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

From: Matthew Wilcox <willy@infradead.org>
To: Eric Biggers <ebiggers@kernel.org>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-block@vger.kernel.org, linux-fscrypt@vger.kernel.org
Subject: Re: [PATCH 3/6] fs: Convert block_read_full_page to be synchronous
Date: Fri, 23 Oct 2020 14:21:38 +0100	[thread overview]
Message-ID: <20201023132138.GB20115@casper.infradead.org> (raw)
In-Reply-To: <20201022234011.GD3613750@gmail.com>

On Thu, Oct 22, 2020 at 04:40:11PM -0700, Eric Biggers wrote:
> On Thu, Oct 22, 2020 at 10:22:25PM +0100, Matthew Wilcox (Oracle) wrote:
> > +static int readpage_submit_bhs(struct page *page, struct blk_completion *cmpl,
> > +		unsigned int nr, struct buffer_head **bhs)
> > +{
> > +	struct bio *bio = NULL;
> > +	unsigned int i;
> > +	int err;
> > +
> > +	blk_completion_init(cmpl, nr);
> > +
> > +	for (i = 0; i < nr; i++) {
> > +		struct buffer_head *bh = bhs[i];
> > +		sector_t sector = bh->b_blocknr * (bh->b_size >> 9);
> > +		bool same_page;
> > +
> > +		if (buffer_uptodate(bh)) {
> > +			end_buffer_async_read(bh, 1);
> > +			blk_completion_sub(cmpl, BLK_STS_OK, 1);
> > +			continue;
> > +		}
> > +		if (bio) {
> > +			if (bio_end_sector(bio) == sector &&
> > +			    __bio_try_merge_page(bio, bh->b_page, bh->b_size,
> > +					bh_offset(bh), &same_page))
> > +				continue;
> > +			submit_bio(bio);
> > +		}
> > +		bio = bio_alloc(GFP_NOIO, 1);
> > +		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));
> > +		bio->bi_end_io = readpage_end_bio;
> > +		bio->bi_private = cmpl;
> > +		/* Take care of bh's that straddle the end of the device */
> > +		guard_bio_eod(bio);
> > +	}
> 
> 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?

Anyway, this patchset needs some more thought because I've just seen
the path from mpage_readahead() to block_read_full_page() that should
definitely not be synchronous.