Re: [PATCH RFC 02/29] iomap: introduce iomap_read/write_region interface

Andrey Albershteyn
2025-07-31T15:51:59Z

On 2025-07-29 15:22:52, Darrick J. Wong wrote:
> On Mon, Jul 28, 2025 at 10:30:06PM +0200, Andrey Albershteyn wrote:
> > From: Andrey Albershteyn <aalbersh@redhat.com>
> >
> > Interface for writing data beyond EOF into offsetted region in
> > page cache.
> >
> > Signed-off-by: Andrey Albershteyn <aalbersh@kernel.org>
> > ---
> > include/linux/iomap.h | 16 ++++++++
> > fs/iomap/buffered-io.c | 99 +++++++++++++++++++++++++++++++++++++++++++++++++-
> > 2 files changed, 114 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> > index 4a0b5ebb79e9..73288f28543f 100644
> > --- a/include/linux/iomap.h
> > +++ b/include/linux/iomap.h
> > @@ -83,6 +83,11 @@ struct vm_fault;
> > */
> > #define IOMAP_F_PRIVATE (1U << 12)
> >
> > +/*
> > + * Writes happens beyound inode EOF
> > + */
> > +#define IOMAP_F_BEYOND_EOF (1U << 13)
> > +
> > /*
> > * Flags set by the core iomap code during operations:
> > *
> > @@ -533,4 +538,15 @@ int iomap_swapfile_activate(struct swap_info_struct *sis,
> >
> > extern struct bio_set iomap_ioend_bioset;
> >
> > +struct ioregion {
> > + struct inode *inode;
> > + loff_t pos; /* IO position */
> > + const void *buf; /* Data to be written (in only) */
> > + size_t length; /* Length of the date */
>
> Length of the data ?

thanks!

>
> > + const struct iomap_ops *ops;
> > +};
>
> This sounds like a kiocb and a kvec...
>
> > +
> > +struct folio *iomap_read_region(struct ioregion *region);
> > +int iomap_write_region(struct ioregion *region);
>
> ...and these sound a lot like filemap_read and iomap_write_iter.
> Why not use those? You'd get readahead for free. Though I guess
> filemap_read cuts off at i_size so maybe that's why this is necessary?
>
> (and by extension, is this why the existing fsverity implementations
> seem to do their own readahead and reading?)
>
> ((and now I guess I see why this isn't done through the regular kiocb
> interface, because then we'd be exposing post-EOF data hiding to
> everyone in the system))
>
> > #endif /* LINUX_IOMAP_H */
> >
> > --
> > 2.50.0
> >
> >
> > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > index 7bef232254a3..e959a206cba9 100644
> > --- a/fs/iomap/buffered-io.c
> > +++ b/fs/iomap/buffered-io.c
> > @@ -321,6 +321,7 @@ struct iomap_readpage_ctx {
> > bool cur_folio_in_bio;
> > struct bio *bio;
> > struct readahead_control *rac;
> > + int flags;
>
> What flags go in here?

