Suppressing SIGBUS signals

Posted Jun 30, 2021 11:57 UTC (Wed) by hmh (subscriber, #3838)
In reply to: Suppressing SIGBUS signals by ilammy
Parent article: Suppressing SIGBUS signals

That looks like it would have been a much more generally useful (and expensive, complicated, etc) feature to implement than suppressing sigbus or zero-extending...

Mmap snapshot (maybe with a read-only result if that would be much easier or cheaper to implement and still cover the use cases).

But until someone offers to do that work...

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Suppressing SIGBUS signals

Posted Jul 1, 2021 2:16 UTC (Thu) by ilammy (subscriber, #145312) [Link]

After doing some research, this idea is implemented in some systems as a MAP_COPY flag for mmap() [1]. That's basically the semantics of MAP_PRIVATE mapping, but with a “snapshot” guarantee that the data won't change, making your copy genuinely private, but avoiding the actual copy if the data does not change.

While thinking of how this could be implemented, I realized that it could be quite expensive, complicated, and full of “spooky action at a distance”. If some process grabs or released a MAP_COPY mapping of a file, then all existing processes must be made aware of it (e.g., by turning all mappings for everyone RO and catching page faults). Any change by the other process forces said process to expend some kernel time doing the copy of the page for the benefit of some other process, which is not particularly fair.

Turns out, adding MAP_COPY into Linux was discussed several times [2][3], but it's still considered a pretty stupid idea.

[1]: https://www.gnu.org/software/hurd/glibc/mmap.html
[2]: https://yarchive.net/comp/linux/map_copy.html
[3]: https://www.spinics.net/lists/linux-mm/msg119339.html