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Date:   Thu, 22 Jul 2021 08:12:46 +0800
From:   Gao Xiang <hsiangkao@linux.alibaba.com>
To:     "Darrick J. Wong" <djwong@kernel.org>
Cc:     linux-erofs@lists.ozlabs.org, linux-fsdevel@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        Christoph Hellwig <hch@lst.de>,
        Matthew Wilcox <willy@infradead.org>,
        Andreas Gruenbacher <andreas.gruenbacher@gmail.com>
Subject: Re: [PATCH v5] iomap: support tail packing inline read
Message-ID: <YPi3/okpVH7Q1O2X@B-P7TQMD6M-0146.local>
Mail-Followup-To: "Darrick J. Wong" <djwong@kernel.org>,
        linux-erofs@lists.ozlabs.org, linux-fsdevel@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>, Christoph Hellwig <hch@lst.de>,
        Matthew Wilcox <willy@infradead.org>,
        Andreas Gruenbacher <andreas.gruenbacher@gmail.com>
References: <20210721082323.41933-1-hsiangkao@linux.alibaba.com>
 <20210721222404.GA8639@magnolia>
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Hi Darrick,

On Wed, Jul 21, 2021 at 03:24:04PM -0700, Darrick J. Wong wrote:
> On Wed, Jul 21, 2021 at 04:23:23PM +0800, Gao Xiang wrote:
> > This tries to add tail packing inline read to iomap, which can support
> > several inline tail blocks. Similar to the previous approach, it cleans
> > post-EOF in one iteration.
> > 
> > The write path remains untouched since EROFS cannot be used for testing.
> > It'd be better to be implemented if upcoming real users care rather than
> > leave untested dead code around.
> 
> I had a conversation with Gao on IRC this morning, and I think I've
> finally gotten up to speed on where he's trying to go with this
> patchset.  Maybe that will make review of this patch easier, or at least
> not muddy the waters further.

Many thanks for your time on this and the detailed long reply.

> 
> Right now, inline data in iomap serves exactly two users -- gfs2 and
> ext4.  ext4 doesn't use iomap for buffered IO and doesn't support
> directio for inline data files, so we can ignore them for now.  gfs2
> uses iomap for buffered IO, and it stores the inline data after the
> gfs2_dinode.
> 
> iomap's inline data functions exist to serve the gfs2 use case, which is
> why the code has baked-in assumptions that iomap->offset is always zero,
> and the length is never more than a page.
> 

Yeah, that is why I need to update the iomap inline code before
convering all buffer I/O stuffs to iomap.

> It used to be the case that we'd always attach an iomap_page to a page
> for blocksize < pagesize files, but as of 5.14-rc2 we're starting to
> move towards creating and dropping them on demand.  IOWs, reads from
> inline data files always read the entire file contents into the page so
> we mark the whole page uptodate and do not attach an iomap_page (unlike
> regular reads).  Writes don't attach an iomap_page to inline data files.
> Writeback attaches an iomap_page.

Yeah.

> 
> Did I get that much right?  Onto the erofs part, now that I've also
> taken the time to figure out what it's doing by reading the ondisk
> format in Documentation/.  (Thanks for that, erofs developers!)
> 
> erofs can perform tail packing to reduce internal block fragmentation.
> Tails of files are written immediately after the ondisk inode, which is
> why Gao wants to use IOMAP_INLINE for this.  Note that erofs tailpacking
> is /not/ same as what reiserfs does, and the inlinedata model is /not/
> the same as what gfs2 does.
> 
> A tail-packed erofs file mapping looks like this:
> 
> x = round_down(i_size, blocksize);
> [0..(x - 1)]:		mapped to a range of external blocks
> [x..(i_size - 1)]:	inline data immediately after the inode
>

Correct.

> The previous discussions have gone a bit afield -- there's only one
> inline data region per file, it won't cross a page boundary because
> erofs requires blocksize == pagesize, and it's always at the end of the
> file.  I don't know how we got onto the topic of multiple inline data
> regions or encoded regions in the middle of a file, but that's not on
> anybody's requirement list today, AFAICT.

Sorry, I just tried to give an example. And I saw it misled the topic,
very sorry about that.

> 
> I suspect that adapting the inlinedata code to support regions that
> don't start at offset zero but are otherwise page-aligned can be done
> with fairly minor changes to the accounting, since I think that largely
> can be done by removing the asserts about offset==0.

I think I could try to figure out a page-size aligned only patch like
this, as long as each one is happy about that.

> 
> Did I get that right?
> 
> The next thing the erofs developers want to do is add support for
> blocksize < pagesize, presumably so that they can mount a 4k blocksize
> erofs volume on a machine with 64k pages.  For that, I think erofs needs
> to be able to read the tail bytes into the middle of an existing page.
> Hence the need to update the per-block uptodate bits in the iomap_page
> from the read function, and all the math changes where we increase the
> starting address of a copy by (iomap->offset - pos).  The end result
> should be that we can handle inline data regions anywhere, though we
> won't really have a way to test that until erofs starts supporting
> blocksize < pagesize.

Correct.