IOMAP_F_BEYOND_EOF

>
> > };
> >
> > /**
> > @@ -387,7 +388,8 @@ static int iomap_readpage_iter(struct iomap_iter *iter,
> > if (plen == 0)
> > goto done;
> >
> > - if (iomap_block_needs_zeroing(iter, pos)) {
> > + if (iomap_block_needs_zeroing(iter, pos) &&
> > + !(iomap->flags & IOMAP_F_BEYOND_EOF)) {
> > folio_zero_range(folio, poff, plen);
> > iomap_set_range_uptodate(folio, poff, plen);
> > goto done;
> > @@ -2007,3 +2009,98 @@ iomap_writepages_unbound(struct address_space *mapping, struct writeback_control
> > return iomap_submit_ioend(wpc, error);
> > }
> > EXPORT_SYMBOL_GPL(iomap_writepages_unbound);
> > +
> > +struct folio *
> > +iomap_read_region(struct ioregion *region)
> > +{
> > + struct inode *inode = region->inode;
> > + fgf_t fgp = FGP_CREAT | FGP_LOCK | fgf_set_order(region->length);
> > + pgoff_t index = (region->pos) >> PAGE_SHIFT;
> > + struct folio *folio = __filemap_get_folio(inode->i_mapping, index, fgp,
> > + mapping_gfp_mask(inode->i_mapping));
> > + int ret;
> > + struct iomap_iter iter = {
> > + .inode = folio->mapping->host,
> > + .pos = region->pos,
> > + .len = region->length,
> > + };
> > + struct iomap_readpage_ctx ctx = {
> > + .cur_folio = folio,
> > + };
> > +
> > + if (folio_test_uptodate(folio)) {
> > + folio_unlock(folio);
> > + return folio;
> > + }
> > +
> > + while ((ret = iomap_iter(&iter, region->ops)) > 0)
> > + iter.status = iomap_read_folio_iter(&iter, &ctx);
>
> Huh, we don't read into region->buf? Oh, I see, this gets iomap to
> install an uptodate folio in the pagecache, and then later we can
> just hand it to fsverity. Maybe?

yes

>
> --D
>
> > +
> > + if (ctx.bio) {
> > + submit_bio(ctx.bio);
> > + WARN_ON_ONCE(!ctx.cur_folio_in_bio);
> > + } else {
> > + WARN_ON_ONCE(ctx.cur_folio_in_bio);
> > + folio_unlock(folio);
> > + }
> > +
> > + return folio;
> > +}
> > +EXPORT_SYMBOL_GPL(iomap_read_region);
> > +
> > +static int iomap_write_region_iter(struct iomap_iter *iter, const void *buf)
> > +{
> > + loff_t pos = iter->pos;
> > + u64 bytes = iomap_length(iter);
> > + int status;
> > +
> > + do {
> > + struct folio *folio;
> > + size_t offset;
> > + bool ret;
>
> Is balance_dirty_pages_ratelimited_flags need here if we're at the dirty
> thresholds?

Ah I see, missed that, I will add it, thanks!

>
> > +
> > + bytes = min_t(u64, SIZE_MAX, bytes);
> > + status = iomap_write_begin(iter, &folio, &offset, &bytes);
> > + if (status)
> > + return status;
> > + if (iter->iomap.flags & IOMAP_F_STALE)
> > + break;
> > +
> > + offset = offset_in_folio(folio, pos);
> > + if (bytes > folio_size(folio) - offset)
> > + bytes = folio_size(folio) - offset;
> > +
> > + memcpy_to_folio(folio, offset, buf, bytes);
> > +
> > + ret = iomap_write_end(iter, bytes, bytes, folio);
> > + if (WARN_ON_ONCE(!ret))
> > + return -EIO;
> > +
> > + __iomap_put_folio(iter, bytes, folio);
> > + if (WARN_ON_ONCE(!ret))
> > + return -EIO;
> > +
> > + status = iomap_iter_advance(iter, &bytes);
> > + if (status)
> > + break;
> > + } while (bytes > 0);
> > +
> > + return status;
> > +}
>
> Hrm, stripped down version of iomap_write_iter without the isize
> updates.

yes, it probably can be merged with a flag to skip the i_size
updates, I can look into this if it make more sense

>
> --D
>
> > +
> > +int
> > +iomap_write_region(struct ioregion *region)
> > +{
> > + struct iomap_iter iter = {
> > + .inode = region->inode,
> > + .pos = region->pos,
> > + .len = region->length,
> > + };
> > + ssize_t ret;
> > +
> > + while ((ret = iomap_iter(&iter, region->ops)) > 0)
> > + iter.status = iomap_write_region_iter(&iter, region->buf);
> > +
> > + return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(iomap_write_region);
>

--
- Andrey