Re: [PATCH 2/2] gfs2: Rework read and page fault locking

Matthew Wilcox
2020-06-19T11:56:11Z

On Fri, Jun 19, 2020 at 11:39:16AM +0200, Andreas Gruenbacher wrote:
> static int gfs2_readpage(struct file *file, struct page *page)
> {
> - struct address_space *mapping = page->mapping;
> - struct gfs2_inode *ip = GFS2_I(mapping->host);
> - struct gfs2_holder gh;
> int error;
>
> - unlock_page(page);
> - gfs2_holder_init(ip->i_gl, LM_ST_SHARED, 0, &gh);
> - error = gfs2_glock_nq(&gh);
> - if (unlikely(error))
> - goto out;
> - error = AOP_TRUNCATED_PAGE;
> - lock_page(page);
> - if (page->mapping == mapping && !PageUptodate(page))
> - error = __gfs2_readpage(file, page);
> - else
> - unlock_page(page);
> - gfs2_glock_dq(&gh);
> -out:
> - gfs2_holder_uninit(&gh);
> - if (error && error != AOP_TRUNCATED_PAGE)
> + error = __gfs2_readpage(file, page);
> + if (error)
> lock_page(page);
> return error;

I don't think this is right. If you return an error from ->readpage, I'm
pretty sure you're supposed to unlock that page. Looking at
generic_file_buffered_read():

error = mapping->a_ops->readpage(filp, page);
if (unlikely(error)) {
if (error == AOP_TRUNCATED_PAGE) {
put_page(page);
error = 0;
goto find_page;
}
goto readpage_error;
}
...
readpage_error:
put_page(page);
goto out;
...
out:
ra->prev_pos = prev_index;
ra->prev_pos <<= PAGE_SHIFT;
ra->prev_pos |= prev_offset;

*ppos = ((loff_t)index << PAGE_SHIFT) + offset;
file_accessed(filp);
return written ? written : error;

so we don't call unlock_page() in generic code, which means the next time
we try to get this page, we'll do ...

page = find_get_page(mapping, index);
...
if (!PageUptodate(page)) {
error = wait_on_page_locked_killable(page);
and presumably we'll wait forever because nobody is going to unlock this
page?

> @@ -598,16 +582,9 @@ static void gfs2_readahead(struct readahead_control *rac)
> {
> struct inode *inode = rac->mapping->host;
> struct gfs2_inode *ip = GFS2_I(inode);
> - struct gfs2_holder gh;
>
> - gfs2_holder_init(ip->i_gl, LM_ST_SHARED, 0, &gh);
> - if (gfs2_glock_nq(&gh))
> - goto out_uninit;
> if (!gfs2_is_stuffed(ip))
> mpage_readahead(rac, gfs2_block_map);
> - gfs2_glock_dq(&gh);
> -out_uninit:
> - gfs2_holder_uninit(&gh);
> }

Not for this patch, obviously, but why do you go to the effort of using
iomap_readpage() to implement gfs2_readpage(), but don't use iomap for
gfs2_readahead()? Far more pages are brought in through ->readahead
than are brought in through ->readpage.

> static ssize_t gfs2_file_read_iter(struct kiocb *iocb, struct iov_iter *to)
> {
> + struct gfs2_inode *ip;
> + struct gfs2_holder gh;
> + size_t written = 0;
> ssize_t ret;
>
> + gfs2_holder_mark_uninitialized(&gh);
> if (iocb->ki_flags & IOCB_DIRECT) {
> ret = gfs2_file_direct_read(iocb, to);

Again, future work, but you probably want to pass in &gh here so you
don't have to eat up another 32 bytes or so of stack space on an unused
gfs2_holder.

> if (likely(ret != -ENOTBLK))
> return ret;
> iocb->ki_flags &= ~IOCB_DIRECT;
> }
> - return generic_file_read_iter(iocb, to);
> + iocb->ki_flags |= IOCB_CACHED;
> + ret = generic_file_read_iter(iocb, to);
> + iocb->ki_flags &= ~IOCB_CACHED;
> + if (ret >= 0) {
> + if (!iov_iter_count(to))
> + return ret;
> + written = ret;
> + } else {
> + switch(ret) {
> + case -EAGAIN:
> + if (iocb->ki_flags & IOCB_NOWAIT)
> + return ret;
> + break;
> + case -ECANCELED:
> + break;
> + default:
> + return ret;
> + }
> + }

I'm wondering if we want to do this in common code rather than making it
something special only a few filesystems do (either because they care
about workloads with many threads accessing the same file, or because
their per-file locks are very heavy-weight).