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

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

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.