Some new bio merging behaviors in __bio_try_merge_page

[Ming Lei]

Hi Gao Xiang,

On Thu, Apr 11, 2019 at 01:47:49PM +0800, Gao Xiang wrote:
> Hi Ming,
> 
> I found a erofs issue after commit 07173c3ec276
> ("block: enable multipage bvecs") is merged. It seems that
> it tries to merge more physical continuous pages in one iovec.
> 
> However it breaks the current erofs_read_raw_page logic since it uses
> nr_iovecs of bio_alloc to limit the maximum number of physical

I believe you can do the limit outside easily, such as by checking
how many pages have been added to the bio.

> continuous blocks as well. It was practicable since the old
> __bio_try_merge_page only tries to merge in the same page.
> it is a kAPI behavior change which also affects bio_alloc...
> 
> ...
> 231                 err = erofs_map_blocks(inode, &map, EROFS_GET_BLOCKS_RAW);
> 232                 if (unlikely(err))
> 233                         goto err_out;
> ...
> 284                 /* max # of continuous pages */
> 285                 if (nblocks > DIV_ROUND_UP(map.m_plen, PAGE_SIZE))
> 286                         nblocks = DIV_ROUND_UP(map.m_plen, PAGE_SIZE);
> 287                 if (nblocks > BIO_MAX_PAGES)
> 288                         nblocks = BIO_MAX_PAGES;
> 289
> 290                 bio = erofs_grab_bio(sb, blknr, nblocks, sb,
> 291                                      read_endio, false);

Previously this bio is allowed to add at most 'nblocks' pages, however,
now we are allowed to add at most 'nblocks' io vecs, instead of pages.

> 292                 if (IS_ERR(bio)) {
> 293                         err = PTR_ERR(bio);
> 294                         bio = NULL;
> 295                         goto err_out;
> 296                 }
> 297         }
> 298
> 299         err = bio_add_page(bio, page, PAGE_SIZE, 0);
> 300         /* out of the extent or bio is full */
> 301         if (err < PAGE_SIZE)
> 302                 goto submit_bio_retry;
> ...
> 
> After commit 07173c3ec276 ("block: enable multipage bvecs"), erofs could
> read more beyond what erofs_map_blocks assigns, and out-of-bound data could
> be read and it breaks tail-end inline determination.

I don't understand why, could you explain a bit why erofs reads more?

The amount depends on how many pages you added to the bio.

> 
> I can change the logic in erofs. However, out of curiosity, I have no idea
> if some other places also are designed like this.
> 
> IMO, it's better to provide a total count which indicates how many real
> pages have been added in this bio. some thoughts?

As I mentioned, you may count how many pages added to bio, or you still
can get the number via bio_segments(bio).

Thanks,
Ming