> 
> Assuming that my assumptions are correct, I think this patch decomposes
> into three more targeted changes, one of which applies now, and the rest
> which will go with the later effort.
> 
> 1) Update the code to handle inline data mappings where iomap->offset is
> not zero but the start of the mapping is always page-aligned.

I could write a patch just for page-aligned cases for now.

> 
> 2) Adapt the inline data code to create and update the iop as
> appropriate.  This could be a little tricky since I've seen elsewhere in
> the v4 discussion thread that people like the idea of not paying the iop
> overhead for pages that are backed by a single extent even when bs < ps.
> I suspect we have enough to decide this from the *iomap/*srcmap length
> in iomap_readpage_actor or iomap_write_begin, though I've not written
> any code that tries that.
> 
> 3) Update the code to handle inline data mappings where iomap->offset
> can point to the middle of a page.

Ok, so I think I will unprioritize 2) and 3) for now. and just address
the page-aligned approach since it's currently what EROFS needs.

> 
> My apologies if everyone else already figured all of this out; for all I
> know I'm merely scrawling this here as notes to refer back to for future
> discussions.
> 
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Darrick J. Wong <djwong@kernel.org>
> > Cc: Matthew Wilcox <willy@infradead.org>
> > Cc: Andreas Gruenbacher <andreas.gruenbacher@gmail.com>
> > Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
> > ---
> > v4: https://lore.kernel.org/r/20210720133554.44058-1-hsiangkao@linux.alibaba.com
> > changes since v4:
> >  - turn to WARN_ON_ONCE() suggested by Darrick;
> >  - fix size to "min(iomap->length + iomap->offset - pos,
> >                     PAGE_SIZE - poff)"
> > 
> >  fs/iomap/buffered-io.c | 58 +++++++++++++++++++++++++++---------------
> >  fs/iomap/direct-io.c   | 13 +++++++---
> >  2 files changed, 47 insertions(+), 24 deletions(-)
> > 
> > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > index 87ccb3438bec..d8436d34a159 100644
> > --- a/fs/iomap/buffered-io.c
> > +++ b/fs/iomap/buffered-io.c
> > @@ -205,25 +205,27 @@ struct iomap_readpage_ctx {
> >  	struct readahead_control *rac;
> >  };
> >  
> > -static void
> > +static int
> >  iomap_read_inline_data(struct inode *inode, struct page *page,
> > -		struct iomap *iomap)
> > +		struct iomap *iomap, loff_t pos)
> >  {
> > -	size_t size = i_size_read(inode);
> > +	unsigned int size, poff = offset_in_page(pos);
> >  	void *addr;
> >  
> > -	if (PageUptodate(page))
> > -		return;
> > -
> > -	BUG_ON(page_has_private(page));
> > -	BUG_ON(page->index);
> > -	BUG_ON(size > PAGE_SIZE - offset_in_page(iomap->inline_data));
> > +	/* inline source data must be inside a single page */
> > +	if (WARN_ON_ONCE(iomap->length > PAGE_SIZE -
> > +			 offset_in_page(iomap->inline_data)))
> > +		return -EIO;
> > +	/* handle tail-packing blocks cross the current page into the next */
> > +	size = min_t(unsigned int, iomap->length + iomap->offset - pos,
> > +		     PAGE_SIZE - poff);
> 
> Part of my confusion has resulted from this comment -- now that I think
> I understand the problem domain better, I realize that the clamping code
> here is not because erofs will hand us a tail-packing iomap that crosses
> page boundaries; this clamp simply protects us from memory corruption.
> 
> 	/*
> 	 * iomap->inline_data is a kernel-mapped memory page, so we must
> 	 * terminate the read at the end of that page.
> 	 */
> 	if (WARN_ON_ONCE(...))
> 		return -EIO;
> 	size = min_t(...);

That sounds much better. I will update like this.

> 
> TBH I wonder if we merely need a rule that ->iomap_begin must not hand
> back an inline data mapping that crosses a page, since I think the
> check in the previous line is sufficient.
> 
> >  	addr = kmap_atomic(page);
> > -	memcpy(addr, iomap->inline_data, size);
> > -	memset(addr + size, 0, PAGE_SIZE - size);
> > +	memcpy(addr + poff, iomap->inline_data - iomap->offset + pos, size);
> 
> I keep seeing this (iomap->inline_data + pos - iomap->offset)
> construction in this patch, maybe it should be a helper?

I'm fine with this, (but I'm not good at naming), may I ask for
some suggested naming? e.g.

static inline void *iomap_adjusted_inline_data(iomap, pos)

does that look good?

