Re: [PATCH 3/6] fs: Convert block_read_full_page to be synchronous

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Oct 23, 2020 at 02:21:38PM +0100, Matthew Wilcox wrote:
> > 
> > The following is needed to set the bio encryption context for the
> > '-o inlinecrypt' case on ext4:
> > 
> > diff --git a/fs/buffer.c b/fs/buffer.c
> > index 95c338e2b99c..546a08c5003b 100644
> > --- a/fs/buffer.c
> > +++ b/fs/buffer.c
> > @@ -2237,6 +2237,7 @@ static int readpage_submit_bhs(struct page *page, struct blk_completion *cmpl,
> >  			submit_bio(bio);
> >  		}
> >  		bio = bio_alloc(GFP_NOIO, 1);
> > +		fscrypt_set_bio_crypt_ctx_bh(bio, bh, GFP_NOIO);
> >  		bio_set_dev(bio, bh->b_bdev);
> >  		bio->bi_iter.bi_sector = sector;
> >  		bio_add_page(bio, bh->b_page, bh->b_size, bh_offset(bh));
> 
> Thanks!  I saw that and had every intention of copying it across.
> And then I forgot.  I'll add that.  I'm also going to do:
> 
> -                           __bio_try_merge_page(bio, bh->b_page, bh->b_size,
> -                                       bh_offset(bh), &same_page))
> +                           bio_add_page(bio, bh->b_page, bh->b_size,
> +                                       bh_offset(bh)))
> 
> I wonder about allocating bios that can accommodate more bvecs.  Not sure
> how often filesystems have adjacent blocks which go into non-adjacent
> sub-page blocks.  It's certainly possible that a filesystem might have
> a page consisting of DDhhDDDD ('D' for Data, 'h' for hole), but how
> likely is it to have written the two data chunks next to each other?
> Maybe with O_SYNC?
> 

I think that's a rare case that's not very important to optimize.  And there's
already a lot of code where filesystems *could* submit a single bio in that case
but don't.  For example, both fs/direct-io.c and fs/iomap/direct-io.c only
submit bios that contain logically contiguous data.

If you do implement this optimization, note that it wouldn't work when a
bio_crypt_ctx is set, since the data must be logically contiguous in that case.
To handle that you'd need to call fscrypt_mergeable_bio_bh() when adding each
block, and submit the bio if it returns false.  (In contrast, with your current
proposal, calling fscrypt_mergeable_bio_bh() isn't necessary because each bio
only contains logically contiguous data within one page.)

- Eric