Re: [PATCH v7] iomap: make inline data support more flexible

["Andreas Grünbacher" <andreas.gruenbacher@gmail.com>]

Am Mo., 26. Juli 2021 um 06:00 Uhr schrieb Gao Xiang
<hsiangkao@linux.alibaba.com>:
> On Mon, Jul 26, 2021 at 12:16:39AM +0200, Andreas Gruenbacher wrote:
> > Here's a fixed and cleaned up version that passes fstests on gfs2.
>
> (cont.
> https://lore.kernel.org/r/YP4fk75mr%2FmIotDy@B-P7TQMD6M-0146.local)
>
> Would you mind listing what it fixed on gfs2 compared with v7?
> IOWs, I wonder which case failed with v7 on gfs2 so I could recheck
> this.

The use of iomap->length or pos in iomap_read_inline_data is
fundamentally broken, that's what I've fixed.

> > I see no reason why the combination of tail packing + writing should
> > cause any issues, so in my opinion, the check that disables that
> > combination in iomap_write_begin_inline should still be removed.
> >
> > It turns out that returning the number of bytes copied from
> > iomap_read_inline_data is a bit irritating: the function is really used
> > for filling the page, but that's not always the "progress" we're looking
> > for.  In the iomap_readpage case, we actually need to advance by an
> > antire page, but in the iomap_file_buffered_write case, we need to
> > advance by the length parameter of iomap_write_actor or less.  So I've
> > changed that back.
> >
> > I've also renamed iomap_inline_buf to iomap_inline_data and I've turned
> > iomap_inline_data_size_valid into iomap_within_inline_data, which seems
> > more useful to me.
> >
> > Thanks,
> > Andreas
> >
> > --
> >
> > Subject: [PATCH] iomap: Support tail packing
> >
> > The existing inline data support only works for cases where the entire
> > file is stored as inline data.  For larger files, EROFS stores the
> > initial blocks separately and then can pack a small tail adjacent to the
> > inode.  Generalise inline data to allow for tail packing.  Tails may not
> > cross a page boundary in memory.
> >
> > We currently have no filesystems that support tail packing and writing,
> > so that case is currently disabled (see iomap_write_begin_inline).  I'm
> > not aware of any reason why this code path shouldn't work, however.
> >
> > 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>
> > Tested-by: Huang Jianan <huangjianan@oppo.com> # erofs
> > Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
> > ---
> >  fs/iomap/buffered-io.c | 34 +++++++++++++++++++++++-----------
> >  fs/iomap/direct-io.c   | 11 ++++++-----
> >  include/linux/iomap.h  | 22 +++++++++++++++++++++-
> >  3 files changed, 50 insertions(+), 17 deletions(-)
> >
> > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > index 87ccb3438bec..334bf98fdd4a 100644
> > --- a/fs/iomap/buffered-io.c
> > +++ b/fs/iomap/buffered-io.c
> > @@ -205,25 +205,29 @@ struct iomap_readpage_ctx {
> >       struct readahead_control *rac;
> >  };
> >
> > -static void
> > -iomap_read_inline_data(struct inode *inode, struct page *page,
> > +static int iomap_read_inline_data(struct inode *inode, struct page *page,
> >               struct iomap *iomap)
> >  {
> > -     size_t size = i_size_read(inode);
> > +     size_t size = i_size_read(inode) - iomap->offset;
> >       void *addr;
> >
> >       if (PageUptodate(page))
> > -             return;
> > +             return 0;
> >
> > -     BUG_ON(page_has_private(page));
> > -     BUG_ON(page->index);
> > -     BUG_ON(size > PAGE_SIZE - offset_in_page(iomap->inline_data));
> > +     /* inline and tail-packed data must start page aligned in the file */
> > +     if (WARN_ON_ONCE(offset_in_page(iomap->offset)))
> > +             return -EIO;
> > +     if (WARN_ON_ONCE(size > PAGE_SIZE - offset_in_page(iomap->inline_data)))
> > +             return -EIO;
> > +     if (WARN_ON_ONCE(page_has_private(page)))
> > +             return -EIO;
> >
> >       addr = kmap_atomic(page);
> >       memcpy(addr, iomap->inline_data, size);
> >       memset(addr + size, 0, PAGE_SIZE - size);
> >       kunmap_atomic(addr);
> >       SetPageUptodate(page);
> > +     return 0;
> >  }
> >
> >  static inline bool iomap_block_needs_zeroing(struct inode *inode,
> > @@ -247,7 +251,6 @@ iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> >       sector_t sector;
