On Wed, Jun 14, 2023 at 01:09:59AM +0530, Ritesh Harjani wrote: > Jan Kara <jack@xxxxxxx> writes: > > > On Tue 13-06-23 09:27:38, Ritesh Harjani wrote: > >> Matthew Wilcox <willy@xxxxxxxxxxxxx> writes: > >> > 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? > >> > >> I don't think so. If we look at truncate code, it first reduces i_size, > >> then call truncate_pagecache(inode, newsize) and then we will call > >> ext4_truncate() which will free the corresponding blocks. > >> Since writeback happens with folio lock held until completion, hence I > >> think truncate_pagecache() should block on that folio until it's lock > >> has been released. > >> > >> - IIUC, if truncate would have completed then the folio won't be in the > >> foliocache for writeback to happen. Foliocache is kept consistent > >> via > >> - first truncate the folio in the foliocache and then remove/free > >> the blocks on device. > > > > Yes, correct. > > > >> - Also the reason we update i_size "before" calling truncate_pagecache() > >> is to synchronize with mmap/pagefault. > > > > Yes, but these days mapping->invalidate_lock works for that instead for > > ext4. > > > >> > Is this potentially a silent data corruptor? > >> > >> - Let's consider a case when folio_pos > i_size but both still belongs > >> to the last block. i.e. it's a straddle write case. > >> In such case we require writeback to write the data of this last folio > >> straddling i_size. Because truncate will not remove/free this last folio > >> straddling i_size & neither the last block will be freed. And I think > >> writeback is supposed to write this last folio to the disk to keep the > >> cache and disk data consistent. Because truncate will only zero out > >> the rest of the folio in the foliocache. But I don't think it will go and > >> write that folio out (It's not required because i_size means that the > >> rest of the folio beyond i_size should remain zero). > >> > >> So, IMO writeback is supposed to write this last folio to the disk. And, > >> if we skip this writeout, then I think it may cause silent data corruption. > >> > >> But I am not sure about the rest of the write beyond the last block of > >> i_size. I think those could just be spurious writes which won't cause > >> any harm because truncate will eventually first remove this folio from > >> file mapping and then will release the corresponding disk blocks. > >> So writing those out should does no harm > > > > Correct. The block straddling i_size must be written out, the blocks fully > > beyond new i_size (but below old i_size) may or may not be written out. As > > you say these extra racing writes to blocks that will get truncated cause > > no harm. > > > > Thanks Jan for confirming. So, I think we should make below change. > (note the code which was doing "size - folio_pos(folio)" in > mpage_submit_folio() is dropped by Ted in the latest tree). > > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c > index 43be684dabcb..006eba9be5e6 100644 > --- a/fs/ext4/inode.c > +++ b/fs/ext4/inode.c > @@ -1859,9 +1859,9 @@ static int mpage_submit_folio(struct mpage_da_data *mpd, struct folio *folio) > */ > size = i_size_read(mpd->inode); > len = folio_size(folio); > - if (folio_pos(folio) + len > size && > + if ((folio_pos(folio) >= size || (folio_pos(folio) + len > size)) && > !ext4_verity_in_progress(mpd->inode)) > - len = size & ~PAGE_MASK; > + len = size & (len - 1); > err = ext4_bio_write_folio(&mpd->io_submit, folio, len); > if (!err) > mpd->wbc->nr_to_write--; > @@ -2329,9 +2329,9 @@ static int mpage_journal_page_buffers(handle_t *handle, > folio_clear_checked(folio); > mpd->wbc->nr_to_write--; > > - if (folio_pos(folio) + len > size && > + if ((folio_pos(folio) >= size || (folio_pos(folio) + len > size)) && > !ext4_verity_in_progress(inode)) > - len = size - folio_pos(folio); > + len = size & (len - 1); > > return ext4_journal_folio_buffers(handle, folio, len); > } > > > I will give it some more thoughts and testing. Why should ext4 be different from other filesystems which simply do: if (folio_pos(folio) >= size) return 0;
