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SubjectRe: [PATCH v5] iomap: support tail packing inline read
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
> >

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