> >
> >       if (iomap->type == IOMAP_INLINE) {
> > -             WARN_ON_ONCE(pos);
> >               iomap_read_inline_data(inode, page, iomap);
> >               return PAGE_SIZE;
> >       }
> > @@ -589,6 +592,15 @@ __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
> >       return 0;
> >  }
> >
> > +static int iomap_write_begin_inline(struct inode *inode,
> > +             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;
> > +     return iomap_read_inline_data(inode, page, srcmap);
> > +}
> > +
> >  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 +630,7 @@ 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, page, srcmap);
> >       else if (iomap->flags & IOMAP_F_BUFFER_HEAD)
> >               status = __block_write_begin_int(page, pos, len, NULL, srcmap);
> >       else
> > @@ -671,11 +683,11 @@ static size_t iomap_write_end_inline(struct inode *inode, struct page *page,
> >       void *addr;
> >
> >       WARN_ON_ONCE(!PageUptodate(page));
> > -     BUG_ON(pos + copied > PAGE_SIZE - offset_in_page(iomap->inline_data));
> > +     BUG_ON(!iomap_within_inline_data(iomap, pos + copied - 1));
> >
> >       flush_dcache_page(page);
> >       addr = kmap_atomic(page);
> > -     memcpy(iomap->inline_data + pos, addr + pos, copied);
> > +     memcpy(iomap_inline_data(iomap, pos), addr + pos, copied);
> >       kunmap_atomic(addr);
> >
> >       mark_inode_dirty(inode);
> > diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> > index 9398b8c31323..c9424e58f613 100644
> > --- a/fs/iomap/direct-io.c
> > +++ b/fs/iomap/direct-io.c
> > @@ -380,21 +380,22 @@ iomap_dio_inline_actor(struct inode *inode, loff_t pos, loff_t length,
> >       struct iov_iter *iter = dio->submit.iter;
> >       size_t copied;
> >
> > -     BUG_ON(pos + length > PAGE_SIZE - offset_in_page(iomap->inline_data));
> > +     if (WARN_ON_ONCE(!iomap_within_inline_data(iomap, pos + length - 1)))
> > +             return -EIO;
>
> I also wonder what is wrong with the previous patch:
>
> +       if (WARN_ON_ONCE(!iomap_inline_data_size_valid(iomap)))
> +               return -EIO;
>
> +/*
> + * iomap->inline_data is a potentially kmapped page, ensure it never crosses a
> + * page boundary.
> + */
> +static inline bool iomap_inline_data_size_valid(const struct iomap *iomap)
> +{
> +       return iomap->length <= PAGE_SIZE - offset_in_page(iomap->inline_data);
> +}
>
> In principle, the relationship of iomap->offset, pos, length and
> iomap->length is:
>
> "       iomap->offset <= pos < pos + length <= iomap->offset +
> iomap->length   "
>
> pos and pos + length are also impacted by what user requests rather
> than the original extent itself reported by fs.
>
> Here we need to make sure the whole extent in the page, so I think
> it'd be better to check with iomap->length rather than some pos,
> length related stuffs.

Yeah okay, the difference is that I've checked the validity of the
range of the extent actually used and iomap_inline_data_size_valid
checks if the entire mapping is valid. So let's stick with Christoph's
previous version.

> >
> >       if (dio->flags & IOMAP_DIO_WRITE) {
> > -             loff_t size = inode->i_size;
> > +             loff_t size = iomap->offset + iomap->length;
>
> and here, since it's the last extent and due to the current limitation
> in practice,
> iomap->offset + iomap->length == inode->i_size,
>
> yet I wonder why this part uses iomap->length to calculate instead of
> using i_size as in iomap_read_inline_data().
>
> My thought is "here it handles the i_size pointer and append write",
> so I think "loff_t size = inode->i_size" makes more sense here.

Hmm, right.

> >               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(iomap, size), 0, pos - size);
> > +             copied = copy_from_iter(iomap_inline_data(iomap, pos), length, iter);
>
> iomap_inline_buf() was suggested by Darrick. From my point of view,
> I think it's better since it's a part of iomap->inline_data due to
> pos involved.

I find iomap_inline_buf a bit more confusing because it's not
immediately obvious that "buf" == "data".

I'll send an updated version.

Thanks,
Andreas