uiomove_object_page: a failed copy can still dirty the page

[alc]

Summary

If we fail in the middle of reading from the source, the destination page may have already been dirtied. For example, the source may cross a page boundary, and the second page is inaccessible. Conservatively mark the destination page as dirty, even on errors.

Reported by: Claude (Opus 5.5)

[alc]

I've been reviewing places where we already sync the icache or might need to.

[kib]

We can compare the old and new uio_resid then.

[kib]

In D60280#1383380, @kib wrote:

We can compare the old and new uio_resid then.

For this to work, uiomove_phys() should also avoid crossing userspace page boundaries, not just phys page boundaries.

[alc]

In D60280#1383380, @kib wrote:

We can compare the old and new uio_resid then.

Suppose that the first copyin() by uiomove_fromphys() will cross a page boundary in the source, but not in the destination. Data will be copied from the first source page to the destination page, and at the crossing in the source copyin() fails. uio_resid won't be updated. uiomove_fromphys() only updates uio_resid after at least one successful copyin().

[alc]

In D60280#1383381, @kib wrote:

In D60280#1383380, @kib wrote:

We can compare the old and new uio_resid then.

For this to work, uiomove_phys() should also avoid crossing userspace page boundaries, not just phys page boundaries.

Yes, which is unlikely to be worth the trouble, unless errors become a lot more common.

[alc]

sys/kern/uipc_shm.c

253–254

I'm going to drop the second sentence from this comment. I don't think that it really adds much.