Re: [PATCH v6 17/19] iomap: Restructure iomap_readpages_actor

Dave Chinner
2020-02-19T03:29:11Z

On Mon, Feb 17, 2020 at 10:46:11AM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>
> By putting the 'have we reached the end of the page' condition at the end
> of the loop instead of the beginning, we can remove the 'submit the last
> page' code from iomap_readpages(). Also check that iomap_readpage_actor()
> didn't return 0, which would lead to an endless loop.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---
> fs/iomap/buffered-io.c | 25 ++++++++++++-------------
> 1 file changed, 12 insertions(+), 13 deletions(-)
>
> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> index cb3511eb152a..44303f370b2d 100644
> --- a/fs/iomap/buffered-io.c
> +++ b/fs/iomap/buffered-io.c
> @@ -400,15 +400,9 @@ iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
> void *data, struct iomap *iomap, struct iomap *srcmap)
> {
> struct iomap_readpage_ctx *ctx = data;
> - loff_t done, ret;
> + loff_t ret, done = 0;
>
> - for (done = 0; done < length; done += ret) {
> - if (ctx->cur_page && offset_in_page(pos + done) == 0) {
> - if (!ctx->cur_page_in_bio)
> - unlock_page(ctx->cur_page);
> - put_page(ctx->cur_page);
> - ctx->cur_page = NULL;
> - }
> + while (done < length) {
> if (!ctx->cur_page) {
> ctx->cur_page = iomap_next_page(inode, ctx->pages,
> pos, length, &done);
> @@ -418,6 +412,15 @@ iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
> }
> ret = iomap_readpage_actor(inode, pos + done, length - done,
> ctx, iomap, srcmap);
> + if (WARN_ON(ret == 0))
> + break;

This error case now leaks ctx->cur_page....

> + done += ret;
> + if (offset_in_page(pos + done) == 0) {
> + if (!ctx->cur_page_in_bio)
> + unlock_page(ctx->cur_page);
> + put_page(ctx->cur_page);
> + ctx->cur_page = NULL;
> + }
> }
>
> return done;
> @@ -451,11 +454,7 @@ iomap_readpages(struct address_space *mapping, struct list_head *pages,
> done:
> if (ctx.bio)
> submit_bio(ctx.bio);
> - if (ctx.cur_page) {
> - if (!ctx.cur_page_in_bio)
> - unlock_page(ctx.cur_page);
> - put_page(ctx.cur_page);
> - }
> + BUG_ON(ctx.cur_page);

And so will now trigger both a warn and a bug....

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com