Hi Richard, On 2018/12/14 19:25, Richard Weinberger wrote: > This is the third place which needs this workaround. > UBIFS, F2FS, and now iomap. > > I agree with Dave that nobody can assume that PG_private implies an additional > page reference. > But page migration does that. Including parts of the write back code.
It seems that it's clearly documented in https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/mm.h#n780 * A pagecache page contains an opaque `private' member, which belongs to the * page's address_space. Usually, this is the address of a circular list of * the page's disk buffers. PG_private must be set to tell the VM to call * into the filesystem to release these pages. * * A page may belong to an inode's memory mapping. In this case, page->mapping * is the pointer to the inode, and page->index is the file offset of the page, * in units of PAGE_SIZE. * * If pagecache pages are not associated with an inode, they are said to be * anonymous pages. These may become associated with the swapcache, and in that * case PG_swapcache is set, and page->private is an offset into the swapcache. * * In either case (swapcache or inode backed), the pagecache itself holds one * reference to the page. Setting PG_private should also increment the * refcount. The each user mapping also has a reference to the page. and when I looked into that, I found https://lore.kernel.org/lkml/3cb3ca93.d1416...@zip.com.au/ Thanks, Gao Xiang
