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
