Merge patch series "iomap: fixes for short copies and integrity bio sizes"

Christian Brauner <brauner@kernel.org> says:

Two iomap fixes that came out of Josef's AI bug scans.

The first one is old. For a buffered write into a hole, an unwritten
extent or past EOF __iomap_write_begin() marks the zeroed blocks
uptodate before the copy from userspace. If the copy comes up short the
uncopied part of the write stays behind in an uptodate block:

        ftruncate(fd, 4096);
        pwrite(fd, p, 3000, 100);    /* 16 bytes at p, then PROT_NONE */
        pread(fd, buf, 4096, 0);

That returns 2984 bytes of whatever was in the page before. Willy
reported this in 2024 and Zhang Yi posted a fix but it never got merged.
I went with the same fix. Josef's version zeroed the partially written
blocks completely and only marked those uptodate. That works too but
it's a lot more code for the same result.

Willy suggested back then to drop the uptodate marking from
__iomap_write_begin() completely. I didn't do that. Blocks that were
read from disk are valid and keeping them uptodate means a retried short
copy doesn't have to read them again.

The second one is from v7.1. When the filesystem handles the integrity
metadata the bio size check wraps if a single chunk is larger than
max_integrity_io_size(). That limit is capped by the maximum segment
size, so it's 64 KiB on lpfc and qla2xxx, and XFS writeback of a large
preallocated file happily builds a single 300 MiB bio and overruns the
2 MiB integrity buffer.

Note that a single chunk above the limit still gets a bio of its own.
That's fine with 4k pages. But with 64k pages a folio can be 128 MiB
which needs 4 MiB of PI with 16 bytes of metadata per 512 byte interval.
That's not new though and needs a separate fix.

Both are tested in a VM. Without the first patch the short copy
reproducer leaks on XFS with 4k and 1k blocks and on ext4 with buffered
iomap, and an fsx-like checker with faulting user buffers fails within
a few hundred operations. With it both pass. The second patch was tested
with scsi_debug DIF/DIX patched to a 64 KiB maximum segment size.

* patches from https://patch.msgid.link/20261029-work-iomap-ai-fixes-v1-0-7eda63bc1b13@kernel.org:
  iomap: keep large chunks from defeating the bio cap
  iomap: don't mark unwritten parts of a folio uptodate

Link: https://patch.msgid.link/20261029-work-iomap-ai-fixes-v1-0-7eda63bc1b13@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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