On Mon, Jun 12, 2023 at 11:55:55PM +0530, Ritesh Harjani wrote: > Matthew Wilcox <willy@xxxxxxxxxxxxx> writes: > I couldn't respond to your change because I still had some confusion > around this suggestion - > > > So do we care if we write a random fragment of a page after a truncate? > > If so, we should add: > > > > if (folio_pos(folio) >= size) > > return 0; /* Do we need to account nr_to_write? */ > > I was not sure whether if go with above case then whether it will > work with collapse_range. I initially thought that collapse_range will > truncate the pages between start and end of the file and then > it can also reduce the inode->i_size. That means writeback can find an > inode->i_size smaller than folio_pos(folio) which it is writing to. > But in this case we can't skip the write in writeback case like above > because that write is still required (a spurious write) even though > i_size is reduced as it's corresponding FS blocks are not truncated. > > But just now looking at ext4_collapse_range() code it doesn't look like > it is the problem because it waits for any dirty data to be written > before truncate. So no matter which folio_pos(folio) the writeback is > writing, there should not be an issue if we simply return 0 like how > you suggested above. > > static int ext4_collapse_range(struct file *file, loff_t offset, loff_t len) > > <...> > ioffset = round_down(offset, PAGE_SIZE); > /* > * Write tail of the last page before removed range since it will get > * removed from the page cache below. > */ > > ret = filemap_write_and_wait_range(mapping, ioffset, offset); > if (ret) > goto out_mmap; > /* > * Write data that will be shifted to preserve them when discarding > * page cache below. We are also protected from pages becoming dirty > * by i_rwsem and invalidate_lock. > */ > ret = filemap_write_and_wait_range(mapping, offset + len, > LLONG_MAX); > truncate_pagecache(inode, ioffset); > > <... within i_data_sem> > i_size_write(inode, new_size); > > <...> > > > However to avoid problems like this I felt, I will do some more code > reading. And then I was mostly considering your second suggestion which > is this. This will ensure we keep the current behavior as is and not > change that. > > > If we simply don't care that we're doing a spurious write, then we can > > do something like: > > > > - len = size & ~PAGE_MASK; > > + len = size & (len - 1); For all I know, I've found a bug here. I don't know enough about ext4; if we have truncated a file, and then writeback a page that is past i_size, will the block its writing to have been freed? Is this potentially a silent data corruptor?
