folio_lock, then you call folio_mkwrite_check_truncate() to make sure it wasn't truncated before you took the folio_lock. Truncation will block on the folio_lock, so you're good unless you release the folio_lock (which you don't, you return it to the MM locked).
