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

Matthew Wilcox
2020-02-19T17:06:48Z

On Wed, Feb 19, 2020 at 05:40:05PM +1100, Dave Chinner wrote:
> Ok, that's what the ctx.cur_page_in_bio check is used to detect i.e.
> if we've got a page that the readahead cursor points at, and we
> haven't actually added it to a bio, then we can leave it to the
> read_pages() to unlock and clean up. If it's in a bio, then IO
> completion will unlock it and so we only have to drop the submission
> reference and move the readahead cursor forwards so read_pages()
> doesn't try to unlock this page. i.e:
>
> /* clean up partial page submission failures */
> if (ctx.cur_page && ctx.cur_page_in_bio) {
> put_page(ctx.cur_page);
> readahead_next(rac);
> }
>
> looks to me like it will handle the case of "ret == 0" in the actor
> function just fine.

Here's what I ended up with:

@@ -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;
-
- 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;
- }
+ loff_t ret, done = 0;
+
+ while (done < length) {
if (!ctx->cur_page) {
ctx->cur_page = iomap_next_page(inode, ctx->pages,
pos, length, &done);
@@ -418,6 +412,20 @@ iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
}
ret = iomap_readpage_actor(inode, pos + done, length - done,
ctx, iomap, srcmap);
+ done += ret;
+
+ /* Keep working on a partial page */
+ if (ret && offset_in_page(pos + done))
+ continue;
+
+ if (!ctx->cur_page_in_bio)
+ unlock_page(ctx->cur_page);
+ put_page(ctx->cur_page);
+ ctx->cur_page = NULL;
+
+ /* Don't loop forever if we made no progress */
+ if (WARN_ON(!ret))
+ break;
}

return done;
@@ -451,11 +459,7 @@ 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);

/*
* Check that we didn't lose a page due to the arcance calling

so we'll WARN if we get a ret == 0 (matching ->readpage), and we'll
BUG if we ever see a page being leaked out of readpages_actor, which
is a thing that should never happen and we definitely want to be noisy
about if it does.