Re: [PATCH 16/18] btrfs: Writeprotect mmap pages on snapshot

On 16:04 23/05, Jan Kara wrote: > On Mon 29-04-19 12:26:47, Goldwyn Rodrigues wrote: > > From: Goldwyn Rodrigues <rgoldwyn(a)suse.com&gt; > > > > Inorder to make sure mmap'd files don't change after snapshot, > > writeprotect the mmap pages on snapshot. This is done by performing > > a data writeback on the pages (which simply mark the pages are > > wrprotected). This way if the user process tries to access the memory > > we will get another fault and we can perform a CoW. > > > > In order to accomplish this, we tag all CoW pages as > > PAGECACHE_TAG_TOWRITE, and add the mmapd inode in delalloc_inodes. > > During snapshot, it starts writeback of all delalloc'd inodes and > > here we perform a data writeback. We don't want to keep the inodes > > in delalloc_inodes until it umount (WARN_ON), so we remove it > > during inode evictions. > > > > Signed-off-by: Goldwyn Rodrigues <rgoldwyn(a)suse.com&gt; > > OK, so here you use PAGECACHE_TAG_TOWRITE. But why is not > PAGECACHE_TAG_DIRTY enough for you? Also why isn't the same needed also for > normal non-DAX inodes? There you also need to trigger CoW on mmap write so > I just don't see the difference... Because dax_writeback_mapping_range() writebacks pages marked PAGECACHE_TAG_TOWRITE and not PAGECACHE_TAG_DIRTY. Should it writeback pages marked as PAGECACHE_TAG_DIRTY as well?