> 
> > +	memset(addr + poff + size, 0, PAGE_SIZE - poff - size);
> >  	kunmap_atomic(addr);
> > -	SetPageUptodate(page);
> > +	iomap_set_range_uptodate(page, poff, PAGE_SIZE - poff);
> > +	return PAGE_SIZE - poff;
> >  }
> >  
> >  static inline bool iomap_block_needs_zeroing(struct inode *inode,
> > @@ -245,19 +247,23 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> >  	loff_t orig_pos = pos;
> >  	unsigned poff, plen;
> >  	sector_t sector;
> > +	int ret;
> >  
> > -	if (iomap->type == IOMAP_INLINE) {
> > -		WARN_ON_ONCE(pos);
> > -		iomap_read_inline_data(inode, page, iomap);
> > -		return PAGE_SIZE;
> > -	}
> > -
> > -	/* zero post-eof blocks as the page may be mapped */
> >  	iop = iomap_page_create(inode, page);
> > +	/* needs to skip some leading uptodate blocks */
> >  	iomap_adjust_read_range(inode, iop, &pos, length, &poff, &plen);
> >  	if (plen == 0)
> >  		goto done;
> >  
> > +	if (iomap->type == IOMAP_INLINE) {
> > +		ret = iomap_read_inline_data(inode, page, iomap, pos);
> > +		if (ret < 0)
> > +			return ret;
> > +		plen = ret;
> > +		goto done;
> > +	}
> > +
> > +	/* zero post-eof blocks as the page may be mapped */
> >  	if (iomap_block_needs_zeroing(inode, iomap, pos)) {
> >  		zero_user(page, poff, plen);
> >  		iomap_set_range_uptodate(page, poff, plen);
> > @@ -589,6 +595,18 @@ __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
> >  	return 0;
> >  }
> >  
> > +static int iomap_write_begin_inline(struct inode *inode, loff_t pos,
> > +		struct page *page, struct iomap *srcmap)
> > +{
> > +	/* needs more work for the tailpacking case, disable for now */
> > +	if (WARN_ON_ONCE(srcmap->offset != 0))
> > +		return -EIO;
> > +	if (PageUptodate(page))
> > +		return 0;
> > +	iomap_read_inline_data(inode, page, srcmap, 0);
> > +	return 0;
> > +}
> > +
> >  static int
> >  iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
> >  		struct page **pagep, struct iomap *iomap, struct iomap *srcmap)
> > @@ -618,7 +636,7 @@ iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, unsigned flags,
> >  	}
> >  
> >  	if (srcmap->type == IOMAP_INLINE)
> > -		iomap_read_inline_data(inode, page, srcmap);
> > +		status = iomap_write_begin_inline(inode, pos, page, srcmap);
> >  	else if (iomap->flags & IOMAP_F_BUFFER_HEAD)
> >  		status = __block_write_begin_int(page, pos, len, NULL, srcmap);
> >  	else
> > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> > index 9398b8c31323..cbadb99fb88c 100644
> > --- a/fs/iomap/direct-io.c
> > +++ b/fs/iomap/direct-io.c
> > @@ -379,22 +379,27 @@ iomap_dio_inline_actor(struct inode *inode, loff_t pos, loff_t length,
> >  {
> >  	struct iov_iter *iter = dio->submit.iter;
> >  	size_t copied;
> > +	void *dst = iomap->inline_data + pos - iomap->offset;
> >  
> > -	BUG_ON(pos + length > PAGE_SIZE - offset_in_page(iomap->inline_data));
> > +	/* inline data must be inside a single page */
> > +	if (WARN_ON_ONCE(length > PAGE_SIZE -
> > +			 offset_in_page(iomap->inline_data)))
> > +		return -EIO;
> 
> 	/*
> 	 * iomap->inline_data is a kernel-mapped memory page, so we must
> 	 * terminate the write at the end of that page.
> 	 */
> 	if (WARN_ON_ONCE(...))
> 		return -EIO;

Ok.

> 
> >  	if (dio->flags & IOMAP_DIO_WRITE) {
> 
> I thought we weren't allowing writes to an inline mapping unless
> iomap->offset == 0?  Why is it necessary to change the directio write
> path?  Shouldn't this be:
> 
> 		/* needs more work for the tailpacking case, disable for now */
> 		if (WARN_ON_ONCE(pos > 0))
> 			return -EIO;

That is because Andreas once pointed out a case in:
https://lore.kernel.org/r/CAHpGcMJ4T6byxqmO6zZF78wuw01twaEvSW5N6s90qWm0q_jCXQ@mail.gmail.com/

"This should be a WARN_ON_ONCE(srcmap->offset != 0). Otherwise, something like:

  xfs_io -ft -c 'pwrite 1 2'

will fail because pos will be 1."

I think that is reasonable to gfs2. So I changed like this.

Thanks,
Gao Xiang

> 
> --D
> 
> >  		loff_t size = inode->i_size;
> >  
> >  		if (pos > size)
> > -			memset(iomap->inline_data + size, 0, pos - size);
> > -		copied = copy_from_iter(iomap->inline_data + pos, length, iter);
> > +			memset(iomap->inline_data + size - iomap->offset,
> > +			       0, pos - size);
> > +		copied = copy_from_iter(dst, length, iter);
> >  		if (copied) {
> >  			if (pos + copied > size)
> >  				i_size_write(inode, pos + copied);
> >  			mark_inode_dirty(inode);
> >  		}
> >  	} else {
> > -		copied = copy_to_iter(iomap->inline_data + pos, length, iter);
> > +		copied = copy_to_iter(dst, length, iter);
> >  	}
> >  	dio->size += copied;
> >  	return copied;
> > -- 
> > 2.24.4
> >